arXiv subjects

Akiyuki Tokuno

Publications and source records attributed to Akiyuki Tokuno.

16 recordsLinked to original sources

arXiv2017

Theory of Laser-Controlled Competing Superconducting and Charge Orders

We investigate the nonequilibrium dynamics of competing coexisting superconducting (SC) and charge-density wave (CDW) orders in an attractive Hubbard model. A time-periodic laser field $\vec{A}(t)$ lifts the SC-CDW degeneracy, since the CDW couples linearly to the field ($\vec{A}$), whereas SC couples in second order ($\vec{A}^2$) due to gauge invariance. This leads to a striking resonance: When the photon energy is red-detuned compared to the equilibrium single-particle energy gap, CDW is enhanced and SC is suppressed, while this behavior is reversed for blue detuning. Both orders oscillate with an emergent slow frequency, which is controlled by the small amplitude of a third induced order, namely $η$ pairing, given by the commutator of the two primary orders. The induced $η$ pairing is shown to control the enhancement and suppression of the dominant orders. Finally, we demonstrate that light-induced superconductivity is possible starting from a predominantly CDW initial state.

cond-mat.str-el↗

arXiv2016

Detection of topological states in two-dimensional Dirac systems by the dynamic spin susceptibility

We discuss dynamic spin susceptibility (DSS) in two-dimensional (2D) Dirac electrons with spin-orbit interactions to characterize topological insulators. The imaginary part of the DSS appears as an absorption rate in response to a transverse ac magnetic field, just as in an electron spin resonance experiment for localized spin systems. We found that when the system is in a static magnetic field, the topological state can be identified by an anomalous resonant peak of the imaginary part of the DSS as a function of the frequency of the transverse magnetic field $ω$. In the absence of a static magnetic field, the imaginary part of the DSS becomes a continuous function of $ω$ with a threshold frequency $ω_{\rm c}$. In this case, the topological and the trivial phases can also be distinguished by the values of $ω_{\rm c}$ and by the line shapes. Thus the DSS is an experimentally observable physical quantity to characterize a topological insulator directly from bulk properties, without observing a topological transition.

cond-mat.str-el↗

arXiv2015

Population-imbalance instability in a Bose-Hubbard ladder in the presence of a magnetic flux

We consider a two-leg Bose-Hubbard ladder in the presence of a magnetic flux. We make use of Gross-Pitaevskii, Bogoliubov, bosonization, and renormalization group approaches to reveal a structure of ground-state phase diagrams in a weak-coupling regime relevant to cold atom experiments. It is found that except for a certain flux $ϕ=π$, the system shows different properties as changing hoppings, which also leads to a quantum phase transition similar to the ferromagnetic XXZ model. This implies that population-imbalance instability occurs for certain parameter regimes. On the other hand, for $ϕ=π$, it is shown that an umklapp process caused by commensurability of a magnetic flux stabilizes a superfluid with chirality and the system does not experience such a phase transition.

cond-mat.quant-gas↗

arXiv2014

Ground States of a Bose-Hubbard Ladder in an Artificial Magnetic Field: Field-Theoretical Approach

We consider a Bose-Hubbard ladder subject to an artificial magnetic flux and discuss its different ground states, their physical properties, and the quantum phase transitions between them. A low-energy effective field theory is derived, in the two distinct regimes of a small and large magnetic flux, using a bosonization technique starting from the weak-coupling limit. Based on this effective field theory, the ground-state phase diagram at a filling of one particle per site is investigated for a small flux and for a flux equal to $π$ per plaquette. For $π$-flux, this analysis reveals a tricritical point which has been overlooked in previous studies. In addition, the Mott insulating state at a small magnetic flux is found to display Meissner currents.

