Superconductivity and ferroelectricity are typically thought of as incompatible because the former needs free carriers, but the latter is usually suppressed by free carriers. This is unless the carrier concentration is sufficiently low to allow for polar distortions and mobile electrons to cooperate. In the case of strontium titanate with low carrier concentration, superconductivity and ferroelectricity have been shown to be correlated via various tuning methods, such as strain. Here, we report theoretically and experimentally evaluated Grüneisen parameters whose divergent giant values under tensile stress indicate that the dominant phonon mode which enhances the superconducting order is the ferroelectric transverse soft phonon mode. This finding puts strong constraints on other phonon modes as the main contributors to the enhanced superconductivity in strained strontium titanate. The methodology shown here can be applied to strain tune and probe properties of other materials with polar distortions including topologically nontrivial ones.

Recently, topological phases in non-Hermitian systems have attracted great attention. In most of these works, disorders are added to the chemical potential or hopping term and the topological transition driven by disordered loss and gain has not been found. In this paper, we investigate the one-dimensional dimerized Kitaev superconductor chain with balanced loss and gain. In the clean limit, the phase diagram is given by calculating the Bloch winding number and real-space energy spectrum. Furthermore, by calculating the disorder-averaged real-space winding number and disorder-averaged real-space energy spectrum, we identify a topological transition driven by the disordered balanced loss and gain. The disorder-averaged inverse participate ratio of each eigenstate, mean inverse participate ratio, and density distribution of the edge mode are also calculated to support the existence of the topological Anderson phase. Finally, we compare our model with the Hermitian disordered Kitaev chain and discuss the possible experimental scheme of our model. Our work offers a minimal start point to explore the interplay between the dissipation and Majorana zero modes in the topological superconductor.

We present a complete topological characterization of a bilayer composite of two Chern insulators (specifically, Haldane models) and explicitly establish the bulk-boundary correspondences. We show that an appropriately defined Chern number accurately maps out all the possible phases of the system and remains well defined even in the presence of degeneracies in the occupied bands. Importantly, our result paves the way for realizing adiabatic preparation of monolayer Chern insulators. This has been a major challenge to date, given the impossibility of unitarily connecting inequivalent topological phases. We show that this difficulty can be circumvented by adiabatically varying the interlayer coupling in such a way that the system remains gapped at all times. In particular, complete knowledge of the phase diagram of the bilayer composite immediately allows one to identify all such adiabatic passages which may connect the different Chern inequivalent phases of the individual monolayers.

Ultrafast laser pulse has provided a systematic way to inspect the dynamics of electrons in condensed matter systems. In this paper, by means of time-dependent density matrix renormalization group, we study an ultrafast laser-driven Kondo lattice model, in which conduction electrons are strongly coupled with magnetically local moments. The single-particle spectral function due to strong correlation effects and photon emission in the nonequilibrium states under laser driving are calculated. We find laser field excited collective doublon-hole pairs and an associated transient melting of Kondo coherence phase, signifying the collapse of Kondo energy gap. Moreover, we show that the photon emission, induced by a strong laser field, exhibits a different intensity characteristics than in the equilibrium Kondo insulator, which could be explained by the Kondo collapse and related suppression of both intraband and interband contribution in Kondo melting liquid. This theoretical insight is accessible with time- and angle-resolved photoemission spectroscopy and high-harmonic generation spectroscopy and will stimulate the investigation of nonequilibrium dynamics and nonlinear phenomenon in heavy fermion systems.

