The nonlinear optical response of monolayer transition-metal dichalcogenides $T{X}_2\left(T=\mathrm{Mo},\mathrm{W};X=\mathrm{S},\mathrm{Se}\right)$ on ${\mathrm{SiO}}_2/\mathrm{Si}\left(001\right)$ substrates was examined using second-harmonic generation (SHG) spectroscopy in combination with differential reflectance (DR) spectroscopy. In the SHG spectroscopy, the second-harmonic intensity corrected by a crystalline quartz reference was obtained for the wavelength of the incident femtosecond laser varied from 750 to 1040 nm. The SHG spectra of all the four ${TX}_2$ were dominated by two-photon resonance around the C peaks in the DR spectra that was previously assigned to interband transitions at critical points formed as a result of band nesting. Multiple peaks were observed in each resonance spectrum, which was then decomposed into a few components. The overall feature of the main components—denoted as $\overline{\mathrm{C}1}$ and $\overline{\mathrm{C}2}$—was common among the four ${TX}_2$ materials. The optical transitions contributing to these components were concluded to occur at different $k$ points in a ring-shaped area around the Г point with the aid of the published results of optical spectrum calculations and the SHG spectrum of bilayer ${\mathrm{MoS}}_2$. In addition, one-photon resonance at the $A$ exciton and two-photon resonance at the $A^{\prime }$ exciton were observed in ${\mathrm{MoSe}}_2$ and ${\mathrm{WSe}}_2$, respectively, and their intensities were proven to be significantly lower than those of the resonances around the C peaks.

We consider a system of two purely capacitively coupled singlet-triplet qubits and numerically simulate the energy structure of four electrons in two double quantum dots with a large potential barrier between them. We calculate the interqubit coupling strength using an extended Hund-Mulliken approach which includes excited orbitals in addition to the lowest-energy orbital for each quantum dot. We show the coupling strength as a function of the qubit separation as well as plotting it against the detunings of the two double quantum dots and show that the general qualitative features of our results can be captured by a potential-independent toy model of the system.

We propose to Floquet engineer Dirac cones at the surface of a three-dimensional topological insulator. We show that a large tunability of the Fermi velocity can be achieved as a function of the polarization, direction, and amplitude of the driving field. Using this external control, the Dirac cones in the quasienergy spectrum may become elliptic or massive, in accordance with experimental evidence. These results help us to understand the interplay of surface states and external ac driving fields in topological insulators. In our work we use the full Hamiltonian for the three-dimensional system instead of effective surface Hamiltonians, which are usually considered in the literature. Our findings show that the Dirac cones in the quasienergy spectrum remain robust even in the presence of bulk states, and therefore, they validate the usage of effective surface Hamiltonians to explore the properties of Floquet-driven topological boundaries. Furthermore, our model allows us to introduce out-of-plane field configurations which cannot be accounted for by effective surface Hamiltonians.

Multiterminal transport setups allow one to realize measurements and functionalities (e.g., transistors) of nanoscale systems that are more complex than those of the simple two-terminal arrangement. Here the steady-state density functional formalism (i-DFT) for the description of transport through nanoscale junctions with an arbitrary number of leads is developed. In a three-terminal setup and in the ideal STM limit where one of the electrodes (the “STM tip”) is effectively decoupled from the junction, the formalism allows one to extract its nonequilibrium spectral function (at arbitrary temperature) while a bias is applied between the other two electrodes. Multiterminal i-DFT is shown to be capable of describing the splitting of the Kondo resonance in an Anderson impurity in the presence of an applied bias voltage, as predicted by numerically exact many-body approaches.

Many-body eigenstates beyond the Gaussian approximation can be constructed in terms of local integrals of motion (IOMs), although their actual computation has been, until now, a daunting task. We present a practical computation of IOMs based on displacement transformations and apply it to the problem of many-body localization in disordered one-dimensional systems. It represents a general and systematic way to extend Hartree-Fock and configuration interaction theories to higher order. Our method combines minimization of energy and energy variance of a reference state with exact diagonalization. We show that our implementation for one-dimensional disordered fermions is able to perform ground-state calculations with high precision for relatively large systems. Since it keeps track of the IMOs forming a reference state, our method is particularly efficient dealing with excited states, both in accuracy and in the number of different states that can be constructed.

