We map out the ground state phase diagram of the spin-$\frac{1}{2}$ isotropic Kekulé-Kitaev model on the honeycomb lattice in the presence of the Heisenberg exchange couplings. Our study relies on large-scale tensor network simulations based on a graph-based projected entangled pair state approach and simple tensor update in the thermodynamic limit. We find that, on top of the quantum spin liquid and conventional magnetically ordered phases which are typical of the Kitaev-Heisenberg model, the Kekulé-Heisenberg phase diagram hosts two plaquette valence bond solid (VBS) phases with vanishing magnetic order. While the VBS phases preserve the symmetries of the original Hamiltonian, they differ markedly from the Kitaev spin liquid by having decorated plaquette ordering which is distinguished by a plaquette order parameter.

The linear band touchings at one-dimensional lines or rings that characterize a nodal line semimetal phase have created much interest in research as they may find use in the next generation of low-dissipation electronics and spintronics. Here we demonstrated the presence of multiple scattering processes including electron-phonon $\left(e\text{−}p\right)$ and Mott's interband scattering within a single crystal of nickel selenide (NiSe) using temperature-dependent resistivity data fitted to Bloch-Gruneisen-Mott formula. Multiple scattering mechanisms were further validated from the observation of transverse magnetoresistance (MR) data that displayed a well scaled in extended Kohler's rule and a deviation from Kohler's rule. Temperature-dependent resistivity at different magnetic fields indicate topological semimetallic nature of NiSe. The observed negative magnetoconductivity data fitted to the Hikami-Larkin-Nagaoka equation describes weak antilocalization. The weak field magnetoconductivity follows the $-\ln \left(B\right)$ scaling behavior, indicating the topological nodal line feature. The density functional theory (DFT) calculations support the marginal domination of electron concentration over hole concentration. The experimental observation of nonlinear Hall resistivity with negative slope indicates that electrons are dominating. It is plausible that the finding of low MR is the result of the size of electron pockets is slightly larger than that of hole pockets. Five different types of nodal lines having $\pi$-Berry phase without spin orbit coupling (SOC) are presented as revealed from the DFT calculations: one infinite endless, one diamond-shaped, two with sixfold rotational symmetry, and one with threefold rotational symmetry. All nodal lines stack perpendicular to the crystallographic $c$ axis, with the exception of the diamond-shaped nodal line. The DFT calculations also manifest the two nodal lines which are still protected in the presence of SOC.

Strong dichroism of the outgoing x rays is demonstrated in $K_{\beta }$ x-ray emission. It is shown that while the integrated intensities are zero, the first moments of the dichroic spectra can be directly related to the orbital moments. The first moment is caused by the Coulomb interaction between the $3p$ core hole and the $3d$ valence electrons. Contributions from other interactions are negligibly small or zero. Calculations for a divalent cobalt ion in the presence of a Jahn-Teller distortion show strong dependencies of the dichroic $K_{\beta }$ emission on the angular momentum and the quadrupolar moment.

A bilayer quantum Hall system with a total filling factor of one was studied in the regime of a heavy layer imbalance in a recent transport experiment (Y. Zeng et al., arXiv:2306.16995), with intriguing new findings. We demonstrate in this paper that (1) the exciton Wigner crystal in this regime can melt into a superfluid phase, giving rise to reentrant superfluid behavior; and (2) in the presence of disorder, electron and hole Wigner crystals in the two layers go through a locking-decoupling transition as the layer separation increases, resulting in a sudden change in the counterflow conductance. A comparison will be made with the findings of the experiments.

