We numerically study level statistics of disordered interacting quantum many-body systems. A two-parameter plasma model which controls the level repulsion exponent $\beta$ and range $h$ of interactions between eigenvalues is shown to reproduce accurately features of level statistics across the transition from the ergodic to many-body localized phase. Analysis of higher-order spacing ratios indicates that the considered $\beta -h$ model accounts even for long-range spectral correlations and allows us to obtain a clear picture of the flow of level statistics across the transition. Comparing the spectral form factors of the $\beta -h$ model and of a system across the ergodic-many-body-localized transition, we show that the range of effective interactions between eigenvalues $h$ is related to the Thouless time which marks the onset of quantum chaotic behavior of the system. Analysis of level statistics of the random quantum circuit which hosts chaotic and localized phases supports the claim that the $\beta -h$ model grasps universal features of level statistics in transition between ergodic and many-body-localized phases also for systems breaking time-reversal invariance.

We present a protocol to achieve double quantum magnetometry at large static magnetic fields. This is a regime where sensitive sample parameters, such as the chemical shift, get enhanced facilitating their characterization. In particular, our method delivers two-tone stroboscopic radiation patterns with modulated Rabi frequencies to achieve larger spectral signals. Furthermore, it does not introduce inhomogeneous broadening in the sample spectrum preventing signal misinterpretation. Moreover, our protocol is designed to work under realistic conditions such as the presence of moderate microwave power and errors on the radiation fields. Albeit we particularize to nitrogen vacancy centers, our protocol is general, thus applicable to distinct quantum sensors.

The transition between the Néel antiferromagnet and the valence-bond solid state in two dimensions has become a paradigmatic example of deconfined quantum criticality, a non-Landau transition characterized by fractionalized excitations (spinons). We consider an extension of this scenario whereby the deconfined spinons are subject to a magnetic field. The primary purpose is to identify the exotic scenario of a Bose-Einstein condensate of spinons. We employ quantum Monte Carlo simulations of the $J-Q$ model with a magnetic field, and we perform a quantum field theoretic analysis of the magnetic field and temperature dependence of thermodynamic quantities. The combined analysis provides evidence for Bose-Einstein condensation of spinons and also demonstrates an extended temperature regime in which the system is best described as a gas of spinons interacting with an emergent gauge field.

Using diffusion and path-integral Monte Carlo methods, we show that liquid ${}^4\mathrm{He}$ confined in a narrow nanopore of liquid radius $R=4\text{Å}$ undergoes a crossover from a one-dimensional (1D) to a two-dimensional (2D) fluid as a function of liquid density. At low liquid density, e.g., a linear density ${\rho }_0=0.15 {\text{Å}}^{-1}$, the liquid energy is at a minimum when the liquid lies in a line at the center of the pore. The pair distribution function $g\left(x\right)$, the one-body density matrix $n\left(x\right)$, and the superfluid fraction ${\rho }_S/{\rho }_0$ all show 1D character that is well described by Luttinger liquid (LL) predictions. As density is increased, there is a crossover to 2D with the minimum energy configuration moving from a line at the center of the pore to a film near the pore walls. At linear density ${\rho }_0>0.40 {\text{Å}}^{-1}$, the ${}^4\mathrm{He}$ lies predominantly in a 2D cylindrical film midway between the center and the nanopore walls. The $g\left(x\right), n\left(x\right)$, and ${\rho }_S/{\rho }_0$ all show 2D character and the film has a low but finite transition temperature. ${}^4\mathrm{He}$ at a bulk liquid density corresponds to ${\rho }_0=0.6 {\text{Å}}^{-1}$ in the pore which lies in the 2D regime.

In this work we study the phase diagram of the Kekulé-Kitaev model. The model is defined on a honeycomb lattice with bond-dependent anisotropic exchange interactions, making it exactly solvable in terms of Majorana representation of spins, in close analogy to the Kitaev model. However, the energy spectrum of Majorana fermions has a multiband structure characterized by Chern numbers 0, $\pm 1$, and $\pm 2$. We obtained the phase diagram of the model in the plane of exchange couplings and in the presence of a magnetic field and found chiral topological and trivial spin-liquid ground states. In the absence of magnetic field most of the phase diagram is a trivial gapped phase continuously connected to an Abelian phase, while in the presence of the magnetic field a topological phase arises. Furthermore, motivated by recent thermal measurements on the spin-liquid candidate $\alpha -{\mathrm{RuCl}}_3$, we calculated the thermal Hall conductivity at different regimes of parameters and temperatures and found the latter is quantized over a wide range of temperatures.