At a fixed magnetic filling fraction, fractional quantum Hall states may display a plethora of interaction-induced competing phases. Effective Chern-Simons theories have suggested the existence of multiple interaction-induced short-range entangled phases also at integer quantum Hall plateaus. Among these, a bosonic phase has been proposed with edge modes carrying representations based on the $E_8$ exceptional Lie algebra. Through a theoretical coupled-wire construction, we provide an explicit microscopic model for this $E_8$ Abelian quantum Hall state, at filling $\nu =16$, and discuss how it is intimately related to topological paramagnets in $3+1$ dimensions. Still using coupled wires, we partition the $E_8$ state into a pair of non-Abelian, long-range entangled states. These two states occur at filling $\nu =8$, demonstrating that even topological order may also exist at integer Hall plateaus. These phases are bosonic, carry chiral edge theories with either $G_2$ or $F_4$ internal symmetries, and host Fibonacci anyonic excitations in the bulk. This suggests that the $\nu =8$ quantum Hall plateau may provide an unexpected platform to realize decoherence-free quantum computation by anyon braiding. We also find that these topological ordered phases are related by a notion of particle-hole conjugation based on the $E_8$ state that exchanges the $G_2$ and $F_4$ Fibonacci states. We argue that these phases can be tracked down by their electric and thermal Hall transport satisfying a distinctive Wiedemann-Franz law $\left({\kappa }_{xy}/{\sigma }_{xy}\right)/\left[\left({\pi }^2{k}_B^{2}T\right)/3e^2\right]<1$, even at integer magnetic filling factors.

In this paper we study how dynamical chiral symmetry breaking is affected by nonzero chiral chemical potential in Dirac semimetals. To perform this study we applied lattice quantum Monte Carlo simulations of Dirac semimetals. Within the lattice simulation we calculated the chiral condensate for various fermion masses, the chiral chemical potentials, and effective coupling constants. For all parameters under consideration we have found that the chiral condensate is enhanced by the chiral chemical potential. Thus our results confirm that in Dirac semimetals the chiral chemical potential plays the role of the catalyst of the dynamical chiral symmetry breaking.

The drag of half-light half-matter quasiparticles, exciton-polaritons, by an electric current is a peculiar mechanism of light-matter interaction in solids. While an ideal superfluid is protected from being dragged by its zero viscosity, here we argue that the state of the superfluid polariton condensate formed by nonresonant optical pumping can be controlled by the electric current. The proposed mechanism is based on the stimulated relaxation of moving uncondensed excitons dragged by the electric current. The stimulated relaxation process favors the formation of a moving condensate in a quantum state that is characterized by the lowest condensation threshold. We also show that the electron-mediated inelastic scattering of the reservoir excitons to the condensate leads to the transfer of nonzero mean momentum to the electron gas, thus contributing to the electric current. We predict the generation of circular electric currents in a micropillar cavity in the presence of nonresonant laser pumping at normal incidence.

We investigate theoretically a refrigerator based on a two-level system (TLS) coupled alternately to two different heat baths. Modulation of the coupling is achieved by tuning the level spacing of the TLS. We find that the TLS, which avoids quantum coherences, creates finite cooling power for one of the baths in sudden cycles, i.e., acts as a refrigerator even in the limit of infinite operation frequency. By contrast, the cycles that create quantum coherence in the sudden expansions and compressions lead to heating of both the baths. We propose a driving method that avoids creating coherence and thus restores the cooling in this system. We also discuss a physical realization of the cycle based on a superconducting qubit coupled to dissipative LC resonators.