The half-Heusler (HH) alloy ($\mathrm{N}{\mathrm{b}}_{1-z}\mathrm{CoSb}$) with a defective 19 valence electron configuration exhibits a short-range ordering of vacancies within its crystal structure, giving low electrical resistivity and thermal conductivity. In this paper, we investigate the thermoelectric properties of the Sn-substituted $\mathrm{N}{\mathrm{b}}_{0.83}\mathrm{CoSb}$ across a temperature range of 300–1123 K. The samples of $\mathrm{N}{\mathrm{b}}_{0.83}\mathrm{CoS}{\mathrm{b}}_{1-x}\mathrm{S}{\mathrm{n}}_x$ ($x=0$, 0.01, 0.03, and 0.05) are synthesized by vacuum arc melting followed by vacuum hot press. The x-ray diffraction analysis identified the main phase as the HH phase. The electron probe microanalyzer revealed a deficiency of Nb content from 0.83 to 0.80. The Sn substitution reduces the number of valence electrons, resulting in an increase in the Seebeck coefficient. Also, density functional theory calculations indicate the Nb vacancy displaces the Co atom from the original tetrahedral position toward Sb, which generates additional scattering channels near Brillouin zone edges, arising from the lifting of degeneracy. Additionally, Sn substitution in the Nb vacancy system restores the Co atom to its original position, effectively eliminating these scattering channels and enhancing the transport properties. The high power factor (PF) of $1.79\mathrm{mW}/\left(\mathrm{m}{\mathrm{K}}^2\right)$ at 1126 K is achieved by the $x=0.03$ sample, surpassing the $x=0$ sample PF $[1.53\mathrm{mW}/\left(\mathrm{m}{\mathrm{K}}^2\right)\mathrm{at}1126\mathrm{K}]$ attributed to the optimized carrier concentration and elimination of scattering channels after Sn substitution. The Debye-Callaway model, applied to lattice thermal conductivity data, identifies Umklapp and point defect scattering as significant phonon scattering mechanisms. Furthermore, our bonding analysis also reveals the presence of a bonding hierarchy, which reduces the lattice thermal conductivity, contributing to improved thermoelectric performance. Due to this low $\kappa$ and high PF, the $x=0.03$ sample achieves a $\mathit{zT}$ of 0.92 at 1126 K.

We investigate the electronic structure and transport properties of twisted bilayer graphene (TBLG) at a twist angle of $\theta \approx 1.{696}^{\circ }$. Using a combination of molecular dynamics and tight-binding calculations, we find two superlattice gaps in the energy spectrum of the bulk, which emerge close to the Fermi level from the atomic rearrangement of the carbon atoms leading to a corrugation of the graphene sheets. Nanoribbons made from $1.{696}^{\circ }$-TBLG show edge-localized states inside the superlattice gaps. Applying the Green's function method, we demonstrate that the edge states carry electronic current with conductance values close to the conductance quantum. The edge states can generate a nonlocal resistance, which is not due to one-way transport at the edges but due to the fact that these states are localized only at certain edges of the system, depending on how the nanoribbon has been cut from the bulk.

The Zhang-Rice exciton state in ${\mathrm{NiPS}}_3$, closely connected to the underlying zigzag antiferromagnetic order, is a representative case of intricate interaction among quasiparticles in van der Waals magnets. ${\mathrm{NiPS}}_3$ is usually described by a XXZ-type Hamiltonian below its Néel temperature of 155 K. The spin-orbit entangled Zhang-Rice exciton state of ${}^1{A}_{1\mathrm{g}}$ symmetry emerges at 1.4756 eV below $T_{\mathrm{N}}$ and becomes exceptionally prominent below about 50 K where the full width at half maximum reaches 0.4 meV, narrower than the thermal fluctuation $k_{\mathrm{B}}T$. Here, we report a second Zhang-Rice exciton state in ${\mathrm{NiPS}}_3$ observed at 1.0975 eV by conducting temperature-dependent high-resolution transmittance measurements on bulk ${\mathrm{NiPS}}_3$ single crystals over the frequency range 0.62–2 eV. We identify the second Zhang-Rice exciton state to result from the initial Zhang-Rice triplet ground state ${}^3{A}_{2\mathrm{g}}$ to the excited Zhang-Rice singlet state ${}^1{E}_{\mathrm{g}}$. In addition, the ${}^3{T}_{2\mathrm{g}}$ state, which overlapped with the ${}^1{E}_{\mathrm{g}}$ state, split to ${}^3{E}_{\mathrm{g}}$ and ${}^3{A}_{1\mathrm{g}}$ states as the temperature decreased, and the temperature dependence of their parameters changed below $T_{\mathrm{N}}$. Our results offer valuable information on the intricate electronic structure of ${\mathrm{NiPS}}_3$, where charge transfer and spin-orbit entanglement bring in an exciting arena to study many-body interactions.

