Department of Physics, The University of Tokyo
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2006 seminars

2007/3/2(Fri) 13:00 Y. Nakata & Y. Watanabe (B4) (in Japanease)

2007/2/20(Tue) 14:30 (* an unusual time)
speaker Prof. Yukio Tanaka (Nagoya Univ.)
title Odd-frequency pairing state in superconducting junctions
abstract We present a general theory of the proximity effect in junctions between diffusive normal metals (DN) and unconventional superconductors in the framework of the quasiclassical Green's function formalism. Various possible symmetry classes in a superconductor are considered which are consistent with the Pauli principle: even-frequency spin-singlet even-parity (ESE) state, even-frequency spin-triplet odd-parity (ETO) state, odd-frequency spin-triplet even-parity (OTE) state and odd-frequency spin-singlet odd-parity (OSO) state. It is shown that the pair amplitude in a DN belongs respectively to an ESE, OTE, OTE and ESE. The generation of the OTE state in the DN attached to the ETO p-wave superconductor is of particular interest in the relevance to the novel proximity effect in Sr2RuO4 junctions.

We also studied about the ballistic normal metal / superconductor junctions based on the standard quasiclassical Green°«s function theory. We demonstrate that, quite generally, the spin-singlet even-parity (spin-triplet odd-parity) pair potential in a superconductor induces the odd-frequency pairing component with spin-singlet odd-parity°°(spin-triplet even-parity) near interfaces. The magnitude of the induced odd-frequency°°component is enhanced in the presence of the midgap Andreev resonant state due to the sign change of the anisotropic°°pair potential at the interface. The Josephson effect should therefore occur between odd- and even-frequency superconductors, contrary to the standard wisdom. A method to probe the odd-frequency superconductors is proposed.

[1]Y. Tanaka and A.A. Golubov, Phys. Rev. Lett. 98 037003 (2007).
[2]Y. Tanaka, A.A. Golubov, S. Kashiwaya and M. Ueda, condmat0610017.

2007/2/16(Fri) 15:00 Yuji Kurotani (M2) (in Japanease)

2007/1/30(Tue) 14:00 (* an unusual time)
speaker Dr. Kazue Kudo (Osaka City University)
title Magnetic domain patterns under time-dependent field
abstract The domain patterns in a ferromagnetic thin film under zero field are usually observed after an external magnetic field is removed. Recently, it was shown by both experiments and numerical simulations that the characteristics of the domain patterns under zero field depend on the sweep rate of the magnetic field [1]. I will talk about how the characteristics of the domain patterns depend on the field and what causes the difference of those domain patterns. I will also talk about the domain patterns under oscillating field. Some interesting domain patterns can be observed under oscillating field.

[1] Kazue Kudo, Michinobu Mino and Katsuhiro Nakamura, J. Phys. Soc. Jpn 76, 013002 (2007).

2007/1/16(Tue) 13:00
speaker Dr. Blair Blakie (University of Otago)
title Classical field theory for finite temperature trapped Bose-Einstein condensates
abstract In this talk I discuss a phase space technique based on the Wigner representation that provides an approximate description of dilute ultra-cold Bose gases at finite temperatures. As the quantum field evolution is represented using equations of motion for classical fields, this has become known as the "classical field method", although it does in general include quantum effects in a controlled degree of approximation. This technique provides a practical quantitative description of both equilibrium and dynamical properties of Bose gas systems. I will provide a phenomenological motivation for the theory and present results of recent applications of the theory to trapped Bose gases. These applications include the shift in critical temperature due to interactions, the thermal activation of vortices in 2D traps, and the calculation of correlation functions.
speaker Yusuke Nishida (U. Tokyo, D3)
title Unitary Fermi gas in the epsilon expansion
abstract Two-component Fermi gas with zero-range interaction at infinite scattering length (unitary Fermi gas) has attracted intense attentionacross many subfields of physics. However, its analytic treatment has been difficult due to the lack of a small expansion parameter. In this talk, we construct systematic expansions for the unitary Fermi gas around four and two spatial dimensions. It is shown that the unitary Fermi gas near four spatial dimensions can be understood as a weakly-interacting system of fermionic and bosonic quasiparticles, while near two spatial dimensions it reduces to a weakly-interacting Fermi gas. We calculate various physical quantities, such as the thermodynamic functions, the quasiparticle spectrum, and the critical temperature, using 4-d or d-2 as a small parameter of the perturbative expansion where d is the dimensionality of space.

