We present a study of Seebeck and Nernst effect in combination with magnetoresistance and Hall measurements of the Kagome superconductor ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. A sizable magneto-Seebeck signal appears once the charge density wave (CDW) order sets in below $T_{\text{CDW}}=94$ K. The Nernst signal peaks at a lower temperature $T^{*}\sim 35\mathrm{K}$, crossing which the Hall coefficient switches sign, which we attribute to the ambipolar transport of compensated bands due to the multiband nature of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. Sublinear Nernst signal and Hall resistivity also develop well below the CDW transition, which becomes apparent below $T^{*}$. These findings suggest that, the multiband electronic profile plays an important role in the transport properties of the CDW state in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$.

We study the conditions for Bloch bands to be spanned by symmetric and strictly compact Wannier states that have zero overlap with all lattice sites beyond a certain range. Similar to the characterization of topological insulators in terms of an algebraic (rather than exponential) localization of Wannier states, we find that there may be impediments to the compact localization even of topologically “trivial” obstructed atomic insulators. These insulators admit exponentially localized Wannier states centered at unoccupied orbitals of the crystalline lattice. First, we establish a sufficient condition for an insulator to have a compact representative. Second, for ${\mathcal{C}}_2$ rotational symmetry, we prove that the complement of fragile topological bands cannot be compact, even if it is an atomic insulator. Third, for ${\mathcal{C}}_4$ symmetry, our findings imply that there exist fragile bands with zero correlation length. Fourth, for a ${\mathcal{C}}_3$-symmetric atomic insulator, we explicitly derive that there are no compact Wannier states overlapping with less than 18 lattice sites. We conjecture that this obstruction generalizes to all finite Wannier sizes. Our results can be regarded as the stepping stone to a generalized theory of Wannier states beyond dipole or quadrupole polarization.

We investigate the optical conductivity and far-infrared magneto-optical response of ${\mathrm{BaNiS}}_2$, a simple square-lattice semimetal characterized by Dirac nodal lines that disperse exclusively along the out-of-plane direction. With the magnetic field aligned along the nodal line the in-plane Landau level spectra show a nearly $\sqrt{B}$ behavior, the hallmark of a conical-band dispersion with a small spin-orbit coupling gap. The optical conductivity exhibits an unusual temperature-independent isosbestic line, ending at a Van Hove singularity. First-principles calculations unambiguously assign the isosbestic line to transitions across Dirac nodal states. Our work suggests a universal topology of the electronic structure of Dirac nodal lines.

It has been a long-standing problem in $N$-electron systems as to how to solve the Dyson equation with the quasiparticle wave functions defined as $\langle {\mathrm{\Psi }}_0^N\left|{\psi }_s\left(\mathit{r}\right)\right|{\mathrm{\Psi }}_{\nu }^{N+1}\rangle$ and $\langle {\mathrm{\Psi }}_{\mu }^{N-1}\left|{\psi }_s\left(\mathit{r}\right)\right|{\mathrm{\Psi }}_0^N\rangle$, which are not mutually orthogonal and have a norm less than 1. We show that quasiparticle wave functions without multiple excitations can exactly be normalized to unity owing to the Ward identity and the vertex function in $(\mathit{q},{\omega }^{\prime }-\omega )\to 0$, although they are not necessarily mutually orthogonal. Since they satisfy an eigenvalue equation with the hermitized self-energy due to Baym-Kadanoff's conservation laws, the present theory can be regarded as an extension of the Kohn-Sham density functional theory, with correlated, interacting, one-electron orbitals.

The discovery of spontaneous magnetism in van der Waal (vdW) magnetic monolayers has opened up an unprecedented platform for investigating magnetism in purely two-dimensional systems. Recently, it has been shown that the magnetic properties of vdW magnets can be easily tuned by adjusting the relative composition of halides. Motivated by these experimental advances, here we derive a model for a trihalide CrClBrI monolayer from symmetry principles and we find that, in contrast to its single-halide counterparts, it can display highly anisotropic nearest- and next-to-nearest neighbor Dzyaloshinskii-Moriya and Heisenberg interactions. Depending on the parameters, the DM interactions are responsible for the formation of exotic chiral spin states, such as skyrmions and spin cycloids, as shown by our Monte Carlo simulations. Focusing on a ground state with a two-sublattice unit cell, we find spin-wave bands with nonvanishing Chern numbers. The resulting magnon edge states yield a magnon thermal Hall conductivity that changes sign as a function of temperature and magnetic field, suggesting chromium trihalides as a candidate for testing topological magnon transport in two-dimensional noncollinear spin systems.

AC susceptibility and neutron scattering measurements are used to study the magnetic field-temperature magnetic phase diagram of a Tb single crystal under uniaxial tension along its hexagonal $c$ axis. An external magnetic field is applied in the basal plane. We focus on the region in the phase diagram that corresponds to the helical antiferromagnetic phase and find that this region collapses when the uniaxial tension is increased beyond a critical value as low as 600 bar. There are strong reasons to associate this collapse with an underlying Lifshitz transition in the Fermi surface of Tb's valence electrons. We use a finite-temperature ab initio theory to analyze our measurements, obtaining a pressure-temperature magnetic phase diagram in very good agreement with experiment. Our calculations indicate that short- and long-range magnetic order has a crucial effect on the Fermi surface nesting and consequent magnetism of Tb.

