2nd German-Vietnamese Symposium on
Frontiers in Material Science
6-9th October 2011
J.W. Goethe-University, Frankfurt (M)
Georg-August-University, Göttingen
Germany
2nd GERMAN‐VIETNAMESE WORKSHOP ON FRONTIERS IN MATERIALS SCIENCE Venue: Goethe‐University Frankfurt am Main, Room GW 1.101 Geowissenschaften; Altenhöferallee 1, 60438 Frankfurt am Main Thursday, October 6, 2011
9:00 – 9:20 Registration
9.20 ‐ 9.45 Welcome addresses by Prof. Dr. R. Valentí, Vice President of the Goethe University Prof. Dr. K. Samwer, Vice President of the German Science Foundation Prof. Dr. Nguyen Huu Duc, Vice President of the Vietnam National University Hanoi Session 1 9.45 – 10:10 10:10 ‐ 10.35 10:35 ‐ 11:05 (Chair: Michael Lang) Bach Thanh Cong Properties of magnetic nanometer films Huynh Dang Chinh Synthesis, structural, and magnetic properties of strontrium hexaferrite nanoparticles with La, Sm doping and core/shell structure by the sol‐gel hydrothermal process
Coffee break Session 2 11:05 ‐ 11:30 11:30 ‐ 11:55 11:55 ‐ 12:20 12:20 – 12:45 12:45 ‐ 14:00 (Chair: Bach Thanh Cong) Konrad Samwer Structural phase transition controlled by electronic correlations in manganites Vasily Moshnyaga Role of interfaces in the manganite physics Le van Hong Structural phase separation and magnetic properties of polycrystalline BaTi1‐xMnxO3multiferroics Jens Müller Investigation of transport and thermodynamic properties of the semimetallic ferromagnet EuB6
B
Lunch Session 3 (Chair: Philipp Gegenwart) 14:00 ‐ 14:25 Nguyen Nang Dinh Characterization of nanostructured composite coatings used for solid‐state lighting 14:25 ‐ 14:50 14:50 ‐ 15:15 Le Quoc Minh Controlling synthesis of photoresponsive nanomaterials for photonic wave guide and biomedical application Vu Dinh Lam Design, fabrication and characterization of the representative planar metamaterials operating at microwave frequency regime 15:15 ‐ 15:45 Coffee break 15:45 ‐ 16:10 16:10 ‐ 16:35 16:35 ‐ 17:00 17:00 ‐ 17:25 17:25 ‐ 18:30 Kurt Schönhammer Quantum quenches: full counting statistics, entanglement entropy and the Jarzynsky relation Vu Nguc Tuoc First principle study on AlN/GaN heterostructure nanowire Vu Van Hung Thermodynamic properties of zinc‐blende AlxGa1‐xAs crystals: temperature dependence Roser Valentí High‐Tc Superconductivity : Quo vadis ? Poster Session and Refreshments 18:30 Dinner Friday, October 7, 2011
Session 4 (Chair: Le Van Hong) 9:00 ‐ 9:25 Nguyen Huu Duc Multiferroics and geomagnetic Metglas/PZT laminates 9:25 ‐ 9:50 9:50 ‐ 10:15 10:15 – 10:45 sensors based on Pham Duc Thang PZT films grown on Si with improved ferroelectric properties Bernd Damaschke Local resistive switching on manganite thin films Coffee break Session 5 (Chair: Jens Müller) 10:45 ‐ 11:10 11:10 ‐ 11:35 Pham Hong Quang Effect of electrodeposition potential on the composition and morphology of CIGS absorber thin films 11:35 ‐ 12:00 12:00 ‐ 12:25 12:25 ‐ 14:00 Nguyen The Binh Preparation of Ag nanoparticles in trisodium citrate (TSC) solution by laser ablation and its antibacterial application against Escherichia coli Wolf Aßmus Stable phases of Cs2CuCl4‐xBrx mixed system Bernd Wolf Ultrasonic investigations near the B‐induced quantum critical point of the triangular‐lattice antiferromagnet Cs2CuCl4
Lunch Session 6 (Chair: Nguyen Huu Duc) 14:00 ‐ 14:25 14:25 – 14:50 Markus Münzenberg Hot spin physics 14:50 – 15:20 Coffee break 15:20 ‐ 15:45 15:45 ‐ 16:10 16:10 ‐ 16:35 16:35 – 16:55 Tran Kim Anh Role of Eu3+, Ce3+, Tb3+, Er3+, and Yb3+ ions in the luminescent nanomaterials and application potential Philipp Gegenwart Unconventional magnetic ordering in the honeycomb spin‐
orbit Mott insulator Na2IrO3
Andreas Honecker Multistep approach to microscopic models of frustrated quantum magnets: the case of the natural mineral Azurite Michael Lang Magnetic cooling through quantum criticality Closing remarks by Bach Thanh Cong and Michael Lang 19:30 Conference Dinner in Restaurant “Lahmer Esel“ Properties of magnetic nanometer thin films Bach Thanh Cong, Pham Huong Thao, Nguyen Thuy Trang Faculty of Physics, Hanoi University of Science, 334 Nguyen Trai, Hanoi, Vietnam Email: congbt@vnu.edu.vn In this report we study influence of size effect (thickness) on magnetic characteristics of few‐
layer thin films at finite temperature (magnetic moment and its fluctuation, Curie temperature…) using the functional integral method and Heisenberg localized spin model. It was shown that Gaussian fluctuation of magnetic moment (the Curie temperature) increases (reduces) with the reduction of thickness of films. Calculation at zero temperature (T=0) for CaMnO3 perovskite using Density Functional Theory (DFT) was also carried out in order to compare between localized spin, and electronic density models. Synthesis, structural, and magnetic properties of strontrium hexaferrite nanoparticles with La, Sm doping and core/shell structure by the sol‐gel hydrothermal process Dang Thi Minh Hue1, Luong Xuan Dien1, Tran Duc Hoang1, Phan Manh Huong2, Huynh Dang Chinh1
1) Department of Inorganic Chemistry, Hanoi University of Science and Technology, Vietnam 2) Department of Physics, University of South Florida, Tampa, Florida 33620, USA Email: chinhhd‐fct@mail.hut.edu.vn SrFe12O19 nanoparticles have been prepared by the sol‐gel hydrothermal method. These SrFe12O19 nanoparticles have been doped with La, Sm and coated with NiFe2O4, CoFe2O4 nanoparticles by the sol‐gel and the self‐assembly method, respectively. Properties of the nanoparticle samples were charaterized by powder X‐ray diffraction with the Rietveld refinement, transmission electron microscope, scanning electron microscopy with energy dispersive spectrometer, and vibrating sample magnetometry. SEM micrograhs of the samples showed the spherical shape of NiFe2O4, CoFe2O4 nanoparticles located above on the surface of SrFe12O19 hexagonal platelet shape. TEM micrographs showed the particle size of core and shell. With the core/shell nanoparticles, the saturation magnetization was decreased but coercivity was increased with SrFe12O19/CoFe2O4 sample. 80