We present the structural and magnetic properties of ${\mathrm{KNaCuP}}_2{\mathrm{O}}_7$ investigated via x-ray diffraction, magnetization, specific heat, and ${}^{31}\mathrm{P}$ and ${}^{23}\mathrm{Na}$ NMR measurements and complementary electronic structure calculations. The temperature-dependent magnetic susceptibility and ${}^{31}\mathrm{P}$ NMR shift could be modeled very well by the uniform spin-$\frac{1}{2}$ Heisenberg antiferromagnetic chain model with a nearest-neighbor interaction $J/k_{\mathrm{B}}\simeq 58.7\mathrm{K}$. The corresponding mapping using first-principles electronic structure calculations leads to $J^{\mathrm{DFT}}/k_{\mathrm{B}}\simeq 59\mathrm{K}$ with negligibly small interchain couplings, further confirming that the system is indeed a one-dimensional uniform spin-$\frac{1}{2}$ Heisenberg antiferromagnet. The diverging trend of NMR spin-lattice relaxation rates $({}^{31}1/T_1 \mathrm{and} {}^{23}1/T_1)$ implies the onset of a magnetic long-range ordering at around $T_{\mathrm{N}}\simeq 1\mathrm{K}$. From the value of $T_{\mathrm{N}}$, the average interchain coupling is estimated to be $J^{\prime }/k_{\mathrm{B}}\simeq 0.28\mathrm{K}$. Moreover, the NMR spin-lattice relaxation rates show the dominant contributions from uniform $(q=0)$ and staggered $(q=\pm \pi /a)$ spin fluctuations in the high- and low-temperature regimes, respectively, mimicking one-dimensionality of the spin lattice. We have also demonstrated that ${}^{31}1/T_1$ in high temperatures varies linearly with $1/\sqrt{H}$, reflecting the effect of spin diffusion on the dynamic susceptibility. The temperature-dependent unit cell volume could be described well using the Debye approximation with a Debye temperature of ${\mathrm{\Theta }}_{\mathrm{D}}\simeq 294\mathrm{K}$, consistent with the heat capacity data.

It was observed recently [Shigekawa et al., PNAS 116, 2470 (2019)] that while monolayer iron chalcigenide FeSe on a ${\text{SrTiO}}_3$ (STO) substrate has a very high critical temperature, its chemical and structural twin material FeS/STO has a very low $T_c$, if any. To explain this, the substrate interfacial phonon model of superconductivity in iron chalcogenides is further developed. The main glue is the oxygen ion ${\mathrm{\Omega }}_s=60\text{meV}$ vibrations longitudinal optical (LO) mode. The mode propagates mainly in the ${\text{TiO}}_2$ layer adjacent to the monolayer (and also generally present in similar highly polarized ionic crystals like ${\text{BaTiO}}_3,$ rutile, and anatase). It has stronger electron-phonon coupling to electron gas in FeSe than a well-known ${\mathrm{\Omega }}_h=100\text{meV}$ harder LO mode. It is shown that while (taking into account screened Coulomb repulsion effects) the critical temperature of FeSe on STO and ${\text{TiO}}_2$ is above $65\text{K}$, it becomes less than $5\text{K}$ for FeS due to two factors suppressing the electron-phonon coupling. The effective mass in the latter is twice smaller and, in addition, the distance between the electron gas in FeSe to the vibrating substrate oxygen atoms is 15% smaller than in FeS, reducing the central peak in electron-phonon interaction. The theory is extended to other ionic insulating substrates.

At ambient pressure, lithium molybdenum purple bronze (${\mathrm{Li}}_{0.9}{\mathrm{Mo}}_6{\mathrm{O}}_{17}$) is a quasi-one-dimensional solid in which the anisotropic crystal structure and the linear dispersion of the underlying bands produced by electronic correlations possibly bring about a rare experimental realization of Tomomaga-Luttinger liquid physics. It is also the sole member of the broader purple molybdenum bronzes family where a Peierls instability has not been identified at low temperatures. The present paper reports a pressure-induced series of phase transitions between 0 and 12 GPa. These transitions are strongly reflected in infrared spectroscopy, Raman spectroscopy, and x-ray diffraction. The most dramatic effect seen in optical conductivity is the metalization of the $c$ axis, concomitant to the decrease in conductivity along the $b$ axis. This indicates that high pressure drives the material away from its quasi-one-dimensional behavior at ambient pressure. Although the first pressure-induced structure of the series is resolved, the identification of the underlying mechanisms driving the dimensional change in the physics remains a challenge.