We develop a self-consistent renormalized spin-wave theory for two-dimensional ferromagnetic ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ monolayer and study the magnon spectrum including the magnon-magnon interaction at finite temperatures. We consider the spin Hamiltonian containing the Heisenberg term, exchange anisotropy, and crystal field effect, with the parameters obtained from both first-principles calculations and recent experimental measurements. We demonstrate that exchange anisotropy and single-ion anisotropy are both important to describe the monolayer ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. A significant renormalization of the magnon spectrum is found to be momentum dependent, especially near the high-symmetry points, which confirms the effects of magnon-magnon interactions as the temperature approaches the Curie temperature. We also observe a notable increase in both the Curie temperature and magnetic anisotropy under the magnetic fields. Due to the suppression of spin fluctuations by the magnetic field, the energy gaps increase significantly as the magnetic fields increase. Our results agree well with recent experiments and deepen our understanding about the magnetic property of two-dimensional magnets.

We show that vertex corrections to Migdal's theorem in general induce an odd-frequency spin-triplet superconducting order parameter, which coexists with its more commonly known even-frequency spin-singlet counterpart. Fully self-consistent vertex-corrected Eliashberg theory calculations for a two-dimensional cuprate model, isotropically coupled to an Einstein phonon, confirm that both superconducting gaps are finite over a wide range of temperatures. The subordinate $d$-wave odd-frequency superconducting gap is found to be one order of magnitude smaller than the primary even-frequency $d$-wave gap. Our study provides a direct proof of concept for a previously unknown generation mechanism of odd-frequency superconductivity as well as for the generic coexistence of both superconducting states in bulk materials.

By varying the disorder realization in the many-body localized (MBL) phase, we investigate the influence of resonances on spectral properties. The standard theory of the MBL phase is based on the existence of local integrals of motion (LIOM), and eigenstates of the time evolution operator can be described as LIOM configurations. We show that smooth variations of the disorder give rise to avoided level crossings, and we identify these with resonances between LIOM configurations. Through this parametric approach, we develop a theory for resonances in terms of standard properties of nonresonant LIOM. This framework describes resonances that are locally pairwise, and is appropriate in arbitrarily large systems deep within the MBL phase. We show that resonances are associated with large level curvatures on paths through the ensemble of disorder realizations, and we determine the curvature distribution. By considering the level repulsion associated with resonances, we calculate the two-point correlator of the level density. We also find the distributions of matrix elements of local observables and discuss implications for low-frequency dynamics.

We present a full density-functional theory-based implementation of the stochastic path-integral approach proposed by Liu et al. [H. Liu, Y. Yuan, D. Liu, X.-Z. Li, and J. Shi, Phys. Rev. Research 2, 013340 (2020)] for estimating the superconducting transition temperature ($T_c$) of a liquid. The implementation is based on the all-electron projector augmented-wave (PAW) method. We generalize Liu et al.'s formalism to accommodate the pseudodescription of electron states in the PAW method. A formula for constructing the overlap operator of the PAW method is proposed to eliminate errors due to the incompleteness of a pseudobasis set. We apply the implementation to estimate $T_c$'s of metallic hydrogen liquids. It confirms Liu et al.'s prediction that metallic hydrogen could form a superconducting liquid.

A quantum model for Josephson-based metamaterials working in the three-wave mixing (3WM) and four-wave mixing (4WM) regimes at the single-photon level is presented. The transmission line taken into account, namely Josephson traveling wave parametric amplifier (JTWPA), is a bipole composed of a chain of rf-SQUIDs, which can be biased by a DC current or a magnetic field to activate the 3WM or 4WM nonlinearities. The model exploits a Hamiltonian approach to analytically determine the time evolution of the system both in the Heisenberg and interaction pictures. The former returns the analytic form of the gain of the amplifier, while the latter allows recovering the probability distributions vs time of the photonic populations, for multimodal Fock and coherent input states. The dependence of the metamaterial's nonlinearities is presented in terms of circuit parameters in a lumped model framework while evaluating the effects of the experimental conditions on the model validity.

We study an inhomogeneous critical Ising chain in a transverse field whose couplings decay exponentially from the center. In the strong inhomogeneity limit, we apply Fisher's renormalization group to show that the ground state is formed by concentric singlets similar to those of the rainbow state of the XX model. In the weak inhomogeneity limit, we map the model to a massless Majorana fermion living in a hyperbolic space-time, where, using conformal field theory techniques, we derive the entanglement entropy and show that it presents a linear scaling. We also study an inhomogeneous noncritical Ising model that for weak inhomogeneity is mapped to a massive Majorana fermion, while for strong inhomogeneity regime, it exhibits trivial and nontrivial topological phases and a separation between regions with high and low entanglement. We also present the entanglement Hamiltonian of the model.