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Magnetization loops at room temperature and TEM of core SrFe12O19, shell of spine ferrite, and core/shell nanoparticles which had been sintered at 950 0 C for 2 h in flowing oxygen gas. Keywords: Hexaferrite magnetic powder, sol‐gel hydrothermal, magnetic nanomaterrials Structural phase transition controlled by electronic correlations in manganites Konrad Samwer Interrelations between global and local structure and magnetism and transport in three‐
dimensional perovskite manganites is reviewed and compared with recent studies on thin films and superlattices. The concept of correlated Jahn‐Teller (JT) polarons is discussed within the phase separation scenario; their role in the local and global structural modifications of manganites is demonstrated. Polaron correlations, affected by external control parameters (temperature, electric and magnetic fields, doping, light, strain) may be very efficient to modify the ground state of manganites. Examples of electronic control of the structure by means of interface modifications, electric field and mechanical strain are highlighted. Role of interfaces in manganite physics V. Moshnyaga Physikalisches Institut, Georg‐August‐Universität Göttingen, Friedrich‐Hund‐Platz 1, 37077 Göttingen, Germany. Perovskite manganites are materials in which strong electronic correlations may be influenced and/or induced by electron‐phonon interaction (Jahn‐Teller effect). As a result a unique interplay between spin, charge and lattice degrees of freedom causes a number of interesting and useful phenomena, like colossal (CMR) and tunneling (TMR) magnetoresistance, resistance switching and, finally, multiferroic behavior. The interfacial and polaronic aspect of all these effects, considering competing different electronic/structural phases, will be highlighted and some insights into a common “polaronic” origin of magneto‐ and electro‐resistance properties in manganites will be discussed. For a prototypical phase separated CMR system, (La1‐yPry)0.7Ca0.3MnO3, very large low‐field CMR is due to exchange (AFM) coupling between ferromagnetic nanodomains, actuated by correlated polarons, which are located at the “intrinsic interfaces” between FM domains. Manganite/titanite superlattices (SL), e.g. [(La0.7Ca0.3MnO3)40u.c./(BaTiO3)20u.c.]10 (LCMO/BTO), are promising as potential artificial multiferroic system, in which orbital, charge and spin reconstructions at the interfaces play an important role. Moreover, epitaxy stress strongly influences both ferromagnetic (manganite) as well as ferroelectric (titanite) polarizations. SL’s have been grown by a metalorganic aerosol deposition technique on MgO(100) and SrTiO3 substrates. The structure of the LCMO/BTO interfaces was studied with atomic resolution by TEM and elemental chemical analysis (EELS). We show a possibility of interface engineering to control magnetotransport and crystal structure of the manganite phase (LCMO) by means of electron‐doping (2 u.c. of LaMnO3) at the interface. This shifts the manganite system away from the FM/AFM phase boundary and yields an electronically homogeneous and basically polaron‐free ferromagnetic metallic ground state. Structural phase separation and magnetic properties of polycrystalline BaTi1‐xMnxO3 multiferroics N. V. Danga,b, T. D. Thanhb, N. X. Nghiab, V. D. Lamb, L. V. Hongb, P. Zhangc, T. L. Phanc a
Faculty of Physics, College of Science, Thai Nguyen University, Thai Nguyen, Viet Nam b
Institute of Materials Science, VAST, Ha Noi, Viet Nam c
Department of Physics, Chungbuk National University, Cheongju 361‐763, Korea This work presents the influences of the Mn doping on structural characterization and magnetic properties of BaTi1‐xMnxO3 (x = 0.0 ‐ 0.1). These samples were synthesized by solid‐
state reaction. The products obtained were studied the crystal structure, phonon spectra, and magnetic properties by means of x‐ray diffraction (XRD), Raman scattering (RS) spectroscopy and a physical properties measurement system (PPMS), respectively. XRD analyses reveal a phase separation from a tetragonal structure to a hexagonal one taking place at a threshold concentration of x = 0.01. An increase in Mn content (x) in BaTi1‐xMnxO3 leads to an increase in crystalline volume fraction of the hexagonal phase. It reaches a maximum value as x ≈ 0.10. This is also further confirmed by RS spectra though the appearance of phonon modes associated with the hexagonal phase, and the disappearance of modes associated with the tetragonal phase. Both XRD and RS studies indicate an incorporation of Mn dopants into BaTiO3 host lattices. The presence of these impurities enhances the room‐temperature ferromagnetism (FM) of BaTi1‐xMnxO3. Values of the saturation magnetization and coercivity depend strongly on the variations structural phases and Mn dopant concentration. Interestingly, the undoped sample (i.e., BaTiO3) also exhibit room‐temperature FM. The heat treatment in Ar ambient enhances remarkably the FM order, particularly for a long annealing time. This conducted to a contribution of the present of Ti3+ due to the oxygen deficiency. Investigation of transport and thermodynamic properties of the semimetallic ferromagnet EuB6 Jens Müller, Pintu Das, Adham Amyan, Jens Brandenburg, Mariano de Souza, Michael Lang Institute of Physics, Goethe‐University Frankfurt, Germany The effect of a material’s magnetic state on the electronic system is at the core of spintronics research. A model system, where the intricate interplay of magnetism and electronic transport is particularly evident, is the rare earth chalcogenide EuB6, a rare example of a low‐carrier density semimetal, which also orders ferromagnetically. In this material, the sensitivity of the electrical transport properties to applied magnetic fields (colossal magnetoresistance effect) is related to the presence of magnetic polarons. In transport and thermodynamic measurements, two consecutive phase transitions at about TC = 12.6 K and TM = 15.3 K are observed: upon lowering the temperature, a charge‐