Berry curvature acts analogously to a magnetic field in the momentum space, and it modifies the flow of charge carriers and entropy. This induces several intriguing magnetoelectric and magnetothermal transport phenomena in Weyl semimetals. Here, we explore the impact of the Berry curvature and orbital magnetization on the thermopower in tilted type-I and type-II Weyl semimetals, using semiclassical Boltzmann transport formalism. We analytically calculate the full magnetoconductivity matrix and use it to obtain the thermopower matrix for different orientations of the magnetic field $\left(B\right)$, with respect to the tilt axis. We find that the tilt of the Weyl nodes induces linear magnetic field terms in the conductivity matrix, as well as in the thermopower matrix. The linear-$B$ term appears in the Seebeck coefficients, when the $B$ field is applied along the tilt axis. Applying the magnetic field in a plane perpendicular to the tilt axis results in a quadratic-$B$ planar Nernst effect, linear-$B$ out-of-plane Nernst effect, and quadratic-$B$ correction in the Seebeck coefficient.

We use inelastic neutron scattering to show that long-range spin waves arising from the static bicollinear antiferromagnetic (AF) order in FeTe, which have twofold rotational symmetry in a fully detwinned crystal, rapidly dissolve above $E\approx 26\mathrm{meV}$ into ridges of scattering with fourfold rotational symmetry and a nearly isotropic magnetic fluctuation spectrum. With increasing temperature above $T_N\approx 68\mathrm{K}$, the twofold spin waves change into broad regions of scattering with fourfold symmetry. Since the scattering patterns from plaquette magnetic order generated within a bilinear biquadratic Hamiltonian have fourfold rotational symmetry consistent with the high-energy, spin-isotropic spin waves of FeTe, we conclude that the bicollinear AF state in FeTe is quasidegenerate with plaquette magnetic order, providing evidence of the strongly frustrated nature of the local moments in the iron chalcogenide family of iron-based superconductors.

Static and dynamic properties of the quasi-two-dimensional antiferromagnet ${\mathrm{K}}_2{\mathrm{V}}_3{\mathrm{O}}_8$ have been investigated by ${}^{51}\mathrm{V}$-NMR experiments on nonmagnetic ${\mathrm{V}}^{5+}$ sites. Above the structural transition temperature $T_S=115$ K, NMR spectra are fully compatible with the $P4bm$ space-group symmetry. The formation of superstructure below $T_S$ causes splitting of the NMR lines, which get broadened at lower temperatures so that individual peaks are not well resolved. Evolution of NMR spectra with magnetic field along the $c$ axis below the magnetic transition temperature $T_N\sim 4\mathrm{K}$ is qualitatively consistent with a simple Néel order and a spin-flop transition. However, a broad feature of the spectra does not rule out possible incommensurate spin structure. The spin-lattice relaxation rate $1/T_1$ below $T_N$ shows huge enhancement for a certain range of magnetic field, which is independent of temperature and attributed to cross relaxation due to anomalously large nuclear spin-spin coupling between ${\mathrm{V}}^{5+}$ and magnetic ${\mathrm{V}}^{4+}$ sites. The results indicate strong gapless spin fluctuations, which could arise from incommensurate orders or complex spin textures.

We study the possibility of topological superconductivity in the noncentrosymmetric monolayer and trilayer ${\mathrm{TaS}}_2$ with out-of-plane mirror symmetry. A time-reversal-invariant nodal $\left(s+f\right)$-wave pairing state with even mirror parity is found to be a promising candidate. This mixed state holds 12 (36) nodes at the Fermi pockets around $\mathrm{\Gamma }$ for the monolayer (trilayer) case and its unconventional superconductivity is consistent with the scanning tunneling microscopy experiments observed in $2H\text{−}{\mathrm{TaS}}_2$ thin flakes. Furthermore, with doping or under uniaxial pressure for trilayer $2H\text{−}{\mathrm{TaS}}_2$, large-Chern-number time-reversal symmetry-breaking mixed states between $\left(d+id\right)$- and $\left(p-ip\right)$-wave pairings can be realized in the phase diagram.

We investigate the interplay between coherent gap dynamics and damping in superconductors taken out of equilibrium by strong optical pulses with subgap terahertz frequencies. A semiphenomenological formalism is developed to include the damping within the electronic subsystem that arises from effects beyond Bardeen-Cooper-Schrieffer mean-field theory, such as interactions between Bogoliubov quasiparticles and decay of the Higgs mode. These processes, conveniently expressed as longitudinal $T_1$ and transverse $T_2$ relaxation times in the standard pseudospin language for superconductors, cause the gap amplitude to be suppressed after the pulse is turned off, but before the timescale where thermalization occurs due to coupling to the lattice. We show that our model quantitatively captures the experimental gap dynamics reported here of NbN and ${\mathrm{Nb}}_3\mathrm{Sn}$ through the picosecond timescale.