We consider a hitherto unexplored setting of a stacked multilayer ($\mathcal{N}$) Lieb lattice which undergoes an unusual topological transition in the presence of intralayer spin-orbit coupling (SOC). The specific stacking configuration induces an effective nonsymmorphic two-dimensional lattice structure, even though the constituent monolayer Lieb lattice is characterized by a symmorphic space group. This emergent nonsymmorphicity leads to multiple doubly degenerate bands extending over the edge of the Brillouin zone (i.e., quadratic band crossing lines). In the presence of intralayer SOC, these doubly degenerate bands typically form three $\mathcal{N}$-band subspaces, mutually separated by two band gaps. We analyze the topological properties of these multiband subspaces, using specially devised Wilson loop operators to compute non-Abelian Berry phases in order to show that they carry a higher Chern number $\mathcal{N}$.

Using dynamical mean-field theory, we investigate the correlated nature of quantum-confined electrons localized in the crystal voids of the zero-dimensional electride $\beta \text{−}{\mathrm{Yb}}_5{\mathrm{Sb}}_3$. In this work, maximally localized Wannier functions were used to obtain an analytical description of the half-filled electride electronic states and to establish a high degree of their localization, with the magnetic moment of $1{\mu }_{\mathrm{B}}$ per anionic cavity. We reproduced the Mott metal-insulator transition, obtained the experimentally observed energy gap, and demonstrated the decrease of resistivity with increasing temperature, typical for semiconductors. The Curie-type temperature dependence of the compound's magnetic susceptibility was calculated and found to be in close agreement with experiment. These results obtained prove that the Coulomb correlations between the localized anionic electrons are the source of the observed electronic and magnetic properties of $\beta \text{−}{\mathrm{Yb}}_5{\mathrm{Sb}}_3$.

We studied the electronic structure of the $\beta \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ insulator within the density-functional theory using the generalized gradient approximation while taking into account strong Coulomb correlations in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band structure method. The $\beta \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ undergoes a pressure-induced structural and magnetic phase transition at $P_c\sim 4$ GPa with symmetry lowering to the monoclinic $C2/c$. The structural phase transition is accompanied by the formation of ${\mathrm{Ir}}_2$ dimers on the zigzag chains, with an Ir-Ir distance of $\sim 2.66$ Å, even shorter than that of metallic Ir. The strong dimerization stabilizes the bonding molecular-orbital state, leads to the collapse of the magnetism, and opens the energy gap with a concomitant electronic phase transition from a Mott insulator to band insulator. The resonant inelastic x-ray scattering spectra (RIXS) at the Ir $L_3$ edge were investigated theoretically from first principles. The calculated results are in good agreement with the experimental data. We show that the drastic reconstruction of the RIXS spectral peak at 0.7 eV associated with the structural $Fddd\to C2/c$ phase transition at $P_c$ can be related to the disappearing of the Coulomb correlations in the high-pressure $C2/c$ phase.

Square-root topological phases have been discussed mainly for systems with chiral symmetry. In this paper, we analyze the topology of the squared Hamiltonian for systems preserving time-reversal and particle-hole symmetry. Our analysis elucidates that two-dimensional systems of class CII host helical edge states due to the nontrivial topology of the squared Hamiltonian in contrast to the absence of ordinary topological phases. The emergence of helical edge modes is demonstrated by analyzing a toy model. We also show the emergence of surface states induced by the nontrivial topology of the squared Hamiltonian in three dimensions.

