| Physics at the University of Virginia | ||||||
| Academics | People | Research | Announcements | Facilities | Administration | Classes |
| Thursday, January 17, 2008 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, January 24, 2008 | RESERVED | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, January 31, 2008 | Bruce Gaulin [Host: Seung-Hun Lee] | |
| 4:00 PM, Room 204 | McMaster University | |
| Physics Building | “Frustrated and Satisfied Ground States in Pyrochlore Magnets” |
| Geometrical frustration arises quite generally when pairwise interactions in magnetic materials are incompatable with their local geometry. This often involves magnetic materials made up of assemblies of triangles or tetrahedra. The frustration is manifest by disordered low temperature states for the magnetic material - some of which are described by spin liquids, spin glasses, and spin ice. I will discuss (mostly) neutron scattering work using DCS at NIST on two magnetic pyrochlores Tb2Ti2O7 and Ho2Ti2O7, which can be thought of as Ising-like moments decorating a network of corner-sharing tetrahedra. Tb2Ti2O7 displays a spin liquid, or cooperative paramagnetic ground state, but can be brought to order in an applied magnetic field. Ho2Ti2O7 displays a static disordered "spin ice" state at low temperatures. |
| Thursday, February 7, 2008 | Taku Sato [Host: Seunghun Lee] | |
| 4:00 PM, Room 204 | University of Tokyo | |
| Physics Building | “Neutron scattering study on static and dynamic spin structures in quasicrystalline magnets” |
| Quasicrystals have distinct spatial symmetry characterized by highly-ordered but non-periodic (quasiperiodic) atomic structure, which differs both from the periodic and random structures. Ordering and excitations of quasiperiodically arranged magnetic moments (spins) are yet fundamental open problems, despite the intensive efforts continuously made since the discovery of the quasicrystal. In this talk I will present recent development of understanding on the static and dynamic spin structures in quasicrystalline magnets, using the extensively studied Zn-Mg-RE systems as typical examples. In magnetization measurements, the Zn-Mg-RE quasicrystals all show spin-glass-like behavior, indicating random freezing of spins at low temperatures, however, well-defined short-range order have been observed in the neutron scattering. The inelastic response of Zn-Mg-RE falls into two classes: For RE = Tb and Dy, a broad inelastic peak has been observed with very weak Q dependence, suggesting an existence of the strongly localized collective excitation modes. On the other hand, for RE = Ho, temperature-independent S(Q, h ω ) was observed in the neutron-energy-gain side (h ω >0) for an incredibly large temperature range (up to 200 K!). The anomalous spin fluctuations may be related to criticality of electron wave functions in the quasiperiodic lattice. |
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Monday, February 11, 2008 Note Special Day |
Gia-Wei Chern [Host: Seunghun Lee] | |
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3:30 PM, Room 204 Note Special Time |
Johns Hopkins University | |
| Physics Building | “Order by distortion and chiral magnetism in CdCr2O4” |
| Thursday, February 14, 2008 | Kazuma Hirota [Host: Seung-Hun Lee] | |
| 4:00 PM, Room 204 | University of Tokyo | |
| Physics Building | “Frustrations in Relaxors” |
| Relaxors have been extensively studied for a variety of applications as ideal dielectric materials because they often exhibit extremely large dielectric and piezoelectric constants while the dielectric loss and temperature dependence are small. Although it is now widely believed that heterogeneity embedded and appearing in relaxors are relevant to various phenomena specific to relaxors, there is still no established microscopic theory for relaxors. The prototypical relaxors Pb(Mg1/3Nb2/3)O3 (PMN) consists of Pb2+ on the A site of the ABO3 Perovskite structure and Mg2+ and Nb5+ on the B site. To keep the charge neutrality, Mg2+ and Nb5+ have to form a solid solution with a ratio of 1:2 so as to have an average valence of 4+. However, it is likely that a large difference between the ionic radius of Mg2+ and that of Nb5+ prefers the 1:1 solid solution resulting in an alternating arrangement of Mg2+ and Nb5+. The 1:2 state and the 1:1 state are mutually exclusive, thus the system falls in a state of frustration. The concept of frustration has been studied almost exclusively in magnetism, e.g., a geometrical frustration in an antiferromagnetic triangular lattice, though phenomena related to frustration are widely seen in nature. Since there is no unique ground state in a frustrated system, the system becomes unstable among various different states, which may cause large fluctuations leading to a extremely large susceptibility against an external field and to a novel exotic phase. We now consider that heterogeneous structures appearing in relaxors can originate from such a frustration. In this presentation, a review is given on a series of neutron and x-ray scattering experiments on spatial structures and dynamics of polar nano regions in relaxors. We then would like to discuss how such experimental results can be understood in the frame work of frustration. We also discuss what we will be able to study the microscopic mechanism of relaxors by controlling the frustration through lattice distortion, charge imbalance and dimensionality. |
| Thursday, February 21, 2008 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, February 28, 2008 | Jack Chan [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “Random Telegraph Signal in Carbon Nanotube Device” |