2006/11/28(Tue) 13:00
speaker Yu Watanabe (B4)
title Quantum Algorithm for N-Queens Problem
abstract I will talk about N-Queens Problem (NQP) that is a kind of puzzle on the chessboard and it is hard to solve on a classical computer, that is, time to solve it increases more than exponentially with the chessboard size n. I will show that quantum algorithm for NQP is faster than the classical one. The main idea of the quantum algorithm is to ap ply the quantum counting technique that uses Grover iteration and quantum Fourier transformation.
speaker Yoshiyuki Nakata (B4)
title Transition temperature of dilute interacting Bose particles
abstract I study a statistical mechanics using a path integral formulation and attempt to calculate the condensation temperature of a dilute interacting Bose gas by using that method. A partition function of a Bose gas can be expressed by that method. If the density of the gas is enough low, the partition function in the system can be calculated and the transition temperature can be estimated. In this seminar, I will talk about the statistical mechanics using a path integral and report the progress of my attempt.

2006/11/21(Tue) 13:00
speaker Dr. Jani-Petri Martikainen (U. Helsinki)
title Polarized superfluid Fermi gas and the physics of three-component trapped Fermi gas
abstract I will discuss the physics of two-component polarized Fermi gas i.e. a system where one of the components has more atoms than the other one. My focus will be on the finitetemperature effects within the context of a mean-field theory and local density approximation. I will demonstrate the appearance of a (stable) polarized Fermi superfluid at intermediate temperatures and a gapless superfluid (Sarma phase) close to Tc. I will then proceed to explore the physics of a three-component Fermi gas. Experiments to probe such systems are under construction, but very little is yet known about such systems. As a by-product of the three-component theory, we also solve the mean-field theory of the two-component system with unequal mass fermions.nbsp; I will outline the expected phase-diagram and show how one can expect co-existing superfluids and unusual superfluid shell structures when trap physics is taken into account. I will conclude by showing how induced many-body effects are expected to be more pronounced when pairing occurs between unequal mass fermions.

2006/11/17(Fri) 13:00
speaker Takahiro Sagawa (M1)
title Jarzynski Equality with Maxwell's Demon
abstract We propose a new thermodynamic equality and several inequalities concerning the relationship between work and information for an isothermal process with Maxwell's demon. Our approach is based on the formulation a la Jarzynski of the thermodynamic engine and on the quantum information-theoretic characterization of the demon. The lower bound of each inequality, which is expressed in terms of the information gain by the demon and the accuracy of the demon's measurement, gives the minimum work that can be performed on a single heat bath in an isothermal process. These results are independent of the state of the demon, be it in thermodynamic equilibrium or not.

2006/11/14(Tue) 13:00
speaker Shuta Nakajima (M1)
title Exploring of strongly correlated quantum gases in optical lattices
abstract Since the experimental realization of Mott-insulator superfluid transiton in optical lattices (2002), many physicists attempt to simulate electrons in a solid-state crystal by using the cold atoms in the optical lattices. Because we can control a lot of parameters of optical lattices (strength of interactions, carrier density, and dimension etc.), the study of ultracold atoms in optical lattices will play a key role in condensed matter physics. In this seminar, I will talk about experiments we are trying to realize. In the first part of the talk I will provide brief explanations about cold atom physics and optical lattices. In the second part of the talk I will talk about the aim of our group: observation of the dynamics of the formation of long-range order in optical lattices. Particularly, I will explain the physics of the Hubbard model for fermionic atoms in optical lattices. Lastly, I will talk about the progress of our experiments and future prospects.

2006/11/7(Tue) 13:00
speaker Yuji Kurotani (M2)
title Quantum measurement with possibility of backcalculation
abstract Recently, noise in the quantum measurement has been reconsidered by M. Ozawa. Quantum measurement is called noiseless when the obtainable probability by the measurement is corresponds with absolute square of the measuring wavefunction. We generalize the class of noiseless by using the concept of possibility of backcalculation. If measurement belongs to the generalized class, we can accurately estimate absolute square of the wavefunction from the outcomes.

