Within many-body perturbation theory, we calculate band offsets for a set of epitaxial interfaces, including AlP/GaP, AlAs/GaAs, Ge/AlAs, Ge/GaAs, Ge/ZnSe, Si/GaP, ZnSe/GaAs, and ${\mathrm{CaF}}_2/\mathrm{Si}$. We consider various quasiparticle self-consistent $GW$ schemes with or without including vertex functions. In particular, we consider two types of effective vertex functions complying with the Ward identity in the long range, one of which additionally carries a short-range part, which has been found to improve ionization potentials. The obtained band offsets correspond to model interface structures that match the experimental lattice parameters of the bulk components. Strain, zero-phonon renormalization, and spin-orbit coupling effects are properly accounted for. For the band offsets of the semiconductor-semiconductor interfaces, all the self-consistent $GW$ schemes yield similar mean absolute errors on the order of 0.2 eV. In the case of the ${\mathrm{CaF}}_2/\mathrm{Si}$ interface, the calculated band offsets show large indetermination spanning an interval up to 1 eV, the discrepancy with respect to experiment being correlated with the error by which the band gap of the insulator is described. Through $GW$ calculations for selected interface models, we further assess the effect of self-consistently updating the charge density. Our result support the practice of relying on semilocal or hybrid-functional schemes for determining the line-up potential.

The levitation of a metallic nanoflake has been experimentally demonstrated in liquid by manipulating the Casimir force with a proper choice of materials, and as a step further, dynamic control of the Casimir force acting on a nanoflake is of great importance. We show that the Casimir force acting on a nanoflake in ethanol can be dynamically controlled by adjusting the chemical potential of a graphene layer sandwiched by the Teflon film and the $\mathrm{Si}{\mathrm{O}}_2$ substrate at a distance away from the nanoflake, where the nanoflake can consist of either monolayer graphene or solid material. We develop a harmonic oscillator model that captures the dynamics of the nanoflake observed in numerical calculations. The time constant in the dynamics for a graphene nanoflake is 3 to 6 times longer than that for a gold nanoflake. The upper limit of the zero-force position of the nanoflake is discussed. Our scheme, through the use of the tunable graphene layer, significantly extends the degrees of freedom for dynamic control of nanoflakes in liquid.

Understanding the nonlinear polarization mechanism of two-dimensional (2D) halide perovskites is essential for developing various nonlinear optical applications. Herein, the terahertz (THz) emission from Ruddlesden-Popper type ${\left(\mathrm{BA}\right)}_2\mathrm{Pb}{\mathrm{I}}_4$ and ${\left(\mathrm{iso}-\mathrm{BA}\right)}_2\mathrm{Pb}{\mathrm{I}}_4$ are investigated by both above-band-gap (400 nm) and below-band-gap (800 nm) femtosecond laser excitation. Under above-band-gap excitation, the dominant mechanism is the linear photogalvanic effect instead of the common photo-Dember effect and surface depletion field effect, suggesting the carrier transport between adjacent conductor layers is forbidden by the organic layers. Under below-band-gap excitation, the THz emission from orthorhombic ${\left(\mathrm{BA}\right)}_2\mathrm{Pb}{\mathrm{I}}_4$ exhibits a remarkable quadruple rotational symmetry on the azimuthal angle, indicating that the nonlinear polarization is driven by a higher-order electric quadrupole term instead of the bulk electric dipole term, which is forbidden by the centrosymmetric structure. In comparison, the low symmetry of THz emission on the azimuthal angle of monoclinic ${\left(\mathrm{iso}-\mathrm{BA}\right)}_2\mathrm{Pb}{\mathrm{I}}_4$ is attributed to the influence of symmetry breaking in the surface region, which raises a surface electric dipole contribution. In this paper, we reveal THz emission spectroscopy as a powerful contactless technique in investigating the nonlinear polarization of advanced materials.

We present the dynamical spin structure factor of the antiferromagnetic spin-$\frac{1}{2} J_1\text{−}{J}_2$ Heisenberg model on a triangular lattice obtained from large-scale matrix-product state simulations. The high frustration due to the combination of antiferromagnetic nearest- and next-nearest-neighbor interactions yields a rich phase diagram. We resolve the low-energy excitations both in the ${120}^{\circ }$ ordered phase and in the putative spin-liquid phase at $J_2/J_1=0.125$. In the ordered phase, we observe an avoided decay of the lowest magnon branch, demonstrating the robustness of this phenomenon in the presence of gapless excitations. Our findings in the spin-liquid phase chime with the field-theoretical predictions for a gapless Dirac spin liquid, in particular the picture of low-lying monopole excitations at the corners of the Brillouin zone. We comment on possible practical difficulties of distinguishing proximate liquid and solid phases based on the dynamical structure factor.