delocalization transition via the overlap of magnetic polarons precedes the ferromagnetic ordering. This behavior is discussed in terms of electronic and magnetic phase separation, and a percolative phase transition [1]. We employ fluctuation (noise) spectroscopy in order to investigate the dynamics of the charge carriers close to the above‐mentioned transitions and their coupling to fluctuating and spatially inhomogeneous magnetic clusters. At temperatures, where the formation of magnetic polarons is expected to start, the resistance noise power spectral density becomes strongly enhanced, and divergingly large resistance fluctuations are observed at the ferromagnetic transition temperature TC. We also discuss the onset of non‐linear electronic transport below TM exhibiting a maximum at TC and its strong dependence on small external magnetic fields. Furthermore, highresolution thermal expansion measurements reveal that lattice degrees of freedom are strongly coupled to both low‐temperature transitions. [1] X. Zhang, L. Yu, S. von Molnár, Z. Fisk and P. Xiong, Phys. Rev. Lett. 103, 106602 (2009). Characterization of nanostructured composite coatings used for solid‐state lighting .Do Ngoc Chung, Tran Thi Thao, Nguyen Phuong Hoai Nam, Nguyen Nang Dinh University of Engineering and Technology, VNU Hanoi, 144 Xuan Thuy Road, Cau Giay Distr., Hanoi, Vietnam Email: dinhnn@vnu.edu.vn With the aim to enhance the photoluminescence efficiency of conjugate polymers, nanocomposite films from Poly[2‐methoxy‐5‐(2'‐ethyl‐hexyloxy)‐1,4‐phenylene vinylene] as MEH‐PPV with nanocrystalline TiO (nc‐TiO ) – (MEH‐PPV+nc‐TiO ) were prepared. MEH‐
2 2
2
PPV+nc‐TiO composites incorporated with Y Al O :Ce (YAG:Ce) were used for covering 2 3
5 12
InGaN blue LED chip to make white LEDs (WLED). The research results showed that both the electrical and spectral properties of the conjugate polymers were enhanced due to the incorporation of nc‐TiO with an optimal concentration embedded in the polymers. The 2 efficiency of the luminous flux and angle intensity distribution of the WLEDs were also obtained. Keywords: Nanocomposite, Photoluminescence (PL), White Light Emitting Diode (WLED). Controlling synthesis of photoresponsive nanomaterials for photonic wave guide and biomedical application 1,2 1,3 Le Quoc Minh* , Le Dac Tuyen , Chia Chen Hsu3, Lam Thi Kieu Giang1,Tran Thu Huong1, Nguyen Thanh Huong1, Hoang Thi Khuyen1, Nguyen Thanh Binh, Vu Duc Tu1,2 Tran Duc Dat1 and Tran Kim Anh1
1) Institute of Materials Science (IMS), Vietnam Academy of Science and Technology (VAST) 2) University of Engineering and Technology (UET), Vietnam National University Hanoi 3) Department of Physics, National Chung Cheng University, Ming Hsiung, Chia Yi 621, Taiwan Email: lequocminh@ims.vast.ac.vn
A survey in brief of the controlling syntheses of photoresponsive nanomaterials towards to construct photonic wave guide and for biomedical application will be presented. The controlling nanosynthesis approach consisted of four consequent stages: seed grow, encapsulate, functionization and conjugation between nanomoisture and expected molecular structures. The controlling synthesis in size, shape and structures of nanomaterials containing rare earth ions, Y, Eu have been investigated for fabrication of active photonic crystal in form of opal like. The TEM image, reflexion and luminescence spectra of the Eu‐active photonic crystals prepared by self assembling process have indicated the construction obtained having highly optical band gap characters. The measurements of luminescent properties of the nanophosphors rods and tubes Y(OH)3: Eu3+, Y2O3:Eu3+ , Eu(OH)3, Eu2O3, and EuPO4 .H2O have been studied to elucidate the emission process of one dimensional (1‐D) nanostructured RE compounds due to space anisotropy in comparison with zero dimensional nanostructures. Based on the large difference of the rates of the transitions from 5D0‐7F2 and 5D0‐7F1 energy levels in nanoparticles and nanotubes of Y(OH)3:Eu3+, Y2O3:Eu3+, Eu(OH)3 , Eu2O3 , and EuPO4 one can state that the shape anisotropy play an important role to the local structures of Eu3+ doped 1‐D nanonanomaterials. The rare earth nanoluminophor used research to biomedical application is intensively underway for development of an novel label in fluorescent immunoassay analysis to viral (vaccine) biomedical industry. Keywords: Controlling nanosynthesis, active photonic crystal, rare earth nanoluminophor, fluorescent biomedlabel Design, fabrication and characterization of the representative metamaterial structures operating at microwave frequency regime Vu Dinh Lam, Do Thanh Viet, Le Thi Quynh, and Nguyen Thanh Tung Institute of Materials Science, Vietnam Academy of Science and Technology, Vietnam We investigated both experimentally and numerically the electromagnetic response of different metamaterial structures from the cut‐wire pair for providing a negative magnetic permeability to the combined structure of cut‐wire pair and continuous wire exhibiting the left‐handed behavior, in other words, the negative refractive index. It was found that the cut‐wire pair structure exhibits both magnetic and electric resonances. Two resonances are separated by a certain frequency range, which strongly depends on the distance between cut‐wire pair layers. The results also show that this effect leads to a decrease in the effective plasma frequency of combined structure when the distance between combined structure layers is increased, resulting in the degraded or even destroyed left‐handed behavior of combined structure. These structures were designed, built, and measured in the microwave frequency range. Quantum quenches: full counting statistics, entanglement entropy and the Jarzynsky relation K. Schönhammmer When separate metallic subsystems are suddenly connected their quantum states become entangled. This is accompanied by electron and energy exchange. For a finite bias between the subsystems not only the average current and its fluctuation but the whole probability distribution of the transferred charge (“full counting statistics”) can be investigated. Neglecting the electron‐electron interaction it can be obtained from the eigenvalues of a time dependent one‐particle operator. These eigenvalues also determine the entanglement entropy. For the probability distribution of the energy transfer the “Crooks relation” holds which generalizes the detailed balance relation valid in the linear response regime. It implies the “Jarzynsky relation” which allows to generally prove that systems in thermal equilibrium are “passive” subject to external perturbations. First principle study on AlN/GaN heterostructure nanowire