Spin carriers propagating along quantum circuits gather quantum spin phases depending on the circuit's size, shape, and spin-orbit coupling (SOC) strength. These phases typically grow monotonically with the SOC strength, as found in Rashba quantum wires and rings. In this work we show that the spin-phase gathering can be engineered by geometric means, viz., by the geometric curvature of the circuits, to be nonmonotonic. We demonstrate this peculiar property by using one-dimensional polygonal models where flat segments alternate with highly curved vertices. The complex interplay between dynamic and geometric spin-phase components—triggered by a series of emergent spin degeneracy points—leads to bounded, global spin phases. Moreover, we show that the particulars of the spin-phase gathering have observable consequences in the Aharonov-Casher conductance of Rashba loops, a connection that passed unnoticed in previous works.

Interlayer excitons are bound states of electrons and holes confined in separate two-dimensional layers. Due to their repulsive dipolar interaction, interlayer excitons can form a correlated liquid. If another electron-hole bilayer is present, excitons from different bilayers can exhibit mutual attraction. We study such a quadrilayer system by a hypernetted chain formalism. We compute ground-state energies, pair correlation functions, and collective mode velocities as functions of the exciton densities. We estimate the critical density for the transition to a paired biexciton phase. For a strongly unbalanced (unequal density) system, the excitons in the more dilute bilayer behave as polarons. We compute energies and effective masses of such exciton-polarons.

Calculating the spectral function of two-dimensional systems is arguably one of the most pressing challenges in modern computational condensed matter physics. While efficient techniques are available in lower dimensions, two-dimensional systems present insurmountable hurdles, ranging from the sign problem in quantum Monte Carlo methods to the entanglement area law in tensor-network-based methods. We hereby present a variational approach based on a Chebyshev expansion of the spectral function and a neural network representation for the wave functions. The Chebyshev moments are obtained by recursively applying the Hamiltonian and projecting on the space of variational states using a modified natural gradient descent method. We compare this approach with a modified approximation of the spectral function which uses a Krylov subspace constructed from the “Chebyshev wave functions.” We present results for the one-dimensional and two-dimensional Heisenberg model on the square lattice and compare them with those obtained by other methods in the literature.

We study theoretically the spatial distribution of the polarizations in the array of resonant electromagnetic dipole emitters coupled to a one-dimensional waveguide. The ratchet effect manifests itself in the spatial asymmetry of the distribution of the emitter occupations along the array under symmetrical pumping from both sides. The occupation asymmetry is driven by the periodic modulation in time of the emitter resonance frequencies. We find numerically and analytically the optimal conditions for maximal asymmetry. We also demonstrate that the ratchet effect can be enhanced due to the formation of topological electromagnetic edge states, enabled by the frequency modulation. Our results apply to classical structures with coupled resonators or arrays of semiconductor quantum wells as well as quantum setups with waveguide-coupled natural or artificial atoms.

We propose the concept of single-valley state in two-dimensional antiferromagnetic lattice, which intrinsically holds only one valley and thus robust valley-polarized physics. Using tight-binding model analysis, the physical mechanism to form the single-valley state is illustrated, and large magnetic moment and strong spin-orbit coupling are unveiled to define it. Furthermore, based on first-principles calculations, the single-valley state is predicted in a real material of single-layer $\mathrm{BiFe}{\mathrm{O}}_3$. The spontaneous valley polarization in single-layer $\mathrm{BiFe}{\mathrm{O}}_3$ is particularly huge, at least one order of magnitude higher than those of all the existing ferrovalley systems. Our work enriches the physics of two-dimensional valleytronics and provides a promising avenue to observe strong valley-polarized phenomena.

We consider weak-coupling superconductivity in the inversion- and rotation-symmetric system of two pseudospin $s=1$ low-energy fermions of opposite chirality. General contact interactions can lead to Cooper instabilities toward $d$-wave or toward a novel $2\times 3 p$-wave matrix order parameter. We compute the Ginzburg-Landau (GL) free energy for the latter. Remarkably, in this case the GL free energy can be minimized exactly, with the resulting ordered state being analogous to the “axiplanar” $p$ wave, which exhibits extra degeneracy and generally breaks time-reversal symmetry. Whereas a generic GL free energy for our order parameter has only $U\left(1\right)\times SO\left(2\right)\times SO\left(3\right)$ symmetry, at weak coupling we find it displaying the enlarged $U\left(1\right)\times SO\left(3\right)\times U\left(1\right)\times SO\left(3\right)$ symmetry broken down to $SO\left(2\right)\times SO\left(2\right)$ in the axiplanar ordered phase and leading therefore to six Goldstone bosons. We show how the lattice, once restored, fixes the allowed values of the magnetization in the superconducting state by locking the spatial directions implicit in the order parameter to its high-symmetry axes.