We analyze experimentally and theoretically the transport spectra of a gated lateral GaAs double quantum dot containing two holes. The strong spin-orbit interaction present in the hole subband lifts the Pauli spin blockade and allows to map out the complete spectra of the two-hole system. By performing measurements in both source-drain voltage directions, at different detunings and magnetic fields, we carry out quantitative fitting to a Hubbard two-site model accounting for the tunnel coupling to the leads and the spin-flip relaxation process. We extract the singlet-triplet gap and the magnetic field corresponding to the singlet-triplet transition in the double-hole ground state. Additionally, at the singlet-triplet transition we find a resonant enhancement (in the blockaded direction) and suppression of current (in the conduction direction). The current enhancement stems from the multiple resonance of two-hole levels, opening several conduction channels at once. The current suppression arises from the quantum interference of spin-conserving and spin-flipping tunneling processes.

Orbitally ordered states exhibit unique features which make them a promising platform for exploring the ultrafast dynamics of long-range order in solids: Their free energy typically has multiple discrete minima, and electric laser fields or selectively excited phonons can exert effective forces that may be used to steer the order parameter through these free-energy landscapes. Moreover, their free energy strongly depends on fluctuations, and in some cases restoring forces close to a minimum are exclusively of entropic origin (order-by-disorder mechanisms). This can open pathways to control the dynamics of the order parameter via nonthermal fluctuations. In this paper, we study the laser-induced nonequilibrium dynamics in a ${120}^{\circ }$ compass model, using time-dependent Ginzburg-Landau theory. We analyze protocols to switch the order parameter between equivalent configurations, with a focus on the interplay between the external force due to the driving field and the nonthermal entropic forces. In particular, we find that remanent nonthermal fluctuations after some excitation can stabilize the high-symmetry phase even when the homogeneous potential has retrieved its low-temperature form, which facilitates laser-induced switching.

The Anderson Impurity Model (AIM) is a canonical model of quantum many-body physics. Here we investigate whether machine learning models, both neural networks (NN) and kernel ridge regression (KRR), can accurately predict the AIM spectral function in all of its regimes, from empty orbital, to mixed valence, to Kondo. To tackle this question, we construct two large spectral databases containing approximately 410 000 and 600 000 spectral functions of the single-channel impurity problem. We show that the NN models can accurately predict the AIM spectral function in all of its regimes, with pointwise mean absolute errors down to 0.003 in normalized units. We find that the trained NN models outperform models based on KRR and enjoy a speedup on the order of ${10}^5$ over traditional AIM solvers. The required size of the training set of our model can be significantly reduced using farthest point sampling in the AIM parameter space, which is important for generalizing our method to more complicated multichannel impurity problems of relevance to predicting the properties of real materials.

Topological Weyl semimetals (WSMs) have been predicted to be excellent candidates for detecting Berry curvature dipole (BCD) and the related nonlinear effects in electronics and optics due to the large Berry curvature concentrated around the Weyl nodes. And yet, linearized models of isolated tilted Weyl cones only realize a diagonal nonzero BCD tensor which sum to zero in the model of WSM with multiple Weyl nodes in the presence of mirror symmetry. On the other hand, recent ab initio work has found that realistic WSMs like TaAs-type or ${\mathrm{MoTe}}_2$-type compounds, which have mirror symmetry, indeed show an off-diagonal BCD tensor with an enhanced magnitude for its nonzero components. So far, there is a lack of theoretical work addressing this contradiction for three-dimensional (3D) WSMs. In this paper, we systematically study the BCD in 3D WSMs using lattice Weyl Hamiltonians, which go beyond the linearized models. We find that the nonzero BCD and its related important features for these WSMs do not rely on the contribution from the Weyl nodes. Instead, they are dependent on the part of the Fermi surface that lie between the Weyl nodes, in the region of the reciprocal space where neighboring Weyl cones overlap. For large-enough chemical potential, such Fermi surfaces are present in the lattice Weyl Hamiltonians as well as in the realistic WSMs. We also show that a lattice Weyl Hamitonian with a nonzero chiral chemical potential for the Weyl cones can also support dips or peaks in the off diagonal components of the BCD tensor near the Weyl nodes themselves, consistent with recent ab initio work. In addition, we predict specific experimental signatures of BCD-induced transport such as nonlinear anomalous Hall, Nernst, and thermal Hall effects in WSMs that can be directly checked in experiments.