Vu Ngoc Tuoc1,2, Nguyen Viet Minh2, Le Thi Hong Lien2, Tran Doan Huan2,3
1) International Center for Computational Materials Science (ICCMS), Hanoi University of Science and Technology, 1 Dai Co Viet Road, Hanoi, Vietnam 2) Department of Theoretical Physics, Institute of Engineering Physics, Hanoi University of Science and Technology, 1 Dai Co Viet Road, Hanoi, Vietnam 3) Department of Physics, University of Basel, Klingelbergstrasse 82, CH‐4056 Basel, Switzerland Email: tuocvungoc@mail.hut.edu.vn
We perform a first‐principles density functional theory study on structural and optical properties of several series of passivated and unpassivated AlGaN/GaN heterostructure superlattice nanowires. The effects of surface relaxation and surface stress which are absent in atomistic models are carefully taken into account. Structural properties, energy bands and optical properties for a class of hexagonal nanowires with various compositional content superlattice structure and size are calculated. Obtained results show that the passivation procedure with pseudo hydrogen, i.e. hydrogen with partial charges, works well and completely suppressed the effect of dangling bond states at the outer surface. With this passivation procedure the band gap increases, while the optical absorption coefficient data shows that the structures are active in the ultraviolet region. Keywords: Wurtzite nano wire, density functional theory, heterostructure, superlattice High‐Tc superconductivity: Quo vadis? Roser Valentí
Goethe‐University Frankfurt am Main One hundred years after the discovery of superconductivity by Kamerlingh Onnes in Leiden, this phenomenon has remained one of the most fascinating subjects in solid state physics. In his talk I will give an historical overview about superconductivity and discuss the observation of this phenomenon in many unexpected materials like oxides, organic systems and recently iron pnictides. I will also discuss possible routes to get high‐Tc superconductors in the future. Multiferroics and geomagnetic sensors based on Metglas/PZT laminates D.T. Huong Giang, P.A. Duc, N.A. Phuong, N.T. Ngoc, N.X. Toan and N.H. Duc
Laboratory for Nano Magnetic Materials and Devices, Faculty of Engineering Physics and Nanotechnology, VNU University of Engineering and Technology, Vietnam National University, Hanoi Email: giangdth@vnu.edu.vn Spintronics is a new branch of physics and nanotechnology, which aims to simultaneously combine both the charge and the spin of electrons in the same device and describes the new physics. Spintronics is mainly related to the magnetization switching phenomena, among which field‐induced magnetization switching, thermally assisted switching and spin transfer switching are well described. Multiferroics are materials which possess simultaneous ferromagnetic and ferroelectric properties as well as interesting magnetoelastic phenomena. Due to the magnetoelectric coupling between these properties, the magnetization or electric polarization can be turned by applying an external electrical field or magnetic field, respectively. This magnetoelectric (ME) effect proposed another magnetization switching type, names as ME‐induced spin reorientation. It can give new advances in spintronics nanostructures. This paper, however, focuses to the geomagnetic sensors. The sensor is developed base on magnetoelectric effect of Ni‐based Metglas/PZT laminates. By optimizing the size, shape and configuration of laminated composites, a new type of magnetic sensor has been designed, fabricated and characterized. An incredibly high output voltage response to a low magnetic field of 871 mV/Oe with a resolution of 3×10‐4 Oe has been achieved in the composite with the dimension of 1×15 mm2. This sensor can measure precisely the not only a strength variation in the range of the Earth's magnetic field but also its orientation. The angular resolution and sensitivity of sensor are determined of 10‐1 degree and 3.86 mV/degree, respectively. The simple, low cost fabrication sensor and high sensitivity as well as accuracy and resolution without amplification make these sensors potential for not only a novel smart compass and global positioning devices but also for magnetic biosensor applications. PZT films grown on Si with improved ferroelectric properties P.D. Thang and M.T.N. Pham Thin films of perovskite‐type oxide Pb(Zr0.53Ti0.47)O3 (PZT) have been grown on Si (001) substrate by pulsed laser deposition (PLD). To minimize the effect of the native amorphous SiOx layer on the Si surface, a thin buffer layer of Yttria‐stabilized zirconia (YSZ) has been pre‐