We investigate several definitions of the time-dependent spectral function $A(\omega ,t)$ of the Anderson impurity model following a quench and within the time-dependent numerical renormalization group (TDNRG) method. In terms of the single-particle two-time retarded Green function $G^r(t_1,t_2)$, the definitions we consider differ in the choice of the time variable $t$ with respect to $t_1$ and/or $t_2$ (which we refer to as the time reference). In a previous study [H. T. M. Nghiem et al., Phys. Rev. Lett. 119, 156601 (2017)], we investigated the spectral function $A(\omega ,t)$, obtained from the Fourier transform of $\mathrm{Im}\left[G^r(t_1,t_2)\right]$ with respect to the time difference $t^{\prime }=t_1-t_2$, with time reference $t=t_2$. Here, we complement this work by deriving expressions for the retarded Green function for the choices $t=t_1$ and the average, or Wigner, time $t=(t_1+t_2)/2$, within the TDNRG approach. We compare and contrast the resulting $A(\omega ,t)$ for the different choices of time reference. While the choice $t=t_1$ results in a spectral function with no time dependence before the quench ($t<0$) (being identical to the equilibrium initial-state spectral function for $t<0$), the choices $t=(t_1+t_2)/2$ and $t=t_2$ exhibit nontrivial time evolution both before and after the quench. Expressions for the lesser, greater, and advanced Green functions are also derived within TDNRG for all choices of time reference. The average-time lesser Green function $G^{<}(\omega ,t)$ is particularly interesting, as it determines the time-dependent occupied density of states $N(\omega ,t)=G^{<}(\omega ,t)/\left(2\pi i\right)$, a quantity that determines the photoemission current in the context of time-resolved pump-probe photoemission spectroscopy. We present calculations for $N(\omega ,t)$ for the Anderson model following a quench, and discuss the resulting time evolution of the spectral features, such as the Kondo resonance and high-energy satellite peaks. We also discuss the issue of thermalization at long times for $N(\omega ,t)$. Finally, we use the results for $N(\omega ,t)$ to calculate the time-resolved photoemission current for the Anderson model following a quench (acting as the pump) and study the different behaviors that can be observed for different resolution times of a Gaussian probe pulse.

The quasi-one-dimensional spin ladder compounds, ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ and ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$, are investigated by infrared spectroscopy and density functional theory (DFT) calculations. We observe strong anisotropic electronic properties and an optical gap in the leg direction that is gradually filled above the antiferromagnetic (AFM) ordering temperature, turning the systems into a metallic phase. Combining the optical data with the DFT calculations we associate the optical gap feature with the $p-d$ transition that appears only in the AFM ordered state. Hence, the insulating ground state along the leg direction is attributed to Slater physics rather than Mott-type correlations.

We report on measurements of the ac Josephson effect in three-dimensional topological insulator (TI) Josephson junctions with Fermi energy tuning using a back gate. We successfully tune the Fermi energy into the bulk gap of the TI. We observe that the Josephson energy as a function of the gate voltage has a dip feature, indicating the presence of specular Andreev reflections. We study the ac Josephson effect with detection of both Shapiro steps and Josephson emission. The obtained results show no signature of Majorana modes. With the support of simulations, we conclude that the observation of the fractional ac Josephson effect may be complicated by both a large parallel capacitance and a small ratio of current carried in $4\pi$ period modes compared to conventional $2\pi$ modes.

Recent experiments have shown surprisingly large thermal time constants in suspended graphene ranging from 10 to 100 ns in drums with a diameter ranging from 2 to 7 $\mu \mathrm{m}$. The large time constants and their scaling with diameter points toward a thermal resistance at the edge of the drum. However, an explanation of the microscopic origin of this resistance is lacking. Here, we show how phonon scattering at a kink in the graphene, e.g., formed by sidewall adhesion at the edge of the suspended membrane, can cause a large thermal time constant. This kink strongly limits the fraction of flexural phonons that cross the suspended graphene edge, which causes a thermal resistance at its boundary. Our model predicts thermal time constants that are of the same order of magnitude as experimental data and shows a similar dependence on the circumference. Furthermore, the model predicts the relative in-plane and out-of-plane phonon contributions to graphene's thermal expansion force, in agreement with experiments. We thus show an unconventional thermal boundary resistance which occurs solely due to strong deformations within a two-dimensional material.