We investigate and compare different approaches for handling the dynamical effects of electron-phonon coupling (EPC) on the superconducting properties of two-dimensional (2D) systems in the presence of plasmons. By calculating the Eliashberg function in terms of dynamically renormalized phonons, we discuss the approximations frequently used in the literature and compare them to a fully dynamical treatment of the screening effects. We emphasize the importance of consistent calculations of charge fluctuations that shape the superconducting properties, not only through the renormalization of phonon frequencies and damping rates but also through structural changes in the phonon spectral function. We find that part of the phonon spectral weight necessarily shifts to low frequencies owing to the coupling with the 2D gapless plasmon. Additionally, we find that the EPC leads to excess phonon spectral weight—i.e., phonon production—which generally tends to enhance the transition temperature.

We compare the various chirality measures most widely used in the literature to quantify chiral symmetry in extended solids, i.e., the continuous chirality measure and the Hausdorff distance. By studying these functions in an algebraically tractable case, we can evaluate their strengths and weaknesses when applied to more complex crystals. Going beyond those classical calculations, we propose a different method to quantify the handedness of a crystal based on a pseudoscalar function, i.e., the helicity during a soft phonon mode driven displacive phase transition from an achiral structure. This quantity, borrowed from hydrodynamics, can be computed from the eigenvector carrying the system from the high-symmetry nonchiral phase to the low-symmetry chiral phase. Different model systems like ${\mathrm{K}}_3{\mathrm{NiO}}_2$, ${\mathrm{CsCuCl}}_3$, and ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ are used as test cases where we show the superior interest of using helicity to quantify chirality in displacive chiral transitions together with the handedness distinction.

We present low-temperature specific heat ($C_{\mathrm{p}}$) measurements of a monoclinic $P{2}_1/c$ crystal formed by quasiplanar molecules of tetrachloro-$m$-xylene. The dynamic disorder frozen at low temperature of the asymmetric unit (formed by a half molecule) consists of reorientation around a threefold-like axis perpendicular to the benzene ring. Such a minimal disorder gives rise to typical glassy anomalies, as a linear contribution in $C_{\mathrm{p}}$ ascribed to two-level systems and a broad maximum around 6.6 K in $C_{\mathrm{p}}/T^3$ (the boson peak). We discuss these results in the framework of other quasiplanar molecular crystals with different accountable numbers of in-plane molecular orientations. We find that the density of two-level systems does not correlate with the degree of orientational disorder. Rather, it is the molecular asymmetry that seems to play a relevant role in the thermal anomalies. Furthermore, we discuss the suggested correlation between the boson peak ($T_{\mathrm{BP}}$) and Debye (${\mathrm{\Theta }}_D$) temperatures. We find that a linear correlation between $T_{\mathrm{BP}}$ and ${\mathrm{\Theta }}_D$ holds for many—but not all—structural glasses and strikingly holds even better for some disordered crystals, including our studied quasiplanar molecular crystals.

We report on a direct mapping of acoustic slender waveguides to the one-dimensional trimer Su-Schrieffer-Heeger model, with neither chiral nor mirror symmetry. Importantly, we can choose to perform this mapping for either the acoustic velocity or pressure. We demonstrate that, for finite systems, this choice is necessarily linked to the boundary conditions. It allows for the unveiling of the edge states of the acoustic system through a phase diagram. An experimental realization of our setup in the audible regime corroborates our theoretical predictions.

The Cu-doped lead-apatite samples reported recently take on the ferromagnetic behavior but the mechanism is unknown. Based on the first-principles calculations, we investigate the magnetic properties of pure and Cu-doped lead apatite with excessive oxygen and copper contents. Three kinds of ferromagnetic compounds are found, including ${\mathrm{Pb}}_{10}{\left({\mathrm{PO}}_4\right)}_6{\mathrm{O}}_2$, ${\mathrm{Pb}}_9\mathrm{Cu}{\left({\mathrm{PO}}_4\right)}_6{\mathrm{O}}_2$ with Cu in the $4f$ site, and ${\mathrm{Pb}}_8{\mathrm{Cu}}_2{\left({\mathrm{PO}}_4\right)}_6\mathrm{O}$ with Cu in the $4f$ site. In the first compound ferromagnetism is triggered by the extra O atom, while in the latter two compounds ferromagnetism is enhanced by the incorporation of extra oxygen or copper atoms. Because of the higher energy of ${\mathrm{Pb}}_9\mathrm{Cu}{\left({\mathrm{PO}}_4\right)}_6{\mathrm{O}}_2$ or ${\mathrm{Pb}}_8{\mathrm{Cu}}_2{\left({\mathrm{PO}}_4\right)}_6\mathrm{O}$ with Cu in the $4f$ site, there is a relatively low probability of them existing in the samples from the viewpoint of energy. Therefore, the ferromagnetic behaviors reported should mainly arise from the oxygen-overdoped lead-apatite ${\mathrm{Pb}}_{10}{\left({\mathrm{PO}}_4\right)}_6{\mathrm{O}}_2$.

