Floquet Engineering of Quantum Materials Oka, Takashi; Kitamura, Sota
Annual review of condensed matter physics,
03/2019, Letnik:
10, Številka:
1
Journal Article
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Odprti dostop
Floquet engineering, the control of quantum systems using periodic driving, is an old concept in condensed matter physics dating back to ideas such as the inverse Faraday effect. However, there is a ...renewed interest in this concept owing to (
a
) the rapid developments in laser and ultrafast spectroscopy techniques, (
b
) discovery and understanding of various "quantum materials" hosting interesting exotic quantum properties, and (
c
) communication with different areas of physics such as artificial matter and nonequilibrium quantum statistical physics. Here, starting from a nontechnical introduction with emphasis on the Floquet picture and effective Hamiltonians, we review the recent applications of Floquet engineering in ultrafast, nonlinear phenomena in the solid state. In particular, Floquet topological states and their application to ultrafast spintronics and strongly correlated electron systems are overviewed.
Graphene irradiated by a circularly polarized laser has been predicted to be a Floquet topological insulator showing a laser-induced quantum Hall effect. A circularly polarized laser also drives the ...system out of equilibrium, resulting in nonthermal electron distribution functions that strongly affect transport properties. Results are presented for the Hall conductance for two different cases. One is for a closed system, such as a cold-atomic gas, where transverse drift due to nonzero Berry curvature can be measured in time-off-light measurements. For this case the effect of a circularly polarized laser that has been suddenly switched on is studied. The second is for an open system coupled to an external reservoir of phonons. While for the former the Hall conductance is far from the quantized limit, for the latter, coupling to a sufficiently low temperature reservoir of phonons is found to produce effective cooling, and thus an approach to the quantum limit, provided the frequency of the laser is large as compared to the bandwidth. For laser frequencies comparable to the bandwidth, strong deviations from the quantum limit of conductance are found even for a very low temperature reservoir, with the precise value of the Hall conductance determined by a competition between reservoir-induced cooling and the excitation of photocarriers by the laser. For the closed system, the electron distribution function is determined by the overlap between the initial wave function and the Floquet states, which can result in a Hall conductance which is opposite in sign to that of the open system.
We propose a local detection scheme for the Majorana zero mode (MZM) carried by a vison in Kitaev's chiral spin liquid (CSL) using scanning tunneling microscopy (STM). The STM introduces a single ...Majorana into the system through hole-charge injection and the Majorana interacts with the MZM to form a stable composite object. We derive the exact analytical expression of single-hole Green's function in the Mott insulating limit of Kitaev's model, and show that the differential conductance has split peaks, as a consequence of resonant tunneling through the vison-hole composite. The peak splitting turns out comparable to the Majorana gap in CSL, well within the reach of experimental observation.
Tuning diamagnetism with currentProperties of materials can be tuned by various means, such as chemical doping, magnetic field, or pressure. Sow et al. used electrical currents of modest density to ...turn the Mott insulator Ca2RuO4 into a semimetal. Concurrently, its diamagnetic response—the ability to counter an externally applied magnetic field—rose to levels higher than in any other nonsuperconducting material. The use of electrical current as a powerful experimental knob may be applicable to other similar materials.Science, this issue p. 1084Mott insulators can host a surprisingly diverse set of quantum phenomena when their frozen electrons are perturbed by various stimuli. Superconductivity, metal-insulator transition, and colossal magnetoresistance induced by element substitution, pressure, and magnetic field are prominent examples. Here we report strong diamagnetism in the Mott insulator calcium ruthenate (Ca2RuO4) induced by dc electric current. The application of a current density of merely 1 ampere per centimeter squared induces diamagnetism stronger than that in other nonsuperconducting materials. This change is coincident with changes in the transport properties as the system becomes semimetallic. These findings suggest that dc current may be a means to control the properties of materials in the vicinity of a Mott insulating transition.
A
bstract
We analyze vacuum instability of strongly coupled gauge theories in a constant electric field using AdS/CFT correspondence. The model is the
1-flavor supersymmetric large
N
c
QCD in the ...strong ’t Hooft coupling limit. We calculate the Euler-Heisenberg effective Lagrangian
(
E
), which encodes the nonlinear response and the quantum decay rate of the vacuum in a background electric field
E
, from the complex D-brane action in AdS/CFT. We find that the decay rate given by Im
(
E
) becomes nonzero above a critical electric field set by the confining force between quarks. A large
E
expansion of Im
(
E
) is found to coincide with that of the Schwinger effects in QED, replacing its electron mass by the confining force. Then, the time-dependent response of the system in a strong electric field is solved non-perturbatively, and we observe a universal thermalization at a shortest timescale “Planckian thermalization time”
. Here,
is an effective temperature which quarks feel in the nonequilibrium state with nonzero electric current, calculated in AdS/CFT as a Hawking temperature. Stronger electric fields accelerate the thermalization, and for a realistic value of the electric field in RHIC experiment, we obtain
τ
th
~ 1 fm/c, which is consistent with the believed value.
We theoretically predict a nonequilibrium phase transition in quantum spin systems induced by a laser, which provides a purely quantum-mechanical way of coherently controlling magnetization. Namely, ...when a circularly polarized laser is applied to a spin system, the magnetic component of a laser is shown to induce a magnetization normal to the plane of polarization, leading to an ultrafast phase transition. We first demonstrate this phenomenon numerically for an S = 1 antiferromagnetic Heisenberg spin chain, where a new state emerges with magnetization perpendicular to the polarization plane of the laser in place of the topologically ordered Haldane state. We then elucidate its physical mechanism by mapping the system to an effective static model. The theory also indicates that the phenomenon should occur in general quantum spin systems with a magnetic anisotropy. The required laser frequency is in the terahertz range, with the required intensity being within a prospective experimental feasibility.
When people attempt to suppress stereotypes, they often end up making stereotypical judgments. The adverse effects of this form of suppression are called "paradoxical effects." This study examined ...the effect of perspective-taking as a strategy to reduce the paradoxical effects related to stereotype suppression. Specifically, this study addressed stereotypes within the context of women's mathematical abilities, with Japanese university students as participants. It was predicted that when participants suppressed the stereotype of a woman, those who engaged in perspective-taking toward that woman would make less stereotypical judgments of other women, compared with those who did not. Moreover, as this study focuses on gender stereotypes, an exploratory analysis was conducted to investigate whether the effects of engaging in perspective-taking about women vary depending on the participants' gender. Although no significant effect was observed and the hypothesis was not supported, and while the results of this study were statistically inadequate, they suggest that among the female participants, those who did not engage in perspective-taking showed the paradoxical effects of stereotype suppression. However, those paradoxical effects were not observed among those who performed perspective-taking.
Abstract
Harmonic generation is a general characteristic of driven nonlinear systems, and serves as an efficient tool for investigating the fundamental principles that govern the ultrafast nonlinear ...dynamics. Here, we report on terahertz-field driven high-harmonic generation in the three-dimensional Dirac semimetal Cd
3
As
2
at room temperature. Excited by linearly-polarized multi-cycle terahertz pulses, the third-, fifth-, and seventh-order harmonic generation is very efficient and detected via time-resolved spectroscopic techniques. The observed harmonic radiation is further studied as a function of pump-pulse fluence. Their fluence dependence is found to deviate evidently from the expected power-law dependence in the perturbative regime. The observed highly non-perturbative behavior is reproduced based on our analysis of the intraband kinetics of the terahertz-field driven nonequilibrium state using the Boltzmann transport theory. Our results indicate that the driven nonlinear kinetics of the Dirac electrons plays the central role for the observed highly nonlinear response.
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