cond-mat.quant-gas↗

arXiv2014

Unconventional superfluidity in quasi-one-dimensional systems

We show that an unconventional superfluid triggered by spin-orbit coupling is realized for repulsively interacting quasi-one-dimensional fermions. A competition between spin-singlet and -triplet pairings occurs due to the breaking of inversion symmetry. We show that both superfluid correlations decay algebraically with the same exponent except for special coupling constants for which a dominant superfluid is controlled by the spin-orbit coupling. We also discuss a possible experiment to observe such phases with cold atoms.

cond-mat.quant-gas↗

arXiv2013

Competition of spin and charge excitations in the Hubbard model

Motivated by recent experiments with ultracold fermionic atoms in optical lattices, we study finite temperature magnetic correlations, as singlet and triplet correlations, and the double occupancy in the one-dimensional Hubbard model. We point out that for intermediate interaction strengths the double occupancy has an intriguing doubly non-monotonic temperature dependence due to the competition between spin and charge modes, related to the Pomeranchuk effect. Furthermore, we determine properties of magnetic correlations in the temperature regimes relevant for current cold atom experiments and discuss effects of the trap on spatially integrated observables. We estimate the entropy and the temperature reached in the experiment by Greif et al., Science 340, 1307 (2013).

cond-mat.quant-gas↗

arXiv2013

Magnetic resonance spectroscopy and characterization of magnetic phases for spinor Bose-Einstein condensates

The response of spinor Bose-Einstein condensates to dynamical modulation of magnetic fields is discussed with linear response theory. As an experimentally measurable quantity, the energy absorption rate (EAR) is considered, and the response function is found to access quadratic spin correlations which come from the perturbation of the quadratic Zeeman term. By applying our formalism to spin-1 condensates, we demonstrate that the EAR spectrum as a function of the modulation frequency is able to characterize the different magnetically ordered phases.

cond-mat.quant-gas↗

arXiv2012

Finite temperature dynamical properties of SU($N$) fermionic Hubbard models in the spin-incoherent regime

We study strongly correlated Hubbard systems extended to symmetric $N$-component fermions. We focus on the intermediate-temperature regime between magnetic superexchange and interaction energy, which is relevant to current ultracold fermionic atom experiments. The $N$-component fermions are represented by slave particles, and, by using a diagrammatic technique based on the atomic limit, spectral functions are analytically obtained as a function of temperature, filling factor and the component number $N$. We also apply this analytical technique to the calculation of lattice modulation experiments. We compute the production rate of double occupancy induced by modulation of an optical lattice potential. Furthermore, we extend the analysis to take into account the trapping potential by use of the local density approximation. We find an excellent agreement with recent experiments on $^{173}$Yb atoms.

cond-mat.quant-gas↗

arXiv2012

Spin correlations and doublon production rate for fermionic atoms in modulated optical lattices

We compute the integrated doublon production rate in response to a lattice modulation for two component fermions in an optical lattice. We derive a general formula for the integrated intensity, valid in the presence of inhomogeneous potentials such as the trap, which gives the integrated intensity in terms of equal time correlation functions only. Such a formula is thus well suited for direct numerical calculations. We show that, in the limit of large repulsion for commensurate fillings, or for temperature ranges for which the hopping is incoherent, the integrated doublon spectrum is directly related to the nearest neighbor spin-spin correlation function. We compute its temperature dependence in this regime using finite temperature quantum Monte Carlo calculation.

cond-mat.quant-gas↗

arXiv2012

Doublon production rate in modulated optical lattices

We study theoretically lattice modulation experiments with ultracold fermions in optical lattices. We focus on the regime relevant to current experiments when interaction strength is larger than the bandwidth and temperature is higher than magnetic superexchange energy. We obtain analytical expressions for the rate of doublon production as a function of modulation frequency, filling factor, and temperature. We use local density approximation to average over inhomogeneous density for atoms in a parabolic trap and find excellent agreement with experimentally measured values. Our results suggest that lattice modulation experiments can be used for thermometry of strongly interacting fermionic ensembles in optical lattices.

cond-mat.quant-gas↗

arXiv2011

Spectroscopy for cold atom gases in periodically phase-modulated optical lattices