Microwave conductivity experiments can directly measure the quasiparticle scattering rate in the superconducting state. We show that this, combined with knowledge of the Fermi surface geometry, can distinguish between closely related superconducting order parameters, e.g., $d_{x^2-y^2}$ and $d_{xy}$ superconductivity. We benchmark this method on ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ and, unsurprisingly, confirm that this is a $d_{x^2-y^2}$ superconductor. We then apply our method to $\kappa -{(\mathrm{BEDT}-\mathrm{TTF})}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Br}$, which we discover is a $d_{xy}$ superconductor.

Synchrotron nanosecond time-resolved x-ray diffraction has been performed on dynamically compressed bismuth along various compression and release paths, hence exploring the Bi phase diagram up to 8 GPa and 600 K. Marked departures from the static phase diagram is observed. The sequence of structural changes is different upon compression and release. The Bi-III complex host-guest structure is never obtained. Instead the Bi-V phase is observed over a large domain. Melting of Bi-V and crystallization of the fluid into Bi-I are clearly identified on stress release. Supercooling and superheating deviations to the equilibrium melting line are observed. These observations on a prototypical system underline the possible deviations between the phase diagram obtained by dynamical compression and the one under static pressure.

In the MnGe chiral magnet, the helimagnetic order and local moment collapse in two steps, showing the succession of high spin (HS) and low spin (LS) states as pressure increases. Here, we use high-pressure neutron diffraction to study the doped compounds ${\mathrm{Mn}}_{0.86}{\mathrm{Co}}_{0.14}\mathrm{Ge}$ and ${\mathrm{Mn}}_{0.9}{\mathrm{Rh}}_{0.1}\mathrm{Ge}$, and show that the evolution of their microscopic magnetic properties is instead continuous. It means that the bulk HS-LS transition is a unique feature of pure MnGe, very sensitive to small changes of the band structure and easily suppressed by chemical substitution. On the other hand, the helimagnetic correlations appear to be strengthened by doping and survive up to larger pressures ($\approx 19\mathrm{GPa}$, to be compared with $\approx 13\mathrm{GPa}$). We discuss these results in the light of other disordered systems with remarkable properties, the so-called Invar alloys.

We have studied the longitudinal spin Seebeck effect (LSSE) in the layered ferromagnetic insulators ${\mathrm{CrSiTe}}_3$ and ${\mathrm{CrGeTe}}_3$ covered by Pt films in the measurement configuration where spin current traverses the ferromagnetic Cr layers. The LSSE response is clearly observed in the ferromagnetic phase and, in contrast to a standard LSSE magnet ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$, persists above the critical temperatures in both ${\mathrm{CrSiTe}}_3/\mathrm{Pt}$ and ${\mathrm{CrGeTe}}_3/\mathrm{Pt}$ samples. With the help of a numerical calculation, we attribute the LSSE signals observed in the paramagnetic regime to exchange-dominated interlayer transport of in-plane paramagnetic moments reinforced by short-range ferromagnetic correlations and strong Zeeman effects.

In Thouless pumping, although a nonflat band has no effects on the quantization of particle transport, it induces wave-packet dispersion which hinders the practical applications of Thouless pumping. Indeed, we find that the dispersion mainly arises from the dynamical phase difference between individual Bloch states. Here we propose two efficient schemes to suppress the dispersion in Thouless pumping: a relocalization echo protocol and a high-order tunneling suppression protocol. In the relocalization echo protocol, we reverse the Hamiltonian in the second pumping cycle to cancel the dynamical phase difference arising from the nonflat band, so that the dispersed wave packet becomes relocalized. In the high-order tunneling suppression protocol, we modulate the nearest-neighbor tunneling to make the Bloch band more flat and suppress the high-order tunneling which causes wave-packet dispersion. Our study paves a way toward the dispersionless Thouless pumping for practical applications in matter transport, state transfer, and quantum communication.