deposited in Ar then O2 environments prior to the deposition. SrRuO3 (SRO) and (La0.7Sr0.3)MnO3 (LSMO) were chosen as the next electrode/buffer layers for subsequent PZT deposition. By replacing the layer of SRO by LSMO and using a suitable substrate temperature, the oriented direction of PZT films can be tuned from (110) to (001) direction. The films exhibit low leakage current in the range of 10‐6 A/cm2 and well‐defined butterfly shape of capacitance‐voltage dependence. The polarization‐voltage measurements show hysteresis loops with high remanent polarization around 15 μC/cm2. Local resistive switching on manganite thin films B. Damaschke Physikalisches Institut, Georg‐August‐Universität Göttingen Perovskite manganites are known for a rich variety of interesting magnetic and electric phenomena; very prominent, for instance, are the metal insulator transition and the colossal magnetoresistance effect. Moreover, they can also show resistive switching phenomena driven by an electric field [1, 2]. We have performed local conductivity measurements by conductive atomic force microscopy and observed bipolar switching of nanometer sized regions [3]. The results were obtained for La Ca MnO thin films revealing spatial properties and providing insight in 0.8
0.2
3 the voltage and pulse width dependence of the switching process (Fig. 1). Our results support a scenario based on local structural changes. A qualitative model for the switching mechanism accounting for different orbital and structural states at the surface is presented. Fig. 1: Local switching experiment with voltage pulses in a conductive atomic force microscope: the current map shows the switched regions (white, high current) in an insulating matrix (black, low current). Pulse height (left to right) and pulse duration (bottom to top) were systematically varied. References: [1] M. Esseling: Grenzflächen in Manganatschichten, Thesis, Göttingen 2008 [2] C. Kalkert, J.‐O. Krisponeit, M. Esseling, V. Moshnyaga, B. Damaschke, and K. Samwer, Resistive switching at manganite/manganite interfaces, accepted for publication in Appl. Phys. Lett. 2011 [3] J.‐O. Krisponeit, C. Kalkert, B. Damaschke, V. Moshnyaga, and K. Samwer Nanoscale resistance switching in manganite thin films: Sharp voltage threshold and pulse‐width dependence, Phys. Rev. B 82, 144440 (2010) Effect of electrodeposition potential on the composition and morphology of CIGS absorber thin film Pham Hong Quanga*, Ngo Dinh Sangb, Le Tuan Tua, Dang Thi Bich Hopa and Nguyen Thanh Nghia
a
Hanoi University of Science, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam b
National University of Civil Engineering, 55 Giai Phong street Hai Ba Trung, Hanoi, Vietnam E‐Mail: phquang2711@yahoo.com CuInGaSe (CIGS) thin films were deposited on Mo/soda‐lime glass substrates by electrodeposition at different potentials ranging from ‐0.3 to ‐1.1 V vs. Ag/AgCl. Cyclic voltamettry (CV) studies of unitary Cu, Ga, In and Se systems, binary Cu‐Se, Ga‐Se and In‐Se systems, and quaternary Cu‐In‐Ga‐Se have been carried out to understand the mechanism of deposition of each constituent. The concentration of the films was determined by EDS. The structure and morphology of the films were characterized by XRD and SEM. The underpotential deposition mechanism of Cu‐Se and In‐Se phases was observed in voltammograms of binary and quaternary systems. The variation in composition with applied potentials was explained by CV data. A suitable potential range from ‐0.8 V to ‐1.0 V was found for obtaining films with desired and stable stoichiometry. In the post annealing films, the chalcopyrite structure starts occurring in the samples deposited at ‐0.5 V and grows with varying the applied potential toward negative direction. Keywords: thin films, cyclic voltammetry, CIGS, solar cell, electrodeposition Preparation of Ag nanoparticles in trisodium citrate (TSC) solution by laser ablation and its antibacterial application against Escherichia coli Nguyen The Binh, Nguyen Hoang Hai, Nguyen The An, Pham Nguyen Hai Faculty of Physics, University of Natural Science, VNU‐Hanoi. Email : thebinh@vnu.edu.vn Silver nanoparticles (AgNP) were prepared by laser ablation of a silver plate in a non‐toxic and biocompatible solution (Trisodium citrate –TSC solution). The role of laser fluence, laser wave length and TSC concentration were studied in the laser ablation process. The silver nanoparticles are rather spherical in shape and their average diameter ranges from 6 to 18nm. Our observation shows that the prepared silver nanoparticles colloids was stable at least during 60 days in TSC solution. We studied qualitatively the antibacterial activity of silver nanoparticles against Escherichia coli DH5α in LB (Luria‐Bertani) broth. The antimicrobial activity of AgNP against E. coli is obtained at very low concentration with MIC (minimal inhibitory concentration) value of 16 μg/ml. Stable Phases of the Cs2CuCl4‐xBrx Mixed Systems N. Krueger, A. A. Haghighirad, F. Ritter, and W. Assmus Physikalisches Institut, Goethe‐Universitaet Frankfurt, Max‐von‐Laue‐Strasse 1, 60438 Frankfurt am Main, Germany In recent years, Cs2CuCl4 has attracted much interest, because it was one of the first systems that permitted the observation of field‐induced Bose‐Einstein condensation of magnons. On the other hand, for the iso‐structural Cs2CuBr4, magnon crystallization is found instead of magnon condensation. The unusual properties of these cesium halogen cuprates are caused by the special arrangement of the constituent CuX4 (X=Cl or Br) tetrahedra and Cs ions, resulting in a characteristic hierarchy of magnetic interactions. Starting from Cs2CuCl4 and Cs2CuBr4, we focus on the growth of the Cs2CuCl4‐xBrx mixed crystals from aqueous solution and the investigation of the occurring structural variations. The well‐known orthorhombic structure (space group Pnma) of the end members of this system is interrupted within the intermediate composition range from Cs2CuCl3Br1 to Cs2CuCl2Br2, if the growth takes place at room temperature. Within this range, a new tetragonal phase is found (space group I4/mmm). However, in case the growth temperature is increased to 50 °C, the existence of the orthorhombic structure can be extended over the whole Cs2CuCl4‐xBrx mixed system. A detailed analysis of the composition dependence of the lattice parameters is used to draw conclusions about the incorporation of Cl and Br ions at different sites, which is important for the magnetic interactions between the Cu ions. The mixed