The response of cold atom gases to small periodic phase modulation of an optical lattice is discussed. For bosonic gases, the energy absorption rate is given, within linear response theory, by imaginary part of the current correlation function. For fermionic gases in a strong lattice potential, the same correlation function can be probed via the production rate double occupancy. The phase modulation gives thus direct access to the conductivity of the system, as function of the modulation frequency. We give an example of application in the case of one dimensional bosons at zero temperature and discuss the link between the phase- and amplitude-modulation.

cond-mat.quant-gas↗

arXiv2009

Discovery of parity-violating Majorana fermions in a chiral superconductor Sr2RuO4

We found parity-violating Majorana fermions in a chiral superconductor Sr2RuO4. The current-voltage curves show an anomalous behavior: The induced voltage is an even function of the bias current. The magnetic field dependent results suggest the excitation of the Majorana fermions along the closed chiral edge current of the single domain under bias current. We also discuss the relationship between a change of the chirality and spontaneous magnetization of the single domain Sr2RuO4.

cond-mat.supr-con↗

arXiv2009

Skyrmion in spinor condensates and its stability in trap potentials

A necessary condition for the existence of a skyrmion in two-component Bose-Einstein condensates with $\mathrm{SU(2)}$ symmetry was recently provided by two of the authors [Phys. Rev. Lett. {\bf 97}, 080403 (2006)], by mapping the problem to a classical particle in a potential subject to time-dependent dissipation. Here we further elaborate this approach. For two classes of models, we demonstrate the existence of the critical dissipation strength above which the skyrmion solution does not exist. Furthermore, we discuss the local stability of the skyrmion solution by considering the second-order variation. A sufficient condition for the local stability is given in terms of the ground-state energy of a one-dimensional quantum-mechanical Hamiltonian. This condition requires a minimum number of bosons, for a certain class of the trap potential. In the optimal case, the minimum number of bosons can be as small $\sim 10^4$.

cond-mat.other↗

arXiv2008

Spontaneous rotation in one-dimensional systems of cold atoms

We theoretically study harmonically trapped one-dimensional Bose gases (e.g., Li, Na, K, Rb, etc.) with multibands occupied, focusing on effects of higher-energy bands. Combining the Ginzburg-Landau theory with the bosonization techniques, we predict that the repulsive interaction between higher-band bosons and the quantum fluctuation can induce the ground state with a finite angular momentum around the trapped axis. In this state, the Z_2 reflection symmetry (clockwise or anticlockwise rotations) is spontaneously broken.

cond-mat.other↗

arXiv2008

Dynamics of one-dimensional Bose liquids: Andreev-like reflection at Y-junctions and absence of the Aharonov-Bohm effect

We study one dimensional Bose liquids of interacting ultracold atoms in the Y-shaped potential when each branch is filled with atoms. We find that the excitation packet incident on a single Y-junction should experience a negative density reflection analogous to the Andreev reflection at normal-superconductor interfaces, although the present system does not contain fermions. In a ring interferometer type configuration, we find that the transport is completely insensitive to the (effective) flux contained in the ring, in contrast to the Aharonov-Bohm effect of a single particle in the same geometry.

cond-mat.other↗

arXiv2005

Inversion phenomenon and phase diagram of the $S=1/2$ distorted diamond chain with the $XXZ$ interaction anisotropy

We discuss the anisotropies of the Hamiltonian and the wave-function in an $S=1/2$ distorted diamond chain. The ground-state phase diagram of this model is investigated using the degenerate perturbation theory up to the first order and the level spectroscopy analysis of the numerical diagonalization data. In some regions of the obtained phase diagram, the anisotropy of the Hamiltonian and that of the wave-function are inverted, which we call inversion phenomenon; the Néel phase appears for the XY-like anisotropy and the spin-fluid phase appears for the Ising-like anisotropy. Three key words are important for this nature, which are frustration, the trimer nature, and the $XXZ$ anisotropy.

cond-mat.stat-mech↗