By expanding the gauge ${\lambda }_n\left(\mathbf{k}\right)$ for magnon band $n$ in harmonics of momentum $\mathbf{k}=(k,\varphi )$, we demonstrate that the only observable component of the magnon orbital angular momentum $O_n\left(\mathbf{k}\right)$ is its angular average over all angles $\varphi$, denoted by $F_n\left(k\right)$. Although $F_n\left(k\right)$ vanishes for antiferromagnetic honeycomb and zigzag ($0<J_1<J_2$) lattices, it is nonzero for the ferromagnetic (FM) versions of those lattices in the presence of Dzyaloshinskii-Moriya interactions. For a FM zigzag model with equal exchange interactions $J_{1x}$ and $J_{1y}$ along the $x$ and $y$ axes, the magnon bands are degenerate along the boundaries of the Brillouin zone with $k_x-k_y=\pm \pi /a$ and the Chern numbers $C_n$ are not well defined. However, a revised model with $J_{1y}\ne J_{1x}$ lifts those degeneracies and produces well-defined Chern numbers of $C_n=\pm 1$ for the two magnon bands. When $J_{1y}=J_{1x}$, the thermal conductivity ${\kappa }^{xy}\left(T\right)$ of the FM zigzag lattice is largest for $J_2/J_1>6$ but is still about four times smaller than that of the FM honeycomb lattice at high temperatures. Due to the removal of band degeneracies, ${\kappa }^{xy}\left(T\right)$ is slightly enhanced when $J_{1y}\ne J_{1x}$.

In recent years, there has been a fervent search for room-temperature superconductivity within the binary hydrides. However, as the number of untested compounds dwindled, it became natural to begin searching within the ternary hydrides. This led to the controversial discovery of room-temperature superconductivity at only 1 GPa in nitrogen-doped lutetium hydride [Dasenbrock-Gammon et al., Nature (London) 615, 244 (2023)] and consequently provided much impetus for the synthesis of nitrogen-based ternary hydrides. Here, we report the synthesis of stable trigonal ${\mathrm{LuH}}_3$ by hydrogenating pure lutetium which was subsequently pressurized to $\sim 2$ GPa in a dilute-${\mathrm{N}}_2$/He-rich pressure medium. Raman spectroscopy and x-ray diffraction were used to characterize the structures throughout. After depressurizing, energy-dispersive and wavelength-dispersive x-ray spectroscopies characterized the final compound. Though our compound under pressure exhibits similar structural behavior to the Dasenbrock-Gammon et al. sample, we do not observe any nitrogen within the structure of the recovered sample at ambient pressure. We observe two cubic structures under pressure that simultaneously explain the x-ray diffraction and Raman spectra observed: The first corresponds well to $Fm\overline{3}m {\mathrm{LuH}}_{2+x}$, while the latter is an $Ia\overline{3}$-type structure.

Transonic defect motion is of interest for high strain-rate plastic deformation as well as for crack propagation. Ever since Eshelby's 1949 prediction [J. D. Eshelby, Proc. Phys. Soc. A 62, 307 (1949)] in the isotropic limit of a “radiation-free” transonic velocity $v_{\text{RF}}=\sqrt{2}{c}_{\text{T}}$, where shock waves are absent, there has been speculation about the significance of radiation-free velocities (if they truly exist) for defect mobility. Here, we argue that they do not play any significant role in dislocation dynamics in metals, based on comparing theoretical predictions of radiation-free velocities for transonic edge dislocations with molecular dynamics simulations for two face-centered cubic metals: Ag, where theory predicts radiation-free states, and Cu, where it does not.