Majorana-based quantum computation seeks to encode information nonlocally in pairs of Majorana zero modes, thereby isolating qubit states from a local noisy environment. In addition to long coherence times, the attractiveness of Majorana-based quantum computing relies on achieving topologically protected Clifford gates from braiding operations. Recent works have conjectured that mean-field BCS calculations may fail to account for nonuniversal corrections to the Majorana braiding operations. Such errors would be detrimental to Majorana-based topological quantum computing schemes. In this work, we develop a particle-number-conserving approach for measurement-based topological quantum computing and investigate the effect of quantum phase fluctuations. We demonstrate that braiding transformations are indeed topologically protected in charge-protected Majorana-based quantum computing schemes.

Two-dimensional (2D) magnetic materials are of great current interest for their promising applications in spintronics. Here we propose the van der Waals (vdW) material ${\mathrm{VI}}_3$ to be a 2D Ising ferromagnet (FM), using density functional calculations, crystal field level diagrams, superexchange model analyses, and Monte Carlo simulations. The $a_{1g}{}^1{e}^{\prime }_{-}^1$ ground state in the trigonal crystal field gives rise to the 2D Ising FM due to a significant single ion anisotropy (SIA) and enhanced FM superexchange both associated with the $S_z=1$ and $L_z=-1$ states of ${\mathrm{V}}^{3+}$ ions. We find that a tensile strain on the ${\mathrm{VI}}_3$ monolayer further stabilizes the $a_{1g}{}^1{e}^{\prime }_{-}^1$ ground state, and its Curie temperature $\left(T_{\mathrm{C}}\right)$ would increase from 70 to 90–110 K under a 2.5%–5% tensile strain. Moreover, we suggest a group of spin-orbital states with a strong SIA which may help to search more 2D Ising magnets.

It has been revealed that a Y-type hexaferrite ${\mathrm{BaSrCo}}_2{\mathrm{Fe}}_{11.1}{\mathrm{Al}}_{0.9}{\mathrm{O}}_{22}$ hosts not only an in-plane $\left(P_{ab}\right)$ but also an out-of-plane $\left(P_c\right)$ component of magnetically induced electric polarization in the double-fan magnetic phase. With an increasing in-plane magnetic field, the magnitude of $P_c$ reaches a maximum at around $2\text{T}$ and decreases as approaching the critical field $\left(3.8\text{T}\right)$ of the transition to the collinear ferrimagnetic phase. $P_c$ has also been verified to show a ${120}^{\circ }$-period oscillation as a function of the in-plane magnetic field direction. These results are reproduced by a model combining the inverse Dzyaloshinskii-Moriya interaction and the spin-dependent $pd$ hybridization mechanisms.

The pseudogap phenomenology is one of the enigmas of the physics of high-$T_{\mathrm{c}}$ superconductors. Many members of the cuprate family have now been experimentally characterized with high resolution in both real and momentum space, which revealed highly anisotropic Fermi arcs and local domains which break rotational symmetry in the ${\mathrm{CuO}}_2$ plane at the intraunit cell level. While most theoretical approaches to date have focused on the role of electronic correlations and doping-induced disorder to explain these features, we show that many features of the pseudogap phase can be reproduced by considering the interplay between electronic and nonlinear electron-phonon interactions within a model of fluctuating Cu-O-Cu bonds. Remarkably, we find that electronic segregation arises naturally without the need to explicitly include disorder. Our approach points not only to the key role played by the oxygen bond in the pseudogap phase, but opens different directions to explore how nonequilibrium lattice excitations can be used to control the properties of the pseudogap phase.