| Due to the low dimensionality of carbon nanotubes (CNTs), charging of a single defect site near a CNT may have a profound effect on modifying carrier transport mobility in a long CNT channel. Random Telegraph Signals (RTS) have been studied in carbon nanotube field effect transistors (CNT-FETs). If the energy of the defect center is close to the Fermi level of the CNT-FET, trapping and detrapping of carriers would modify the carrier transport in the channel, and give rise to RTS. RTS is observed as a switching between discrete current levels, representing a carrier being trapped and detrapped successively in the defect center. We speculate that RTS spectra could provide a characteristic signature of specific adsorbates or adducts on the nanotube channel. This capability is of interest not only for potential sensing technology but also provides a way to introduce controllable quantum interference resonances in the channel transport. |
| Thursday, March 13, 2008 | RESERVED | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, March 20, 2008 | Zhiqiang Mao [Host: Seunghun Lee] | |
| 4:00 PM, Room 204 | Tulane University | |
| Physics Building | “Fascinating Exotic Phenomena in Layered Ruthenates” |
| Perovskite ruthenates (Sr,Ca) n+1 Ru n O 3n+1 exhibit a rich variety of fascinating ordered ground states. Spin-triplet superconductivity, metamagnetic quantum criticality, itinerant ferromagnetism, antiferromagnetic Mott insulating, and half-metallic behavior were all found in close proximity to one another. These diverse ground states originate from the strong interplay of charge, spin, lattice, and orbital degrees of freedom. They offer a unique opportunity to tune the system and study the physics of novel quantum phases. In this talk, I will first give a brief overview on studies in this area, and then present our recent work on double layered ruthenates (Sr 1-x Ca x ) 3 Ru 2 O 7 (0 ≤ x ≤ 1). We have established a magnetic phase diagram for this system using the high quality single crystals grown by the floating-zone technique; this phase diagram exhibits significant new phenomena. We find a very unique magnetic state in close proximity to a two-dimensional ferromagnet with T c =0 K for 0.1 < x < 0.4. This state exhibits a surprisingly large Wilson ratio RW (e.g. R W ≈ 700 for x = 0.2); it freezes into a cluster glass (CG) at low temperatures. Furthermore, we observe evidence of non-Fermi liquid behavior as the frozen temperature of the CG phase approaches zero near x = 0.1. The origin of such a state will be discussed. |
| Thursday, March 27, 2008 | Jack Simonson [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “Investigation of high temperature TE compounds” |
| Thursday, April 3, 2008 | Peter Gehring [Host: Seunghun Lee] | |
| 4:00 PM, Room 204 | NIST | |
| Physics Building | “Effects of Short-Range Order on the Structure and Dynamics of Relaxor Ferroelectrics” |
| Relaxors are disordered perovskite (ABO3) oxides, typically characterized by the presence of mixed valence B-site cations, that have found widespread use in numerous device applications because they exhibit low hysteresis and record-setting piezoelectric coefficients. Relaxors derive their name from an unusually frequency-dependent dielectric susceptibility, but they also display a rich variety of unusual physical phenomena including simultaneously soft zone-center and zone-boundary phonons, temperature dependent diffuse scattering, and an anomalous thermal expansion where a transition to a low-temperature invar-like behavior is observed. Recent neutron elastic and inelastic scattering results on the lead-based relaxors PbMg1/3Nb2/3O3 (PMN),PbZn1/3Nb2/3O3 (PZN), and their solid solutions with PbTiO3 will be discussed that indicate the development of static, short-range polar order at high temperatures is central to these phenomena. These results can be understood by analogy with random-field models in which just two temperature scales are required to describe the essential features of relaxor compounds. |
| Thursday, April 10, 2008 | Yize (Stephanie) Li [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “The Mysterious Metallic Phase in 2D Superconductors and the Resulting Phase Diagram” |
| Conventional treatments of electronic transport predict that no metallic phase exists in two-dimensional (2D) superconducting materials at zero temperature (T=0). This view has been challenged by the observation of magnetic field (B) induced metallic behavior in amorphous MoGe and Ta thin films. We have demonstrated that the metallic phase in Ta thin films has an intrinsic origin and associates with nonlinear voltage-current (I-V) characteristics that are qualitatively different from those of superconducting and insulating phases. Based on transport measurement, we can map the phase diagram of Ta thin films in B-T-Disorder space and study the physics it reveals. We also investigated the nature of the B-induced insulating phase. We found that the peak structure of differential IV traces displayed a non-monotonic change as a function of B, which might be a signature for the localized Cooper pairs. |
| Thursday, April 24, 2008 | Wei Chen [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “Exploration of Novel Tunnel Barrier Materials for STT-RAM ” |
| The basic structure of magnetic tunnel junctions (MTJs) consists of two ferromagnetic(FM) layers sandwiched by an ultra thin insulating barrier, and it shows high or low resistance depending on the relative direction of the the magnetization of two FM layers. Conventional Magnetic random access memory (MRAM) using MTJs as storage units are switched using external filed, so it has the scaling problem beyond 65nm node. A new switching mechanism called Spin Torque Transfer (STT) has been proposed and experimentally confirmed. In our work, new tunnel barrier materials are being explored to enhance the performance of this new STT-RAM technology. One of the particular tunnel barrier materials VO2 has the metal-insulator transition close to room temperature, and we're trying to incorporate VO2 into MTJs stack as the smart barrier for the STT switching so that the MTJs could be switched in low resistance state and read at high resistance state. |
To add a speaker, send an email to jy2b@Virginia.EDU. Include the seminar type (e.g. Condensed Matter Seminars), date, name of the speaker, title of talk, and an abstract (if available). [Please send a copy of the email to phys-seminars@Virginia.EDU.]
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