Intrinsically low lattice thermal conductivity ${\kappa }_{\mathrm{L}}$ in halide perovskites is of great interest for energy conversion applications. Here, based on first-principles calculations, we systematically study the lattice thermal conductivity of the recently synthesized layered perovskite ${\mathrm{Cs}}_3{\mathrm{Bi}}_2{\mathrm{I}}_6{\mathrm{Cl}}_3$. By using renormalized force constants extracted from lattice dynamics, our calculated ${\kappa }_{\mathrm{L}}$ is 0.227 and 0.130 ${\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ along the in-plane and cross-plane directions at 300 K, respectively, which agrees well with the experimental values (0.223 and 0.209 ${\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ parallel and perpendicular to the Bridgman growth direction). Meanwhile, ${\kappa }_{\mathrm{L}}$ follows a nonstandard ${\kappa }_{\mathrm{L}}\propto T^{-0.237}$ dependence on heating, originating from the dual particle-wave behavior of heat-carrying phonons where wavelike tunneling dominates $>72$% of the contribution to the total ${\kappa }_{\mathrm{L}}$ when $T>$ 300 K. Further analyses imply that ${\mathrm{Cs}}_3{\mathrm{Bi}}_2{\mathrm{I}}_6{\mathrm{Cl}}_3$ manifests the coexistence of metavalent bonding, loosely bonded rattling atoms with thermally induced large-amplitude vibrations, and stereochemical lone pair activity, which induces strong anharmonicity with the soft low-lying modes, causes a mixed crystalline-liquid state, and, finally, produces unexpectedly glassy thermal conductivity. Our work pinpoints the microscopic origin of ultralow ${\kappa }_{\mathrm{L}}$ in ${\mathrm{Cs}}_3{\mathrm{Bi}}_2{\mathrm{I}}_6{\mathrm{Cl}}_3$, which is important for designing efficient materials in halide perovskites for energy conversion.

Frustration is a ubiquitous phenomenon in many-body physics that influences the nature of the system in a profound way with exotic emergent behavior. Despite its long research history, the analytical or numerical investigations on frustrated spin models remain a formidable challenge due to their extensive ground-state degeneracy. In this paper, we propose a unified tensor network theory to numerically solve the frustrated classical spin models on various two-dimensional (2D) lattice geometry with high efficiency. We show that the appropriate encoding of emergent degrees of freedom in each local tensor is of crucial importance in the construction of the infinite tensor network representation of the partition function. The frustrations are thus relieved through the effective interactions between emergent local degrees of freedom. Then the partition function is written as a product of a one-dimensional (1D) transfer operator, whose eigenequation can be solved by the standard algorithm of matrix product states rigorously, and various phase transitions can be accurately determined from the singularities of the entanglement entropy of the 1D quantum correspondence. We demonstrated the power of our general theory by numerically solving 2D fully frustrated XY spin models on the kagome, square, and triangular lattices, giving rise to a variety of thermal phase transitions from infinite-order Brezinskii-Kosterlitz-Thouless transitions, second-order transitions, to first-order phase transitions. Our approach holds the potential application to other types of frustrated classical systems like Heisenberg spin antiferromagnets.

We present isothermal ferroelectric (FE) and quasiadiabatic phase diagrams for single-crystal ${\mathrm{Co}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ with magnetic fields ($H$) along the $c$ axis, obtained through magnetization, polarization ($P$), and magnetocaloric effect measurements under pulsed magnetic fields up to 30 T. The FE diagram reveals two FE phases, denoted as FE-I and FE-II, with overlapping boundaries at low temperatures ($T$). At 2 K, the $P$ signal along $c$ exhibits a positive value in FE-I, transitioning to a negative value in FE-II with increasing $H$. During the descending-$H$ process, $P$ reverses, with FE-II displaying positive polarization and FE-I displaying negative, indicative of dynamic ferroelectric behavior. Above 3 K, a paraelectric (PE) phase emerges, effectively separating the two FE phases. This PE phase correlates with the 1/2 plateau observed in the magnetization curve. Simultaneously, as $T$ increases, the negative $P$ signals of FE-I and FE-II sequentially reverse to positive, leading to the gradual diminishment of the signal reversal of $P$ along the $c$ axis due to the history of $H$ sweeping. The magnetic Grüneisen parameter diverges near the field on both sides of the 1/2 plateau, providing evidence of multiferroic quantum criticality. Employing symmetry analysis, we propose a chiral “water-strider” type spin configuration to elucidate the origin of ferroelectricity and the magnetization process. The high-field FE phase may be associated with the quintuplet excitation of the Co1 dimer in the presence of magnetic fields.

The formation of a heavy Fermi liquid in metals with local moments is characterized by multiple energy and temperature scales, most prominently the Kondo temperature and the coherence temperature, characterizing the onset of Kondo screening and the emergence of Fermi-liquid coherence, respectively. In the standard setting of a wide conduction band, both scales depend exponentially on the Kondo coupling. Here we discuss how the presence of flat, i.e., dispersionless, conduction bands modifies this situation. Focussing on the case of the kagome Kondo lattice model, we utilize a parton mean-field approach to determine the Kondo temperature and the coherence temperature as a function of the conduction-band filling $n_c$, both numerically and analytically. For $n_c$ values corresponding to the flat conduction band located at the Fermi level, we show that the exponential is replaced by a linear dependence for the Kondo temperature and a quadratic dependence for the coherence temperature, while a cubic law emerges for the coherence temperature at $n_c$ corresponding to the band edge between the flat and dispersive bands. We discuss the implications of our results for pertinent experimental data.