crystal series Cs2CuCl4‐xBrx is well suited to serving as a model system for magnon condensation and crystallization phenomena because the orthorhombic structure type exists within the whole composition range. An especially attractive feature of this system is given by the selectivity of the halogen sites that allows the study of the specific influence of a composition variation at one of these sites. The phase diagram of the CsX/ CuX2/aqueous solution system (X=Cl or Br) leading to mixed crystal system turns out to be richer than expected. For most of the concentration range, more than one structural modification can be obtained, depending on the growth conditions. If the growth temperature is lowered to 8 °C the concentration range for this tetragonal phase seems to extend to the pure end member Cs2CuCl4.We have observed for the first time a tetragonal water free structure type for Cs2CuCl4, which has to be clarified by further investigations. Details of latest results will be shown. This project is supported by Deutsche Forschungsgemeinschaft SFB/TR 49. Reference: N. Krueger et al., Crystal Growth & Design, Vol. 10, Issue 10, pp 4456‐4462 (2010) Ultrasonic investigations near the B‐induced quantum critical point of the triangular‐lattice antiferromagnet Cs2CuCl4
B. Wolf, P. T. Cong, N. Krüger, F. Ritter, W. Assmus and M. Lang Physikalisches Institut, Goethe‐Universität, SFB/TR 49, Frankfurt, Germany The quantum magnet Cs2CuCl4 represents the spatially anisotropic version of a quasi‐
twodimensional triangular‐lattice spin‐1/2 Heisenberg antiferromagnet with moderate magnetic coupling constants. Of particular interest in this system are the anomalous physical properties, resulting from the interplay of strong quantum fluctuations and geometric frustration. In addition, Cs2CuCl4 has attracted much attention due to its spin‐liquid properties and the B‐induced quantum‐critical point (QCP) at the saturation field Bs. Anomalous physical properties, even at finite temperatures, are expected to be observed around Bs as a consequence of the underlying quantum phase transition. Measurements of the longitudinal elastic constants c11, c22 and c33 and the ultrasonic attenuation α in Cs2CuCl4 reveal distinct anomalies near the QCP. These anomalies are particularly strongly pronounced in the ultrasonic attenuation. In isothermal field sweeps performed at low temperatures 0.027 K ≤ T ≤ 0.3 K around the saturation field Bs, the ultrasonic attenuation of all three modes shows a pronounced double‐peak structure, indicating two anomalies of different origin. Upon cooling, however, both features merge suggesting a coincidence at the QCP. While one peak, which can be attributed to the ordering temperature TN(B), becomes critically enhanced upon approaching the QCP, the other one reduces in size and narrows upon cooling. The latter effect has been tentatively assigned to the material’s spin‐liquid features which precede the long‐range antiferromagnetic ordering. Hot spin physics Marvin Walter1, Anissa Zeghuzi1, Mirco Marahrens1, Vladyslav Zbarskyy1, Jakob Walowski1, Andreas Mann1, Unai Atxitia2, Oksana Chubykalo‐Fesenko2, Daniel Ebke3, Andy Thomas3, GünterReiss3, Stefan Maat4, Matthew J. Carey4, Jeffrey R. Childress4, C. Heiliger5, Markus Münzenberg1
1
I. Physikalisches Institut, Universität Göttingen, Germany 2
Instituto de Ciencia de Materiales de Madrid, CSIC, Spain 3
Department of Physics, Universität Bielefeld, Germany 4
San Jose Research Center, Hitachi Global Storage Technologies, San Jose, CA, USA, 5Justus‐Liebig Universität Gießen, Germany In my talk, I present two areas of our work with hot spins in half metallic materials: ultrafast femtosecond demagnetization experiments and the generation of thermovoltages via the magneto‐Seebeck effect. In half metallic materials, the electronic structure is basically different for both spin directions – one side of the spin system for example the majority channel acts as metal, the other spin direction e.g. minority channel has a band gap and electron transport is blocked. Ultrafast femtosecond demagnetization experiments allow meanwhile to determine the microscopic origin of the intrinsic spin‐flip processes. They originate from a spin‐orbit interaction or exchange scattering type. A single spin‐flip rate τel‐
sp together with intrinsic spin fluctuations gives us meanwhile a reasonable understanding of the ultrafast demagnetization [1]. By either promoting or inhibiting spin‐flip processes, a faster [2] or slower [3] demagnetization is expected respectiveley. In contrast to ferromagnetic metals, in half‐
metals the minority‐spin gap blocks the spin scattering, therefore inhibiting spin‐flip processes. As a consequence the demagnetization time τM is slowed down to the ps or even ns range. We proved this behavior for different materials relevant for spin electronics and developed a prediction of this effect directly relating τel‐sp to the spin polarization P [3]. In the transition region between clear half metallic behavior and a ferromagnetic metal with high P, the effects on the demagnetization time τM are generally smaller. For highly spin polarized metallic Heusler films with P=86% we find demagnetization times of up to 400 fs as predicted by our model [3]. The strong electronic assymetry at around the Fermi level, has a second consequence, namely results in a strong spin‐Seebeck effect. Heating a half metallic tunnel junction allows therefore to produce large thermal magneto‐Seebeck voltages for switching the magnetic electrodes from the parallel to the antiparallel orientation [4]. For both, the ultrafast demagnetization of half metals and the magneto‐Seebeck effect in tunnel junctions, the underlying reason is the semiconducting behavior of the half metallic material in one spin channel. [1] U. Atxitia, O. Chubykalo‐Fesenko, J. Walowski, A. Mann, and M. Münzenberg, Phys. Rev. B 81, 174401 (2010). [2] J. Walowski, G. Müller, M. Djordjevic, M. Münzenberg, M. Kläui, C. A. F. Vaz and J. A. C. Bland, Phys. Rev. Lett. 101, 237401 (2008). [3] G. M. Müller, M. Djordjevic, G.‐X. Miao, A. Gupta, A. V. Ramos, K. Gehrke, V. Moshnyaga, K. Samwer, J. Schmalhorst, A. Thomas, A. Hütten, G. Reiss, J. S. Moodera, M. Münzenberg, Nature Mater. 8, 56 (2009). [4] M. Walter, J. Walowski, V. Zbarsky, M. Münzenberg, M. Schäfers, D. Ebke, G. Reiss, A. Thomas, P. Peretzki, M. Seibt, M. Czerner, M. Bachmann, C. Heiliger, Nature Mater., advance online (2011), doi:10.1038/nmat3076.html. Website: http://www.uni‐goettingen.de/de/124076.html Role of Eu3+, Ce3+,Tb3+, Er3+ and Yb3+ ions in the Luminescent Nanomaterials and Application Potential Tran Kim Anh*1, Tran Thu Huong1, Nguyen Vu1, Lam Thi Kieu Giang1, Dinh Xuan Loc1, Wieslaw Strek3 and Le Quoc Minh1,2
