We demonstrate a methodology for quantitative estimation of electron-phonon coupling (EPC) for each phonon mode taking base from second-order correction in polaron-energy resulting bandgap renormalization of the semiconductor by treating EPC as perturbation. This gives coupling constant (${\alpha }_p$) for the $p\mathrm{th}$ phonon mode in terms of phonon energy and renormalized bandgap due to EPC. These theoretical inferences are experimentally explored using combination of optical absorption and Raman spectroscopies for $\mathrm{\Gamma }$ phonons of temperature-varied GaN and ${\mathrm{TiO}}_2$; results reveal a systematic enhancement in ${\alpha }_p$ with temperature that exhibits unique percent variation of ${\alpha }_p$ for each mode. A higher percent change is observed for the mode known to exhibit greater coupling strength, thereby supporting the theoretical discussions. In this paper, we provide a simpler method to directly probe quantum level phonon-specific interactions with electrons, otherwise probed with much advanced techniques.

Normal-state transport properties (2–300 K) of the polycrystalline series ${\mathrm{Sn}}_{1.03-\delta -x}{\mathrm{In}}_x\mathrm{Te} (0\le x\le 0.07$; $\delta \le 0.0025$) were investigated by means of electrical resistivity, thermopower, Hall effect, and thermal conductivity measurements. The distortion of the valence-band structure by the In-induced resonant level (RL) has a profound influence on the evolution of the normal-state properties with $x$ and on the emergence of superconductivity evidenced by specific-heat measurements down to 0.35 K. In addition to a nearly 40-fold increase in the residual electrical resistivity ${\rho }_0$ on going from $x=0.0$ to 0.05, the thermopower $\alpha$ shows a nonlinear, complex behavior as a function of both temperature and $x$. While Hall measurements indicate a dominant holelike response across the entire composition and temperature ranges, $\alpha$ changes sign below about 100 K and remains negative down to 5 K for $0.0015\le x\le 0.0045$. Additional measurements under magnetic fields ${\mu }_{0}H$ of up to 14 T further shows that $\alpha \left({\mu }_{0}H\right)$ gradually shifts towards positive values, suggestive of a dominant holelike contribution to $\alpha$. Superconductivity emerges for $x=0.02$ at a critical temperature $T_{\mathrm{c}}=0.67$ K, with $T_{\mathrm{c}}$ increasing with $x$ to reach 1.73 K for $x=0.07$. The variations in the superconducting parameters with $x$, notably the specific-heat jump at $T_{\mathrm{c}}$, confirm the results reported in prior studies and suggests a nontrivial role of the RL on the electron-phonon coupling strength. The striking similarities between this series and the canonical resonant system ${\mathrm{Pb}}_{1-x}{\mathrm{Tl}}_x\mathrm{Te}$ provide an excellent experimental opportunity to gain a deeper understanding of the close interplay between resonant level, anomalous transport properties, and superconductivity.