The Nieh-Yan anomaly is the anomalous breakdown of the chiral U(1) symmetry caused by the interaction between torsion and fermions. We study this anomaly from the point of view of torsional Landau levels. It was found that the torsional Landau levels are gapless, while their contributions to the chiral anomaly are canceled, except those from the lowest torsional Landau levels. Hence, the dimension is effectively reduced from $(3+1)$-dimensional to $(1+1)$-dimensional. We further show that the coefficient of the Nieh-Yan anomaly is the free-energy density in $(1+1)$ dimensions. Especially, at finite temperature, the thermal Nieh-Yan anomaly is proportional to the central charge. The anomalous thermal Hall conductance in Weyl semimetals is then shown to be proportional to the central charge, which is the experimental fingerprint of the thermal Nieh-Yan anomaly.

We present a stochastic thermodynamics analysis of an electron spin resonance pumped quantum dot device in the Coulomb-blocked regime, where a pure spin current is generated without an accompanying net charge current. Based on a generalized quantum master equation beyond secular approximation, quantum coherences are accounted for in terms of an average spin in the Floquet basis. Elegantly, this average spin undergoes a precession about an effective magnetic field, which originates from the nonsecular treatment and energy renormalization. It is shown that the interaction between average spin and effective magnetic field may have the dominant roles to play in both energy transport and irreversible entropy production. In the stationary limit, the energy and entropy balance relations are also established based on the theory of counting statistics.

We derived simple rules for the sign of ${180}^{\circ }$ superexchange interaction based on the multielectron calculations of the superexchange interaction in the transition metal oxides that are valid both below and above spin crossover under high pressure. The superexchange interaction between two cations in $d^n$ configurations is given by a sum of partial contributions related to the electron-hole virtual excitations to the different states of the $d^{n+1}$ and $d^{n-1}$ configurations. Using these rules, we have analyzed the sign of the ${180}^{\circ }$ superexchange interaction of a number of oxides with magnetic cations in electron configurations from $d^2$ until $d^8$: the iron, cobalt, chromium, nickel, copper, and manganese oxides with increasing pressure. The most interesting result concerns the magnetic state of cobalt and nickel oxides CoO, ${\mathrm{Ni}}_2{\mathrm{O}}_3$ and also ${\mathrm{La}}_2{\mathrm{CoO}}_4, {\mathrm{LaNiO}}_3$ isostructural to well-known high-$T_C$ and colossal magnetoresistance materials. These oxides have a spin $\frac{1}{2}$ at the high pressure. Change of the interaction from antiferromagnetic below spin crossover to ferromagnetic above spin crossover is predicted for oxide materials with cations in $d^5\left({\mathrm{FeBO}}_3\right)$ and $d^7\left(\mathrm{CoO}\right)$ configurations, while for materials with the other $d^n$ configurations spin crossover under high pressure does not change the sign of the ${180}^{\circ }$ superexchange interaction.

The correlations, entanglement entropy, and fidelity susceptibility are calculated for a one-dimensional spin-1/2 XXZ chain with anisotropic power-law long-range interactions by employing the density matrix renormalization group method. In particular, this long-range interaction is assigned to ferromagnetic for transversal components, while it can be either ferro- or antiferromagnetic for the longitudinal spin component. Two ground-state phase diagrams are established versus the anisotropy of the interactions, which not only changes the phase boundaries of the counterparts with short-range interactions, but also leads to the emergence of exotic phases. We found that the long-range interactions of the $z$ component results in a Wigner crystal phase, whereas the transversal one may break a continuous symmetry, resulting in a continuous symmetry-breaking phase.

We study systematically the scattering processes and the conductance spectra in nodal-line semimetal-superconductor junctions using the extended Blonder-Tinkham-Klapwijk theory. The coexistence of peculiar quadruple reflections are found, which are the specular normal reflection, the retro-normal reflection, the specular Andreev reflection and the retro-Andreev reflection. The incident angle dependence and the quasiparticle energy dependence of the double normal reflections and the double Andreev reflections are investigated under various values of parameters such as the interfacial barrier height, the chemical potentials, and the orbital coupling strength. It is found that the appearance and the disappearance of the reflections and their magnitudes can be controlled through tuning these parameters. The scattering mechanism for the reflections are analyzed in details from the viewpoint of the band structure. We also investigate the conductance spectra for the junctions, which show distinctive features and strong anisotropy about the orientation relationships of the nodal line and interface. The unique scattering processes and conductance spectra found in the junctions are helpful in designing superconducting electronic devices and searching for the nodal line in materials experimentally.