The structural and magnetic properties of a spin-$\frac{1}{2}$ averievite ($\mathrm{CsBr}){\mathrm{Cu}}_5{\mathrm{V}}_2{\mathrm{O}}_{10}$ are investigated by means of temperature-dependent x-ray diffraction, magnetization, heat capacity, and ${}^{51}\mathrm{V}$ nuclear magnetic resonance (NMR) measurements. The crystal structure (trigonal, $P\overline{3}$) features a frustrated capped-kagome lattice of the magnetic ${\mathrm{Cu}}^{2+}$ ions. Magnetic susceptibility analysis indicates a large Curie-Weiss temperature of ${\theta }_{\mathrm{CW}}\simeq -175\mathrm{K}$. Heat capacity signals the onset of a magnetic long-range-order (LRO) at $T_{\mathrm{N}}\simeq 21.5\mathrm{K}$ at zero magnetic field due to the presence of significant inter-planer coupling in this system. The magnetic LRO below 27 K is further evident from the drastic change in the ${}^{51}\mathrm{V}$ NMR signal intensity and rapid enhancement in the ${}^{51}\mathrm{V}$ spin-lattice relaxation rate in a magnetic field of 6.3 T. The frustration index $f=|{\theta }_{\mathrm{CW}}|/T_{\mathrm{N}}\simeq 8$ ascertains strong magnetic frustration in this compound. From the high-temperature value of the ${}^{51}\mathrm{V}$ NMR spin-lattice relaxation rate, the leading antiferromagnetic exchange interaction between the ${\mathrm{Cu}}^{2+}$ ions is calculated to be $J/k_{\mathrm{B}}\simeq 136\mathrm{K}$.

Triangular lattice antiferromagnets are prototypes for frustrated magnetism and may potentially realize novel quantum magnetic states such as a quantum spin-liquid ground state. A recent work suggests ${\mathrm{NdTa}}_7{\mathrm{O}}_{19}$ with rare-earth triangular lattice is a quantum spin-liquid candidate and highlights the large family of rare-earth heptatantalates as a framework for quantum magnetism investigation. In this paper, we report the structural and magnetic characterization of ${\mathrm{CeTa}}_7{\mathrm{O}}_{19}$ and ${\mathrm{YbTa}}_7{\mathrm{O}}_{19}$. Both compounds are isostructural to ${\mathrm{NdTa}}_7{\mathrm{O}}_{19}$ with no detectable structural disorder. For ${\mathrm{CeTa}}_7{\mathrm{O}}_{19}$, the crystal field energy levels and parameters are determined by inelastic neutron scattering measurements. Based on the crystal field result, the magnetic susceptibility data could be well fitted and explained, which reveals that ${\mathrm{CeTa}}_7{\mathrm{O}}_{19}$ is a highly anisotropic Ising triangular-lattice antiferromagnet ($g_z/g_{xy}\sim 3$) with very weak exchange interaction ($\mathrm{J}\sim 0.22$ K). For ${\mathrm{YbTa}}_7{\mathrm{O}}_{19}$, millimeter-sized single crystals could be grown. The anisotropic magnetization and electron spin resonance data show that ${\mathrm{YbTa}}_7{\mathrm{O}}_{19}$ has a contrasting in-plane magnetic anisotropy with $g_z/g_{xy}\sim 0.67$ similar as that of ${\mathrm{YbMgGaO}}_4$. The above results indicate that ${\mathrm{CeTa}}_7{\mathrm{O}}_{19}$ and ${\mathrm{YbTa}}_7{\mathrm{O}}_{19}$ with pseudospin-1/2 ground states might either be quantum spin-liquid candidate materials or find applications in adiabatic demagnetization refrigeration due to the weak exchange interaction.