${\mathrm{NaTiSi}}_2{\mathrm{O}}_6$ is an exemplary compound, showing an orbital assisted spin-Peierls phase transition at $T_c=210$ K. We present the results of ${}^{29}\mathrm{Si}$ and ${}^{23}\mathrm{Na}$ nuclear magnetic resonance (NMR) measurements of ${\mathrm{NaTiSi}}_2{\mathrm{O}}_6$. The use of magic angle spinning (MAS) techniques unambiguously shows that only one dynamically averaged silicon site can be seen at $T$ $>T_c$. At cooling, the ${}^{29}\mathrm{Si}$ MAS NMR spectrum shows interesting changes. Immediately below $T_c$, the spectrum gets very broad. Cooling further, it shows two broad lines of unequal intensities which become narrower as the temperature decreases. Below 70 K, two narrow lines have chemical shifts that are typical for diamagnetic silicates. The hyperfine couplings for the two sites are ${}^{29}\mathrm{H}$ ${}_{\text{hf}}=7.4$ kOe/${\mu }_B$ and 4.9 kOe/${\mu }_B$. In the paramagnetic state at high temperature, the spin-lattice relaxation of ${}^{29}\mathrm{Si}$ was found to be weakly temperature dependent. Below $T_c$ the Arrhenius-type temperature dependence of the relaxation rate indicates an energy gap $\mathrm{\Delta }$/${\mathrm{k}}_B=1000\left(50\right)$ K. In the temperature region from 120 to 300 K, the relaxation rate was strongly frequency dependent. At room temperature, we found a power law dependence $T_1^{-1}\propto {\omega }_L^{-0.65\left(5\right)}$. For 70 K $<$ $T$ $<$ 120 K, the relaxation appeared to be nonexponential, which we assigned to a relaxation due to fixed paramagnetic centers. Simulation of the magnetization recovery curve showed activation type temperature dependence of the concentration of these centers. The NMR spectrum of ${}^{23}\mathrm{Na}$ shows the line with typical shape for the central transition of a quadrupolar nucleus. A small frequency shift of ${}^{23}\mathrm{Na}$ resonance corresponds to a very small hyperfine coupling ${}^{23}\mathrm{H}$ ${}_{\text{hf}}=0.32$ kOe/${\mu }_B$. In addition, at $T$ $>T_c$ the ${}^{23}\mathrm{Na}$ spectrum shows another Lorentzian shaped resonance which we attribute to the Na sites where the quadrupolar coupling is partly averaged by ionic motion.

The characterization and the experimental measurement of the Berry curvature in solids have become an increasingly relevant task in condensed matter physics. We present the theoretical prediction of a gate tunable anomalous Hall effect (AHE) in a nonmagnetic oxide interface as a hallmark of a nontrivial Berry curvature. The observed AHE at low temperatures in the presence of a planar magnetic field comes from a multiband low-energy model with a generalized Rashba interaction that supports characteristic out-of-plane spin and orbital textures. We also discuss strategies for reconstructing the Berry curvature from the AHE nonlinearities in (111) ${\mathrm{SrTiO}}_3$ heterostructure interfaces.

Fabrication of isostructural isovalent alloys is a powerful approach to discovering novel materials with emergent properties. This approach has been well established in traditional bulk materials, yet remains to be fully exploited in two-dimensional systems. Here we use black- phosphorus (BP)-like group-VA binary alloyed monolayers on BP-like group IV-VI substrates to demonstrate the enabling power of isomorphism and isovalency in fabricating two-dimensional alloyed monolayers with tunable properties. We achieve the formation of nearly freestanding BP-like ${\mathrm{Sb}}_{1-x}{\mathrm{Bi}}_x (0\le x\le 1)$ monolayers on the GeSe substrate, with a semiconductor-to-metal transition and tunable moiré superstructures as $x$ increases. In contrast, BP-like ${\mathrm{Pb}}_x{\mathrm{Bi}}_{1-x}$ monolayers can be stabilized only for low Pb concentrations, and the contrasting behaviors are attributable to preservation or breakdown of isovalency in the two cases. This study establishes a generic approach to growing alloyed monolayers via van der Waals imprinting, opening a new avenue for materials discoveries with desirable functionalities.

The zero-point lattice expansion (ZPLE) is a small variation of the lattice parameters induced by the presence of phonons in a material compared to the static lattice picture. It contributes significantly to the zero-point renormalization (ZPR) of the band gap energy, but its consequences have not been investigated as thoroughly as those stemming from electron-phonon interactions. In the usual first-principles approach, one evaluates the ZPLE by minimizing the $T=0$ K Helmholtz free energy. In this work, we show that the formalism based on the Grüneisen parameters, which commonly neglects zero-point effects, can be efficiently used to compute the ZPLE for both isotropic and anisotropic materials at a much lower computational cost. We systematically test this formalism on 22 cubic and wurtzite materials, and we obtain excellent agreement with free energy minimization results for both the ZPLE and the resulting band gap ZPR. We use our results to validate an empirical expression estimating the ZPLE-induced ZPR, and we unveil its sensitivity to the temperature range involved in estimating the ZPLE from experimental data. Our findings finally reveal that the ZPLE contribution to the band gap ZPR can reach from 20% to more than 80% of the electron-phonon interaction contribution for heavier or more ionic materials, including materials containing light atoms. Considering both contributions on an equal footing is thus essential should one attempt to compare theoretical ZPR results with experimental data.