1) Institute of Materials Science (IMS), Vietnam Academy of Science and Technology (VAST) 2) University of Engineering and Technology (UET), Vietnam National University Hanoi 3) Institute of Low Temperature and Structure Research, Polish Academy of Science, 2 Okolna, Wroclaw, Poland Email:minhkimanh@gmail.com Role of rare earth ions Eu3+, Ce3+, Tb3+, Er3+ , Yb3+ in the nanoluminophor in the hosts YVO4, Y2O3, PO4 or NaYF4 were investigated in detail in order to chose new materials for apply in the lighting, security printing, solar energy conversion as well as biolabeling. Lanthanides activated materials are widely used for luminescent lamps, flat displays, solid state laser, optical fiber and other fields due to the unique solid state electronic properties, Eu3+, Tb3+, Er3+ to emit photons efficiently in visible and near infrared. Eu3+is strong red emission. Eu3+ spectroscopy used as well known structural probes in Y2O3: Eu. The TbPO4 nanorods (diameter about 15‐30 nm, lengths about 300‐400 nm) were successfully prepared for the first time by hydrothermal method in autoclave. They have strong green luminescent (5D4‐
7Fj) in the case pH=2, enough apply in labels for medical. Used Hecxadecyl trimethyllammonium bromide (HTBA), Sodiumdodecyl sunfate (SDS) Dioctyl sulfosuccinate sodium salt (AOT) as soft template, the size of YVO4:Eu3+ nanoparticles have small diameter of 8‐12 nm, used SDS have strong luminescent intensity than the case used AOT and HTBA. To compare the luminescent spectra of YVO4: Eu3+; YVO4: Eu3+ @ silica, YVO4: Eu3+ @ silica‐
NH2; YVO4: Eu3+ @ silica‐SCN; YVO4: Eu3+ @ silica‐SCN/Biotin show that YVO4: Eu3+ @ silica and YVO4: Eu3+ @ silica‐NH2 have strong red luminescent intensity. Energy transfer effect Tb ‐ Eu in Y2O3, Ce‐Tb in CePO4: Tb and up‐conversion of Y2O3: Er,Yb, NaYF4: Er, Yb will be discussed on the study of photoluminescent spectra and lifetime. In solid phases electrons in the 4fn orbital of lanthanides are localized, do not exhibit quantum confinement even in nanocrystals. Influence of activator concentration, conditions of synthesis such as temperature, pH or core /shell structure in the case of CePO4: Tb/LaPO4 to the size, luminescent spectra and lifetime will be presented. The up conversion mechanism, the competition between red and green emission in the case of Y2O3: Er, Yb, NaYF4: Er have studied. Our materials to apply for security printing (YVO4: Eu, CePO4: Tb) and infrared cards (Y2O3: Er, Yb, NaYF4: Er). Nevertheless a promising application such as nanophosphors for high resolution display and biomedicine will be discussed. Photoluminescent spectra of YVO4: Eu3+; YVO4: Eu3+ @ silica‐NH2; YVO4: Eu3+ @ silica‐SCN; YVO4: Eu3+ @ silica‐SCN/Biotin , 370 nm excited Keywords: Eu3+, Ce3+,Tb3+, Er 3+,Yb3+, Energy transfer , Up conversion Unconventional magnetic ordering in the honeycomb Spin‐orbit Mott insulator Na2IrO3
P. Gegenwart I. Physikalisches Institut, Georg‐August University Goettingen Iridates displaying a Mott insulating state caused by the interplay of electronic correlations and strong spin‐orbit coupling have recently attracted considerable attention. We focus on the honeycomb material A2IrO3 (A=Na, Li), in which the topology of the underlying lattice leads to interesting magnetic properties [1]. The strong spin‐orbit coupling in this 5d transition metal system is expected to result in orbital‐dependent highly anisotropic magnetic in‐plane exchange [2]. The combination of Jeff = 1/2 and the underlying honeycomb lattice makes A2IrO3 a promising candidate for the exactly solvable Kitaev model, which is exactly solvable and has a spin‐liquid ground state. Our experimental data on Na2IrO3 prove a Mott insulating state of effective J=1/2 moments with predominant antiferromagnetic coupling, indicated by a Weiss temperature of θ= −120 K. A bulk antiferromagnetic transition occurs at a much reduced temperature of TN = 15 K and the reduced magnetic entropy suggests strong magnetic frustration and/or low‐
dimensional magnetic interactions. The nature of the ordered phase has also been studied by resonant x‐ray spectroscopy near the Ir‐L3 edge, providing evidence for an unconventional, most‐likely zig‐zag‐type spin ordering [3]. The latter may be related to a substantial next‐nearest neighbour exchange or a substantial Kitaev contribution in the Heisenberg‐Kitaev model [2]. Upon replacing Na with the smaller Li, one may enhance the relative importance of the Kitaev contribution. For Mott insulating Li2IrO3 we observe a similar ordering temperature of 15 K, while the negative Weiss temperature is drastically reduced. These observations are compatible with an enhancement of the Kitaev contribution compared to the Na‐system, suggesting that Li2IrO3 is located close to the Kitaev limit [5]. Work in collaboration with Yogesh Singh, Soham Manni, X. Liu and J.P. Hill. [1] Yogesh Singh and P. Gegenwart, Phys. Rev. B. 82, 064412 (2010). [2] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205, (2009). [3] X. Liu et al., Phys. Rev. B 83, 220403(R) (2011). [4] Yogesh Singh, S. Manni, P. Gegenwart, arXiv:1106.0429v1. [5] J. Reuther, R. Thomale, S. Trebst, arXiv:1105.2005. Multistep Approach to Microscopic Models for Frustrated Quantum Magnets: The Case of the Natural Mineral Azurite Andreas Honecker,a Shijie Hu,a Robert Peters,b Johannes Richter,c Thomas Pruschke,a
