| Date & time | Speaker & affiliation | Talk title & abstract |
| Oct 4 4:00pm | Professor Han-Young Choi (SungKyunKwan Univ) | Search for a new state of superconductivity: odd frequency triplet pairing in superconductor-ferromagnet layers Abstract: Superconductivity is due to pairing and can be characterized in terms of the paired state. The antisymmetricity of the Cooper pair wave-function requires that the total spin S=0 (singlet) state have an even angular momentum l, and S=1 (triplet) state an odd l. If the pairing is generalized to include the different time pairing, the triplet state can be even frequency odd l triplet, or odd frequency even l triplet states. Recently it was realized that the odd frequency s-wave triplet state may be induced in superconductor-ferromagnet (S-F) layers. The odd frequency triplet state, however, has not been directly observed yet although there have been some hints pointing to its existence. We propose that the tunneling experiments on F/S/F trilayers should produce a clear evidence of the triplet state when the magnetization directions of the two F's are properly controlled. I will first give qualitative arguments and then back them up with detailed calculations using the diffusive Usadel equation. |
| Oct 11 4:00pm | Jing Shi (UCR) | Experimental study of the Nerst effects in Ga1-xMnxAs ferromagnetic
semiconductors Abstract: Thermoelectric transport coefficients (e.g. magneto-thermopower and the Nernst effect) together with their electrical transport counterparts (magnetoresistance and the Hall effect) are very important physical quantities in solid state physics. In particular, the anisotropic magneto-thermopower (AMTP) and the anomalous Nernst effect (ANE), both are consequences of the spin-orbit interaction in ferromagnets; therefore they can be used to study the spin-orbit interaction in ferromagnetic metals and semiconductors. In this talk, I will present our experimental work on thermoelectric transport in Mn-doped ferromagnetic semiconductors. In the first part, I will focus on AMTP and a planar Nernst effect we found in GaMnAs thin films when an external magnetic field is in the sample plane, and show you that these two effects are intimately related to each other. In the second part, I will show our more recent results on ANE in the standard Nernst geometry in similar materials. Our objective is to elucidate the nature of the anomalous Hall and anomalous Nernst effects, i.e. extrinsic (skew-scattering) vs. intrinsic (quantum mechanical). |
| Oct 20 Friday 4:00pm Note the unusual day |
Isao Watanabe (RIKEN, Japan) | A quasi static and small internal field in
La2-xSrxCuO4 around 100 K
revealed by μSR Abstract: Precise zero- (ZF) and longitudinal-field (LF) μSR measurements have been carried out to see what is happening around higher around 100 K which is well above Tc. The μSR has been carried out at the RIKEN-RAL Muon Facility in the UK using the world-wide intense pulsed muon beam. The ZF-μSR time spectrum in this temperature range shows a Gaussian shape, indicating that muon spins depolarize mainly by nuclear dipole fields randomly distributed at the muon site. A slight deviation of the shape of the ZF-μSR time spectrum from the Gaussian one has been observed with decreasing temperature. This deviation indicates the appearance of a change of the dynamics of internal fields at the muon site which could be correlated with the dynamics of Cu spins. Since holes in the normal state have been reported to be localized below the similar temperature region, the present result of ZF-μSR can conclude that the degree of freedom of holes is coupled with the one of spins and that the localization of holes causes the suppression of Cu-spin fluctuations. In order to achieve more information on the dynamics of the internal field at the muon site, the dynamic depolarization rate of muon spins was measured in LF. It has been found that the LF dependence of the dynamic depolarization rate around 100 K is well described by the Redfield equation. The amplitude of the fluctuating internal field was estimated to be a few Gauss and its fluctuating frequency was the order of 10-5 sec. This result indicates that a small and quasi static internal field at the muon site appears at around 100 K. Possibilities of the origin of this quasi static and small internal field will be discussed. |
| Oct 25 4:00pm | Charles Reichhardt (LANL) | Colloids as a Model System to Explore Complex Matter Abstract: There are a wide variety of applications for colloidal particle assemblies such as the creation of synthetic photonic band gap materials, sensor arrays, and as self-assembled templates for the mass fabrication of nanostructures. In addition to these practical applications, colloids are also ideal for studying the general problem of complex matter arising in systems with competing interactions. We show that such competing interactions can give rise to novel self-organized stripe, labyrinth and network structures which may be relevant to charge ordering in cuprates and manganites. We also demonstrate that colloids can be used to create artificial nuclei systems, including a colloidal version of the so-called pasta phases that may exist in dense nuclear matter. When the colloids are exposed to a periodic optical trap array, numerous statistical mechanics models can be readily created including Ising, Potts, and artificial spin ice systems, as well as new phases. Work performed in collaboration with Cynthia Olson Reichhardt, Alan Bishop, Zohar Nusinov, Leonid Pryadko, J.-X Lin, and Ivar Martin |
| Nov 1 4:00pm | Brigitte Leridon (ESPCI, France) | Resistivity
fluctuations in Cuprates Abstract: The evidence for a version of superconducting amplitude fluctuations (Aslamazov-Larkin) suitable for a layered system for cuprates will be shown. In high fields, evidence for a vortex confinement-deconfinement transition at a value of resistance equal to 1/4 the quantum of resistance as well as the logarithmic resistivity as a function of temperature will also be presented. |
| Nov 8 4:00pm | Zhiqiang Li (UCSD) | Infrared probe of charge dynamics in field-effect transistors based on
organic molecular crystals and polymers