Recent realization of well-controlled quantum simulators of the triangular-lattice Fermi-Hubbard model, including the triangular optical lattices loaded with ultracold Fermions and the heterostructures of the transition-metal dichalcogenides, as well as the more advanced techniques to probe them, pave the way for studying frustrated Fermi-Hubbard physics. Here, we theoretically predict asymmetric photoemission and tunneling spectroscopies for a lightly hole-doped and electron-doped triangular Mott antiferromagnet, and reveal two distinct types of magnetic polarons: a lightly renormalized quasiparticle with the same momentum as the spin background and a heavily renormalized quasiparticle with a shifted momentum and a nearly flat band, using both analytical and unbiased numerical methods. We propose these theoretical findings to be verified in frustrated optical lattices and Moiré superlattices by probing various observables including the spectral function, the density of states, the energy dispersion, and the quasiparticle weight. Moreover, we reveal the asymmetric response of the spin background against charge doping, demonstrating that the interplay between the local spin and charge degrees of freedom plays a vital role in doped triangular Mott antiferromagnets.

Antiferromagnetic (AFM) pyrochlore iridates are a fertile ground to hunt for a Weyl semimetallic state characterized by a linear crossing of two nondegenerate bands near the Fermi level $E_F$. Here, we demonstrate evidence of low-temperature anomalous and topological Hall effects in an antiferromagnetic ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$(111) epitaxial thin film. The observed anomalous Hall effect is explained in terms of the momentum space Berry curvature associated with the Weyl nodes in the electronic band structure. The topological Hall effect reaches a large value ${\rho }_{xy}^{\text{THE}}\sim 10\mu \mathrm{\Omega }\text{cm}$ at 2 K. The topological nature of the Hall effect is attributed to the nonzero scalar spin chirality of the all-in-all-out/all-out-all-in (AIAO/AOAI) type noncoplanar AFM spin structure. The magnetoresistance (MR) shows a prominent negative variation ($\mathrm{MR}\sim 10\%$ at 2 K) below 10 K. The field dependence of MR below 5 K varies quadratically in the low-field regime, and above 40 kOe it shows a linear trend. The quadratic to linear crossover of the MR is explained by the field-induced ($H$) spin canting (spin chirality) of the static spins in the AIAO/AOAI spin structure. In the intermediate-temperature region MR of $15-25$ K exhibits a hysteretic response which is associated with field-induced domain switching of the AIAO/AOAI spin structure. This work highlights the interplay of magnetism and topology in a spin-orbit-coupled correlated electron system and unravels the possibility for realizing the Weyl phase in antiferromagnetic ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$(111) thin films.

Bloch's theorem is the centerpiece of topological band theory, which itself has defined an era of quantum materials research. However, Bloch's theorem is broken by a perpendicular magnetic field, making it difficult to study topological systems in strong flux. For the first time, moiré materials have made this problem experimentally relevant, and its solution is the focus of this paper. We construct gauge-invariant irreps of the magnetic translation group at $2\pi$ flux on infinite boundary conditions, allowing us to give analytical expressions in terms of the Siegel theta function for the magnetic Bloch Hamiltonian, non-Abelian Wilson loop, and many-body form factors. We illustrate our formalism using a simple square lattice model and the Bistritzer-MacDonald Hamiltonian of twisted bilayer graphene, obtaining reentrant ground states at $2\pi$ flux under the Coulomb interaction.

Efforts to understand metallic behavior have led to important concepts such as those of strange metals, bad metals, or Planckian metals. However, a unified description of metallic resistivity is still missing. An empirical analysis of a large variety of metals shows that the parallel resistor formalism used in the cuprates, which includes $T$-linear and $T$-quadratic dependence of the electron scattering rates, can be used to provide a phenomenological description of the electrical resistivity in all metals, where these two contributions are shown to correspond to the first two terms of a Taylor expansion of the resistivity, detached from their physics origin, and thus valid for any metal. Here, we show that the different metallic classes are then determined by the relative magnitude of these two components and the magnitude of the extrapolated residual resistivity. These two parameters allow us to categorize a few systems that are notoriously hard to ascribe to one of the currently accepted metallic classes. This approach also reveals that the $T$-linear term has a common origin in all cases, strengthening the arguments that propose the universal character of the Planckian dissipation bound.