Tanusri Saha‐Dasgupta,d Hena Das,d Helge Rosner,e Oleg Janson,e Hem Kandpal,f
Xiaoqun Wang,g Andreas Brühl,h Bernd Wolf,h Michael Lang,h Ingo Opahle,i
Roser Valentí,i Harald Jeschkei
a
Institut für Theoretische Physik, Georg‐August‐Universit at G ottingen, Germany b
Department of Physics, Kyoto University, Japan c
Institut f ur Theoretische Physik, Otto‐von‐Guericke Universit at Magdeburg, Germany d
Bose National Centre for Basic Sciences, Kolkata, India e Max‐Planck‐Institut für Chemische Physik fester Stoffe, Dresden, Germany, f IFW Dresden, Germany g
Department of Physics, Renmin University of China, Beijing, China h
Physikalisches Institut, Goethe‐Universit at Frankfurt am Main, Germany i
Institut für Theoretische Physik, Goethe‐Universit at Frankfurt am Main, Germany The natural mineral azurite has been used as a blue pigment since ancient times. More recently, a plateau at 1/3 of the saturation magnetization has been discovered in the high‐
field magnetization curve of azurite Cu3(CO3)2(OH)2. By first‐principles computations, we have identified eight relevant exchange constants [1]. We argue that, to a first approximation, the interchain exchange can be neglected for the low‐energy properties and derive an effective generalized spin‐1/2 diamond chain model. Using numerical results, we then demonstrate that a consistent description can be obtained for various physical properties of azurite [1]: (i) the low‐temperature magnetization curve, (ii) inelastic neutron scattering on the 1/3 magnetization plateau, (iii) nuclear magnetic resonance measurements on the 1/3 magnetization plateau, and (iv) the magnetic susceptibility as well as the specific heat. Our results not only resolve previous controversies on the modeling of azurite but place it in a highly frustrated parameter regime. We point out that this implies in particular an unusually large low‐temperature entropy at high magnetic fields, thus predicting an enhanced magnetocaloric effect [2]. [1] H. Jeschke, I. Opahle, H. Kandpal, R. Valent
, H. Das, T. Saha‐Dasgupta, O. Janson, H. Rosner, A. Br uhl, B. Wolf, M. Lang, J. Richter, S. Hu, X. Wang, R. Peters, T. Pruschke, A. Honecker, Phys. Rev. Lett. 106 (2011) 217201 [2] A. Honecker, S. Hu, R. Peters, J. Richter, J. Phys.: Condens. Matter 23 (2011) 164211 Magnetic cooling through quantum criticality M. Langa, B. Wolfa, Y. Tsuia,b, D. Jaiswal‐Nagara, U. Tutscha, A. Honeckerc, P. T. Conga, A. Prokofievc, and W. Assmusa
a
Physics Institute, Goethe University, SFB/TR 49, Frankfurt, Germany b
Present address: Department of Physics, Durham University, United Kingdom c
Institut of Theoretical Physics, Georg‐August University, Göttingen, Germany d
Institute of Solid State Physsics, Vienna University of Technology, Austria The proximity of a quantum critical point can significantly affect a material’s thermodynamic properties even at finite temperatures. Here we show that the accumulation of entropy around a B‐induced quantum critical point opens up new possibilities for realizing a very efficient low‐T magnetic coolant [1]. For the proof of principle, we focus on a simple model substance ‐ a Cu2+‐containing coordination polymer [Cu(μ‐C2O4)(4‐aminopyridine)2(H2O)]n ‐ a very good realization of a spin‐1/2 antiferromagnetic Heisenberg chain with a weak intra‐
chain coupling constant J/kB = (3.2 ± 0.1) K, corresponding to a saturation field Bs = 4.09 T. To B
B
investigate its potential as a coolant, demagnetization experiments have been performed from Bi > Bs under almost adiabatic conditions. While the cooling process is initially linear in B ‐ such as is seen in standard paramagnets ‐ it becomes superlinear upon approaching the QCP at Bs. In addition, the quantum critical system excels by its high efficiency ΔQc/ΔQm, which exceeds the figures found in state‐of‐the‐art paramagnetic coolants by a factor 2‐3. Here ΔQc is the heat the material can absorb after demagnetization to a final field Bf, and ΔQm the heat of magnetization released to a precooling stage held at a temperature Ti, the initial temperature of the cooling process. While the experimental and theoretical results on the spin‐1/2 antiferromagnetic Heisenberg chain have enabled a proof‐of‐principle demonstration of magnetic cooling through quantum criticality, extensions of this concept are obvious and should be further explored. As one of the promising routes in the search for materials with further improved cooling performance, the combination of reduced dimensionality with geometric frustration [1] has been proposed. As a consequence of the frustration, a large absolute variation of the entropy with magnetic field and hence a large ∂T/∂B can be expected. As a first step in this direction, we have studied the magnetothermal response of Cs2CuCl4, a layered triangular‐
lattice Heisenberg antiferromagnet. In this material, the frustration effects derive from a dominant antiferromagnetic exchange coupling J/kB = 4.3 K along the in‐plane b‐axis and a B
second in‐plane coupling of J’~ J/3 along a diagonal bond in the bc‐plane. Further couplings in this material include an inter‐plane interaction J’’ ~ J/20 as well as a small anisotropic Dzyaloshinskii‐Moriya interaction D ~ J/20. Estimates based on a model for an isotropic triangular lattice system indicates that the efficiency can be substantially enhanced in these geometrically frustrated magnets close to a quantum phase transition. [1] B. Wolf, Y. Tsui, D. Jaiswal‐Nagar, U. Tutsch, A. Honecker, K. Removic‐Langer, G. Hofmann, A. Prokofiev, W. Assmus, G. Donath, M. Lang, Proc. Nat. Acad. Sci., USA, 108, 6862 (2011). [2] M. Lang, B. Wolf, A. Honecker, Y. Tsui, D. Jaiswal‐Nagar, U. Tutsch, G. Hofmann, A. Prokofiev, P. T. Cong, N. Krüger, F. Ritter, W. Assmus, submitted to the Proceedings of the 26th Conference on Low Temperature Physics, LT26, Bejing 2011, China.