We report the physical properties of GdAuGe single crystals, which were grown using Bi flux. The powder x-ray diffraction data shows that the compound crystallizes in hexagonal NdPtSb-type structure (space group $P{6}_{3}mc$). Magnetization measurements performed for field configuration $H\parallel c$ and $H\perp c$ show that GdAuGe orders antiferromagnetically at the Néel temperature, $T_N=17.2\mathrm{K}$. Around this temperature, heat capacity and electrical resistivity data exhibit prominent anomaly due to the antiferromagnetic (AFM) transition. In addition to an AFM phase transition, the magnetization data for $H\parallel c$ display the signature of field-induced metamagnetic (MM) transitions below $T_N$. The critical field range for these transitions vary from 0.2 to 6.2 T. The critical fields for the MM transitions decrease with increasing temperature and approach to zero value for temperature approaching $T_N$. For instance, in high-field MM transition, critical field changes from 6.2 T at 1.7 K to 1.8 T at 16 K. Interestingly, the magnetoresistance (MR) data (for $H\parallel c$) record a sharp increase in values at the critical fields that coincide with those seen in magnetization data, tracking the presence of MM transitions. MR is positive and large ($\approx 169$% at 9 T and 2 K) at low temperatures. Above $T_N$, MR becomes small and switches to negative values. Hall resistivity data reveal the predominance of hole charge carriers in the system. In addition, we observe an emergence of step-like feature in the Hall resistivity data within the field range of second MM, and a significantly large anomalous Hall conductivity of $\approx 1270{\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$ at 2 K. The $H\text{−}T$ phase diagram constructed from our detailed magnetization and magnetotransport measurements reveals multiple intricate magnetic phase transitions. The electronic and magnetic structure of GdAuGe are also thoroughly investigated using first-principles methods. The electronic band structure calculations reveal that GdAuGe is a Dirac nodal-line semimetal.

We analyze potential nonanalytic terms in the Landau diamagnetic susceptibility, ${\chi }_{\text{dia}}$, at a finite temperature $T$ and/or in-plane magnetic field $H$ in a two-dimensional (2D) Fermi liquid. To do this, we express the diamagnetic susceptibility as ${\chi }_{\text{dia}}={(e/c)}^2{lim}_{Q\to 0}{\mathrm{\Pi }}_{\perp }^{\text{JJ}}\left(Q\right)/Q^2$, where ${\mathrm{\Pi }}_{\perp }^{\text{JJ}}$ is the transverse component of the static current-current correlator, and evaluate ${\mathrm{\Pi }}_{\perp }^{\text{JJ}}\left(Q\right)$ for a system of fermions with Hubbard interaction to second order in Hubbard $U$ by combining self-energy, Maki-Thompson, and Aslamazov-Larkin diagrams. We find that at $T=H=0$, the expansion of ${\mathrm{\Pi }}_{\perp }^{\text{JJ}}\left(Q\right)/Q^2$ in $U$ is regular, but at a finite $T$ and/or $H$, it contains $U^{2}T$ and/or $U^2\left|H\right|$ terms. Similar terms have been previously found for the paramagnetic Pauli susceptibility. We obtain the full expression for the nonanalytic $\delta {\chi }_{\text{dia}}(H,T)$ when both $T$ and $H$ are finite, and we show that the $H/T$ dependence is similar to that for the Pauli susceptibility.

The three-dimensional Weyl semimetal ${\mathrm{Mn}}_3\mathrm{Sn}$ has attracted tremendous attention due to its great application potential. However, the complex magnetic structures at different temperature intervals make it extremely difficult to unravel the underlying electronic structures of ${\mathrm{Mn}}_3\mathrm{Sn}$. Here, we perform temperature-dependent optical spectroscopy measurements on single-crystalline ${\mathrm{Mn}}_3\mathrm{Sn}$ to investigate its charge dynamics. We find that both the optical reflectivity $R\left(\omega \right)$ and conductivity ${\sigma }_1\left(\omega \right)$ evolve very smoothly across the magnetic phase transition at $T_M=285$ K, where the giant anomalous Hall effect (AHE) at room temperature drops significantly. Furthermore, linearly increasing segments of ${\sigma }_1\left(\omega \right)$ are observed in the whole temperature range from 300 K to 10 K, indicating that the existence of Weyl fermions is very robust against the magnetic phase transition. In addition, the Weyl points closest to the Fermi level $E_F$ are identified to be located about 101 meV away from $E_F$ at 10 K, and the associated Fermi velocity is about $2.50\times {10}^7$ cm/s. Our results reveal that the phase transition at $T_M$ only generates subtle modification to the band structure, which helps to further uncover the mechanism of the dramatic change of AHE in ${\mathrm{Mn}}_3\mathrm{Sn}$.