Absolute negative mobility (ANM) in nonequilibrium systems depicts the possibility of particles propagating toward the opposite direction of an external force. We uncover in this work a phenomenon analogous to ANM regarding eigenstate localization and particle transport in non-Hermitian systems under the influence of the non-Hermitian skin effect (NHSE). A coherent coupling between two non-Hermitian chains individually possessing the same preferred direction of NHSE is shown to cause a direction reversal of NHSE for all eigenmodes. This concept is further investigated in terms of time evolution dynamics using a non-Hermitian quantum walk platform within reach of current experiments. Our findings are explained both qualitatively and quantitatively. The possible direction reversal of NHSE can potentially lead to interesting applications.

We study the transition from a many-body localized phase to an ergodic phase in spin chain with correlated random magnetic fields. Using multiple statistical measures like gap statistics and extremal entanglement spectrum distributions, we find the phase diagram in the disorder-correlation plane, where the transition happens at progressively larger values of the correlation with increasing values of disorder. We then show that one can use the average of sample variance of magnetic fields as a single parameter which encodes the effects of the correlated disorder. The distributions and averages of various statistics collapse into a single curve as a function of this parameter. This also allows us to analytically calculate the phase diagram in the disorder-correlation plane.

Despite extensive investigations, a topological state that hosts Majorana edge modes in the magnetic field-induced quantum disordered state of the Kitaev candidate material $\alpha \text{−}{\mathrm{RuCl}}_3$ has been hotly debated. To gain more insight into this issue, we measured the thermal Hall conductivity ${\kappa }_{xy}$ of various samples grown by the Bridgman method. The results show that the half-integer quantum thermal Hall effect is intimately related to the magnitude of longitudinal thermal conductivity and the Néel temperature at zero field, both of which are sample dependent. Samples exhibiting the half-integer quantum thermal Hall effect have larger zero-field thermal conductivity values than a threshold value, implying that a long mean free path of heat carriers is an important prerequisite. In addition, we find that samples with a higher Néel temperature exhibit a higher magnetic field at which quantization starts to occur. These results indicate that the quantization phenomenon is significantly affected by the impurity scatterings and the non-Kitaev interactions.

Motivated by recent experiments on the kagome metals $A{\text{V}}_3{\text{Sb}}_5$ with $A=\text{K}$, Rb, and Cs, which show a charge density wave (CDW) at $\sim 100$ K and superconductivity at $\sim 1$ K, we explore the onset of the superconductivity, taking the perspective that it descends from a parent CDW. We argue that viewing the superconductivity as a weak-coupling instability of a reconstructed (by the CDW) Fermi surface naturally explains the experimentally observed “multidome” nonmonotonic dependence on pressure, with the “peaks” in the superconducting critical temperature being associated with the Van Hove singularities of the reconstructed Fermi surface. This “parent-child relationship” also naturally explains the large separation of energy scales between the superconductivity and the CDW. We discuss different possible pairing mechanisms and speculate that the CDW or reconstructed Pomeranchuk fluctuations may mediate the pairing interaction.

Zirconium pentatelluride $\mathrm{Zr}{\mathrm{Te}}_5$ is a topological semimetal. The presence of a temperature-induced Lifshitz transition, in which the Fermi level goes from the conduction band to the valence band with increasing temperature, provides unique opportunities to study the interplay between Fermi-surface topology, dynamics of Dirac fermions, and Berry curvature in one system. Here we present a combined experimental and theoretical study and show that a low-energy model can be used to understand the complicated Hall response and large anomalous Hall effect observed in $\mathrm{Zr}{\mathrm{Te}}_5$ over a wide range of temperature and magnetic field. We found that the anomalous Hall contribution dominates the Hall response in a narrow temperature window around the Lifshitz transition, away from which the orbital contribution dominates. Moreover, our results indicate that a topological phase transition coexists with the Lifshitz transition. Our model provides a unifying framework to understand the Hall effect in semimetals with large Zeeman splitting and nontrivial topology.