The nature of superconductivity in ${\mathrm{BiS}}_2$-based superconductors has been controversial while ab initio calculations proposed this system in close proximity to a charge-density-wave (CDW) phase. Using high-energy high-flux x-ray diffraction, we reveal an intrinsic and long-range CDW phase coexisting with superconductivity in ${\mathrm{NdO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$ superconductor $(x=0.37 \mathrm{and} 0.3)$. The CDW wave vector in ${\mathrm{NdO}}_{0.63}{\mathrm{F}}_{0.37}{\mathrm{BiS}}_2$ correspond ${\mathbf{Q}}_{\mathrm{CDW}}=(0.17, 0.17, 0.5)$ and is associated with transverse atomic displacements. Interestingly, this wave vector does not match theoretical expectations based on either phonon softening or Fermi surface nesting. In ${\mathrm{NdO}}_{0.7}{\mathrm{F}}_{0.3}{\mathrm{BiS}}_2$, where the superconducting transition temperature is highest, the CDW satellites are slightly broader and weaker compared to ${\mathrm{NdO}}_{0.63}{\mathrm{F}}_{0.37}{\mathrm{BiS}}_2$, possibly suggesting the competition with the superconductivity. Last, we measure a thermal diffuse scattering across the superconducting transition temperature and find no meaningful changes. Our result suggests the importance of understanding CDW which might hold a key to the superconductivity in the ${\mathrm{BiS}}_2$-based superconductor.

We investigate transport properties and the electronic structure of BiCuSO, a thermoelectric material which was predicted as a possible parent compound for unconventional superconductivity. For the $p$-type BiCuSO samples studied in this paper, our transport measurements show metallic behavior down to 2 K, and we observe an increase of saturated magnetic moment around 4 K, which could be due to the reorientation of the magnetic easy axis. Using angle-resolved photoemission spectroscopy measurements, we acquired the comprehensive electronic structure of BiCuSO including the predicted flat band, and by carrying out photon polarization dependent measurements we further investigated the orbital characters of bands near the Fermi level. Compared with its sister compound, BiCuSeO, we find the flat band could be the origin of high figure of merit $\left(ZT\right)$ value in the $\mathrm{BiCu}Ch\mathrm{O}$ $(Ch=\mathrm{S}, \mathrm{Se})$ family.

We carried out a comprehensive study of the structural, electrical transport, thermal, and thermodynamic properties in ferrimagnetic ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ single crystals. Mn and Te $K$-edge x-ray absorption spectroscopy and synchrotron powder x-ray diffraction were measured to provide information on the local atomic environment and the average crystal structure. The dc and ac magnetic susceptibility measurements indicate a second-order paramagnetic to ferrimagnetic transition at $T_c\sim 74$ K, which is further confirmed by the specific heat measurement. ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ exhibits semiconducting behavior along with a large negative magnetoresistance of $-87\%$ at $T_c$ and a relatively high value of thermopower up to $\sim 10$ mV/K at 5 K. Besides the rapidly increasing resistivity $\rho \left(T\right)$ and thermopower $S\left(T\right)$ below 20 K, the large discrepancy between the activation energy for resistivity $E_{\rho }$ and thermopower $E_S$ above 20 K indicates the polaronic transport mechanism. Furthermore, the thermal conductivity $\kappa \left(T\right)$ of ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ is notably rather low, comparable to ${\mathrm{Cr}}_2{\mathrm{Si}}_2{\mathrm{Te}}_6$, and is strongly suppressed in the magnetic field across $T_c$, indicating the presence of strong spin-lattice coupling, also similar with ${\mathrm{Cr}}_2{\mathrm{Si}}_2{\mathrm{Te}}_6$.