I established the criteria and obtained analytical results for the pseudogap at the Van Hove (antinodal) point on the Fermi surface in two dimensions. The original criterion $\xi \gg {\xi }_{th\_db}=v_F/\pi T$ is not applicable in this case since Fermi velocity $v_F=0$. It turns out that the characteristic length for the pseudogap crossover at the Van Hove point ${\xi }_{th\_vh}\propto 1/T^{1/2}$ is significantly shorter than the one at the regular Fermi surface points ${\xi }_{th\_db}\propto 1/T$. In particular, ${\xi }_{th\_vh}$ is between one and two lattice spacings in the intermediate interaction regime of the Hubbard model. I have also identified the regime where there is still a single maximum in the spectral function, but single particle properties are abnormal. Specifically, the imaginary part of the self-energy has a minimum instead of maximum at the Fermi level and the slope of the real part of the self-energy is positive instead of negative. The important advantages of an analytical approach is that it provides results in both the Matsubara frequency representation and in the real frequencies representation. I compare Matsubara frequency results with the exact numerical results of the Monte Carlo methods in the Hubbard model and then show what they correspond to in the real frequency self-energy and spectral function.

We explore the role of interfacial antiferromagnetic interaction in coupled soft and hard ferromagnetic layers to ascribe the complex variety of magnetotransport phenomena observed in ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_3/{\text{SrRuO}}_3$ (LSMO/SRO) superlattices (SLs) within a one-band double-exchange model using Monte Carlo simulations. Our model Hamiltonian incorporates magnetocrystalline anisotropy interactions and superexchange interactions of the constituent materials, and two types of antiferromagnetic interactions between Mn and Ru ions at the interface: (i) carrier-driven and (ii) Mn–O–Ru bond superexchange to investigate the properties along hysteresis loop. We find that the antiferromagnetic coupling at the interface induces LSMO and SRO layers to align in antiparallel orientation at low temperatures. Our results reproduce the positive exchange bias of the minor hysteresis loop of LSMO/SRO SL at low temperatures, as reported in experiments. In addition, conductivity calculations show that the carrier-driven antiferromagnetic coupling between the two ferromagnetic layers steers the SL towards a metallic (insulating) state when LSMO and SRO are aligned in antiparallel (parallel) configuration, in good agreement with the experimental data. This demonstrates the necessity of carrier-driven antiferromagnetic interactions at the interface to understand the one-to-one correlation between magnetic and transport properties observed in experiments. For high temperature, just below the ferromagnetic $T_C$ of SRO, we unveil the unconventional three-step flipping process along the magnetic hysteresis loop. We emphasize the key role of interfacial antiferromagnetic coupling between LSMO and SRO to understand these multiple-step flipping processes along the hysteresis loop.

Establishing the underlying links between the diverse landscape of theoretical frameworks for simulating strongly correlated matter is crucial for advancing our understanding of these systems. In this work, we focus on the ghost Gutzwiller approximation (gGA), an extension of the Gutzwiller approximation (GA) based on the variational principle. We derive a framework called “ghost density matrix embedding theory” (gDMET) from quantum embedding (QE) principles similar to those in density matrix embedding theory (DMET), which reproduces the gGA equations for multiorbital Hubbard models with a simpler implementation. This derivation highlights the crucial role of the ghost degrees of freedom, not only as an extension to the GA, but also as the key element in establishing a consistent conceptual connection between DMET and the gGA. This connection further elucidates how gGA overcomes the systematic accuracy limitations of standard GA and achieves results comparable to dynamical mean field theory. Furthermore, it offers an alternative interpretation of the gGA equations, fostering new ideas and generalizations.

Motivated by the wealth of proposals and realizations of nontrivial topological phases in ${\mathrm{EuCd}}_2{\mathrm{As}}_2$, such as a Weyl semimetallic state and the recently controversially discussed semimetallic versus semiconductor behavior in this system, we analyze in this work the role of the delicate interplay of Eu magnetism, strain, and pressure on the realization of such phases. For that we invoke a combination of a group theoretical analysis with ab initio density functional theory calculations and uncover a rich phase diagram with various nontrivial topological phases beyond a Weyl semimetallic state, such as axion and topological crystalline insulating phases, and discuss their realization.