X-ray photoelectron spectroscopy (XPS) measures electron removal energies, providing direct access to core and valence electron binding energies, hence probing the electronic structure. In this Letter, we benchmark the ab initio many-body $GW$ approximation on the complete electron binding energies of noble-gas atoms (He-Rn), which span 100 keV. Our results demonstrate that $GW$ achieves an accuracy within 1.2% in XPS binding energies, by systematically restoring the underestimation from density-functional theory (error of 14%) or the overestimation from Hartree-Fock (error of 4.7%). Such results also imply the correlations of $d$ electrons are very well described by $GW$.

Magnetic moments near zigzag edges in graphene allow complex nanostructures with customized spin properties to be realized. However, computational costs restrict theoretical investigations to small or perfectly periodic structures. Here, we demonstrate that a machine-learning approach, using only geometric input, can accurately estimate magnetic moment profiles, allowing arbitrarily large and disordered systems to be quickly simulated. Excellent agreement is found with mean-field Hubbard calculations, and important electronic, magnetic, and transport properties are reproduced using the estimated profiles. This approach allows the magnetic moments of experimental-scale systems to be quickly and accurately predicted, and will speed up the identification of promising geometries for spintronic applications. While machine-learning approaches to many-body interactions have largely been limited to exact solutions of complex models at very small scales, this Letter establishes that they can be successfully applied at very large scales at mean-field levels of accuracy.

Nitrogen-vacancy (NV) spin ensembles in diamond provide an advanced magnetic sensing platform, with applications in both the physical and life sciences. The development of isotopically engineered ${}^{15}\mathrm{NV}$ diamond offers advantages over naturally occurring ${}^{14}\mathrm{NV}$ for magnetometry, due to its simpler hyperfine structure. However, for sensing modalities requiring a bias magnetic field not aligned with the sensing NV axis, the absence of a quadrupole moment in the ${}^{15}\mathrm{N}$ nuclear spin leads to pronounced envelope modulation effects in time-dependent measurements of ${}^{15}\mathrm{NV}$ spin evolution. While such behavior in spin echo experiments are well studied, analogous effects in Ramsey measurements and the implications for magnetometry remain underexplored. Here, we derive the modulated ${}^{15}\mathrm{NV}$ Ramsey response to a misaligned bias field, using a simple vector description of the effective magnetic field on the nuclear spin. The predicted modulation properties are then compared to experimental results, revealing significant magnetic sensitivity loss if unaddressed. We demonstrate that double-quantum coherences of the NV $S=1$ electronic spin states dramatically suppress these envelope modulations, while additionally proving resilient to other parasitic effects such as strain heterogeneity and temperature shifts.

We study the response of an isolated quantum system governed by the Hamiltonian drawn from the Gaussian Rosenzweig-Porter random matrix ensemble to a perturbation controlled by a small parameter. We focus on the density of states, local density of states, and the eigenfunction amplitude overlap correlation functions which are calculated exactly using the mapping to the supersymmetric nonlinear sigma model. We show that the susceptibility of eigenfunction fidelity to the parameter of perturbation can be expressed in terms of these correlation functions and is strongly peaked at the localization transition: It is independent of the effective disorder strength in the ergodic phase, grows exponentially with increasing disorder in the fractal phase, and decreases exponentially in the localized phase. As a function of the matrix size, the fidelity susceptibility remains constant in the ergodic phase and increases in the fractal and in the localized phases at modestly strong disorder. We show that there is a critical disorder strength inside the insulating phase such that for disorder stronger than the critical, the fidelity susceptibility decreases with increasing the system size. The overall behavior is very similar to the one observed numerically in a recent work by Sels and Polkovnikov [Phys. Rev. E 104, 054105 (2021)] for the normalized fidelity susceptibility in a disordered XXZ spin chain.