2006/10/31(Tue) 13:00
speaker Yuki Kawaguchi (PD)
title Spontaneous spin texture and mass current in spinor dipolar BECs
abstract The dipole-dipole interaction in gaseous Bose-Einstein condensates (BECs) is expected to yield rich phenomena when combined with the spin degree of freedom, because dipole-induced spin textures can generate mass transport via spin-gauge symmetry. In this talk, I will talk about ground-state spin textures in ferromagnetic BECs. Even the weak dipolar coupling of alkali atoms can induce spin texure, and there are three ground-state phases in zero magnetic field. Moreover, a substantial orbital angular momentum emerges spontaneously in a certain parameter regime. I will also talk about the texture-formation dynamics in the presence of the external magnetic field.

2006/10/24(Tue) 13:00
speaker Hiroaki Terashima (PD)
title Reversible quantum measurement with arbitrary spins
abstract Even though a quantum measurement is widely believed to have intrinsic irreversibility, it is not necessarily irreversible when all the information about the premeasurement stateis preserved during the measurement. A quantum measurement is said to be physically reversible if the premeasurement state can be recovered from the postmeasurement state by means of a second measurement, referred to as a reversing measurement, with a nonzero probability. In this talk, I will propose a physically reversible quantum measurement with two arbitrary spins and an Ising interaction. The reversing measurement is explicitly constructed and the degree of reversibility is evaluated in terms of fidelity. In view of recent advances in experimental techniques, this model would be experimentally feasible using an ensemble of atoms and two-mode photons.

2006/10/17(Tue) 13:00
speaker Dr. Axel Griesmaier (U. Stuttgart)
title From Feshbach resonances to dipole-dipole interaction: Tuneable isotropic and anisotropic interactions in a chromium BEC
abstract The chromium Bose-Einstein condensate (BEC) is the first system where a mechanical effect of long-range dipole-dipole interaction has been observed in a gas. In chromium, the dipole-dipole interaction stems from the large magnetic moments of 6 (IB5 (BB of chromium atoms. In contrast to other BECs, the magnetic dipole-dipole interaction (MDDI) of chromium is of comparable strength as the short-range and isotropic contact interaction. We are therefore able to observe and measure MDDI that manifests as a modification of the expansion dynamics of the condensate. Furthermore Feshbach resonances in collisions of ultracold chromium atoms have been observed. Together with the possibility to tune also the strength of the dipole-dipole interaction, these resonances open up the fascinating possibility to perform experiments with a quantum gas that has tuneable long-range and short-range interactions.

2006/10/10(Tue) 13:00
speaker M. Tezuka (U. Tokyo, D3)
title Superconductivity in correlated electron systems coupled to phonons studied by the density-matrix renormalization group
abstract Each of electron-electron and electron-phonon interactions has been extensively studied as the source of pairing force between electrons in superconductors. What happens when they are both strong, however, remains relatively unknown and offers many fundamental questions. We have theoretically studied to answer these questions, using density-matrix renormalization group (DMRG). We have adopted the Holstein-Hubbard (HH) model, where electrons both interact with each other and are coupled to an on-site phonon at each site. Calculation of correlation functions for the one-dimensional half-filled chain shows that the superconducting correlation does not dominate over the charge correlation. However, this situation changes when we change the lattice structure or the electron density to break electron-hole symmetry [1][2]. In this seminar I would like to present our results, after reviewing some of the previous works in the field and giving a short introduction to DMRG. I will explain how DMRG can be applied to study time-dependent quantities and systems with bosonic degrees of freedom. Finally, I will briefly discuss our preliminary results on the HH model in the limit of infinite dimensions.

[1] Masaki Tezuka, Ryotaro Arita and Hideo Aoki, Physica B 359-361, 708 (2005).
[2] Masaki Tezuka, Ryotaro Arita and Hideo Aoki, Phys. Rev. Lett. 95, 226401 (2005).

2006/10/3(Tue) 13:00
speaker Dennis Dickerscheid (PD)
title Quantum phases in Optical Lattices
abstract In this talk I will talk about the physics of ultracold gases in optical lattices. Specifically, in the first part of the talk I will present the theory for ultracold atomic gases in an optical lattice near a Feshbach resonance. In the single-band approximation the theory describes atoms and molecules which can both tunnel through the lattice. Moreover, an avoided crossing between the two-atom and the molecular states occurs at every site. We determine the microscopic parameters of the generalized Hubbard model that describes this physics, using the experimentally known parameters of the Feshbach resonance in the absence of the optical lattice.In the second part of the talk I will consider some applications of the theory, namely, the Bose gas near a Feshbach resonance, theBose-Fermi mixture and confinement induced molecules.