Abstract: While the electronically active element of a field-effect transistor (FET) can be made from a wide variety of novel materials, many new physical phenomena produced by electrostatic doping in these devices are yet to be explored. Employing infrared spectroscopy, we recently have succeeded in directly probing the electronic excitations in FET devices based on organic molecular crystals and polymers. Our studies of single crystal rubrene based FETs reveal band-like charge transport in these devices. Infrared measurements uncover anisotropic optical conductivities of these transistors in agreement with earlier transport studies. Infrared microscopy measurements show uniform charge injection over macroscopic length scales of several millimeters. These results for molecular crystals FETs will be compared with the data for polymer thin film FETs. I will discuss several new aspects of the charge dynamics in organic molecular crystals uncovered by this work. |
| Nov 15 4:00pm | Daniel Podolosky (UC, Berkeley) | The Nernst effect and Diamagnetism in a
phase-fluctuating superconductor Abstract: When a superconductor is warmed above its critical temperature Tc, superconductivity is destroyed by fluctuations in the superconducting order parameter. These fluctuations are seen in a variety of experimental probes, including the electrical conductivity, diamagnetism, and the Nernst effect -- a thermoelectric analogue of the Hall effect. In this talk, I will discuss the experimental signatures of a normal state characterized by well-formed Cooper pairs, but where superconductivity has been destroyed by phase fluctuations due to the proliferation of vortices. Such a description applies to Josephson junction arrays and granular superconductors, where superconductivity on individual grains is robust, while Tc is given by the weak Josephson coupling between grains. This may also be a good model of the underdoped cuprates, where strong phase fluctuations reduce Tc well below its mean-field value. I'll show that the Nernst effect and the diamagnetic response in a phase-only model have a sharp temperature dependence, which can be distinguished from Gaussian fluctuations characteristic of a normal state with depairing. I'll compare these results to measurements on the underdoped cuprates. |
| Nov 20 Monday 4:00pm Note the unusual day |
Zohar Nussinov (Washington University in St. Louis) | Generalized symmetries and finite temperature topological orders Abstract: We prove sufficient conditions for topological order (TO) at finite temperatures. These conditions rely on the existence of "gauge like symmetries" (GLSs) which generally lie half-way between global and local symmetries. Such symmetries appear in numerous systems: the Kitaev model, orbital-dependent spin exchange Hamiltonians in 3d orbital systems, Jahn-Teller effects in transition metal compounds, liquid crystalline phases of Quantum Hall systems, p+ip superconducting arrays, Bose metals, some theories of glasses, sliding Luttinger liquids (stripes), and solvable frustrated spin systems with a deconfined quantum critical point. These GLSs (and perturbations about them) allow us to go beyond standard topological field theories to engineer new physical models with robust finite temperature TO. |
| Nov 29 4:00pm | Frank Pollmann (Max Plank Institute, Dresden) | Correlated fermions on a checkerboard lattice Abstract:Geometrical frustration of lattices can lead to a macroscopic degeneracy in the classical limit and thus to many interesting physical effects. In contrast to magnetic properties, one began only recently to explore the charge degrees of freedom on frustrated lattices. For the systematic study of charge degrees of freedom, we consider a model of spinless fermions with nearest-neighbor hopping t and Coulomb repulsion V. Quantum fluctuations reduce the classical (t=0) macroscopic degeneracy. For the strongly correlated limit V>> |t|, it has been predicted that an added electron can decay into two mobile quasi-particles, leading to fractional charges of e/2 in 2D and 3D systems. For a deeper understanding of these charge degrees of freedom, we calculated numerically the properties of static and dynamic charges on the 2D checkerboard lattice. We present numerical indications for an ordered ground state by means of exact diagonalization. Evidence for a weak mutual confinement of two fractional charges is found, leading to excitations with very large spatial extend. We can show that in the considered limit the ground state and low-energy excitations of the fermionic system are the same as in the corresponding hard-core bosonic system. The low-energy physics of the undoped system can then be described by a confining U(1) gauge theory in d=2+1 |
| Dec 6 4:00pm | Michael Fogler (UCSD) | Power-laws in one-dimensional transport:
Luttinger liquid or disorder? Abstract: When the conductance of a 1D wire shows a power-law dependence on temperature T and voltage V, it is often attributed to Luttinger liquid or related interaction effects. For this explanation to hold the power-law exponents in T and V must be equal. This condition is however systematically violated in long wires (typically, longer than 10 micron), where the thermal exponent is much larger than the voltage one. To shed light on the physics involved we study a theoretical model of a long 1D wire with a finite density of strong random impurities that convert it into a chain of weakly-coupled quantum dots. Electron transport in such a system is shown to exhibit a rich dependence on V and T due to the interplay of sequential and co-tunneling. Remarkably, we indeed find a broad parameter range where the conductance exhibits an algebraic dependence on T and V with unequal exponents, in agreement with the experiments. At much lower temperatures the conductance eventually crosses over to the stretched exponential laws typical of the variable-range hopping. Reference: M. M. Fogler, S. V. Malinin, T. Nattermann, "Coulomb blockade and transport in a chain of one-dimensional quantum dots," Phys. Rev. Lett. 97, 096601 (2006). |
| Dec 13 4:00pm | open | Abstract: |
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