Many-body localized systems exhibit a unique characteristic of avoiding thermalization, primarily attributed to the presence of a local disorder potential in the Hamiltonian. In recent years there has been an interest in simulating these systems on quantum devices. However, actual quantum devices are subject to unavoidable decoherence that can be modeled as coupling to a bath or continuous measurements. The quantum Zeno effect is also known to inhibit thermalization in a quantum system, where repeated measurements suppress transport. In this work we study the interplay of many-body localization and the many-body quantum Zeno effect. In a prethermal regime, we find that signatures of many-body localization are enhanced when the system is coupled to a bath that contains measurements of local fermion population, subject to the appropriate choice of system and bath parameters.

The scaling of magnetic memory into nanometer size calls for a theoretical model to accurately predict the switching current. Previous models show a large discrepancy with experiments in studying the spin-orbit torque switching of the perpendicular magnet. In this work, we observed that the trajectory of magnetization shows a smooth transition during the switching. This contradicts the magnetization dynamics proposed in previous models, which suggest that magnetization first aligns with the spin polarization (σ) before switching can occur. We demonstrate that aligning magnetization to σ requires a very large current, resulting in the unsatisfactory fitting between the previous models and experiments. In contrast, the smooth transition permits a lower switching current. Guided by this refined physical picture, we pinpoint the reversal of precession chirality as a pivotal factor for achieving deterministic switching. This insight contributes to the development of an analytical model that shows good alignment with the experimental data. We offer a clearer understanding of the current-induced magnetization switching process, which will benefit the advancements in spin-orbit torque magnetic random-access memory.

Slow dynamics have been experimentally studied in various spin systems, but most of the studies so far have been conducted on the systems with spin frustration. In the present study, the dynamics of magnetization in a series of ${\mathrm{La}}_5{\mathrm{Mo}}_{4-x}{\mathrm{Mn}}_x{\mathrm{O}}_{16}$ have been investigated, in which the spins on quasi-square lattices have extremely large two-dimensional anisotropy in the magnetic interactions. It was found that slow dynamics are dominated by a nucleation-growth process with interfacial energy caused by in-plane magnetic interactions. A detailed analysis suggests the formation of domain walls at the interface.

Josephson scanning tunneling microscopy (JSTM) is a powerful probe of the local superconducting order parameter, but studies have been largely limited to cases where the superconducting sample and superconducting tip both have the same gap symmetry—either $s$-wave or $d$-wave. It has been generally assumed that, in an ideal $s$-to-$d$ JSTM experiment, the critical current would vanish everywhere, as expected for ideal $c$-axis planar junctions. We show here that this is not the case. Employing first-principles Wannier functions for ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$, we develop a scheme to compute the Josephson critical current ($I_c$) and quasiparticle tunneling current measured by JSTM with subangstrom resolution. We demonstrate that the critical current for tunneling between an $s$-wave tip and a superconducting cuprate sample has the largest magnitude above O sites and it vanishes above Cu sites. $I_c$ changes sign under $\pi /2$ rotation and its average over a unit cell vanishes, as a direct consequence of the $d$-wave gap symmetry in cuprates. Further, we show that $I_c$ is strongly suppressed in the close vicinity of a Zn-like impurity owing to suppression of the superconducting order parameter. More interestingly, $I_c$ acquires nonvanishing values above the Cu sites near the impurity. The critical current modulations produced by the impurity occur at characteristic wave vectors distinct from the quasiparticle interference (QPI) analog. Furthermore, the quasiparticle tunneling spectra in the JSTM setup shows coherence peaks and impurity-induced resonances shifted by the $s$-wave tip gap. We discuss the similarities and differences in JSTM observables and conventional STM observables, making specific predictions that can be tested in future JSTM experiments.

Our study delves into the modifications observed in Vavilov-Cherenkov radiation when its generating charged particle moves parallel to an interface formed by two generic magnetoelectric media, focusing on topological insulators. We compute the electromagnetic field through the Green's function. Applying the far-field approximation and the steepest descent method, we derive analytical expressions for the electric field, revealing contributions from spherical and lateral waves with topological origins. Subsequently, we analyze the angular distribution of the radiation, particularly focusing on parallel motions in close proximity to the interface. Our findings indicate that the radiation along the Vavilov-Cherenkov cone is inhomogeneous and asymmetric. We analyze the radiated energy at both sides of the interface. Finally, we discuss the particle's retarding force, which is notably enhanced in the ultrarelativistic regime. We illustrate these results for the topological insulator ${\mathrm{TlBiSe}}_2$ and the magnetoelectric ${\mathrm{TbPO}}_4$.