Quantum oscillations in both linear and Hall resistivities and weak antilocalization (WAL) are barely observed in bulk single crystals. Here we report the transport properties of a CaCuSb single crystal that crystallizes in the hexagonal crystal structure. The magnetotransport studies reveal WAL and $\mathrm{Shubnikov}-\mathrm{de}$ Haas (SdH) quantum oscillations with a unique frequency at 314 T. A cusplike behavior in the low-field regime of magnetotransport for $J\parallel \left(ab\right)$-plane and $B\parallel \left[0001\right]$ confirms the WAL in CaCuSb. Angular-dependent normalized magnetoconductance and SdH oscillations studies reveal that the observed phenomena originate from the $2\mathrm{D}$ transport channels. The high magnetic-field (up to 45 T) experiments demonstrate plateaulike features in the Hall measurements. The first-principles calculations unfold that CaCuSb is a nontopological semimetal with dominant hole carriers at the Fermi level. Our study reveals that CaCuSb is a promising candidate to explore the quasi-2D quantum transport phenomenon in the transition metal pnictide materials.

The archetypical heavy-fermion superconductor $\mathrm{Ce}{\mathrm{Cu}}_2{\mathrm{Si}}_2$ is known to present two distinct superconducting phases under pressure. In the low-pressure region, the superconductivity is mediated by spin fluctuations while the superconducting phase observed in the high-pressure region could be associated with a first-order valence transition (FOVT). However, the critical end point (CEP) of the FOVT was shown to be located at negative temperature and only a continuous valence change (crossover regime) was so far observed at 14 K, i.e., far above the optimal superconducting temperature $\left(T_c=2.3\mathrm{K}\right)$. Here we present x-ray absorption measurements under pressure and applied magnetic field at the Ce $L_3$ edge at 2.7 K, i.e., close to the optimal $T_{\mathrm{c}}$. It was expected that the applied magnetic field could shift the CEP to positive temperature with the possibility to observe a FOVT. Our data indicate the valence of Ce increases continuously (crossover regime) from 3.11 at ambient pressure up to 3.20 at 8.5 GPa likewise for any applied magnetic field up to 6 T.

Ferroquadrupole order of local atomic orbitals provides a specific realization of electronic nematic order. ${\mathrm{TmVO}}_4$ is an insulator and undergoes ferroquadrupolar order associated with the local Tm $4f$ orbitals at $T_Q=2.15$ K. The material is a model system to study nematic order and the roles played by nematic fluctuations. Here we present ${}^{51}\mathrm{V}$ nuclear magnetic resonance data as a function of field orientation in a single crystal. Although the spectra are well understood in terms of direct dipolar hyperfine couplings, the spin-lattice relaxation rate exhibits strong anisotropy that cannot be understood in terms of magnetic fluctuations. We find that the spin-lattice relaxation rate scales with the shear elastic constant associated with the ferroquadrupole phase transition, suggesting that quadrupole (nematic) fluctuations dominate the spin-lattice relaxation for in-plane fields.

Axion electrodynamics in condensed matter could emerge from the formation of charge density waves (CDWs) in Weyl semimetals. ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_2\mathrm{I}$ was proposed to be the first material platform for realizing axionic CDW that may host topological defects with one-dimensional in-gap modes. The real-space modulation of the CDW phase and the existence of in-gap modes remain elusive. Here, we present a comprehensive scanning tunneling microscopic and spectroscopic study of a CDW on a ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_2\mathrm{I}$ (110) surface. The tunneling spectroscopic measurements reveal a CDW gap of $\sim 200$ meV, in good agreement with prior studies, while the spectroscopy of CDW edge dislocation shows no indication of in-gap states. The bias-dependent scanning tunneling microscopy images indicate that the CDW in ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_2\mathrm{I}$ is dominated by a large periodic lattice distortion instead of charge modulation, suggesting a non-Peierls mechanism of the CDW instability.

Praseodymium-based 1-2-20 cage compounds $\mathrm{Pr}{T}_2{X}_{20}$ ($T$ is generally Ti, V, Nb, Ru, Rh, Ir; and $X$ is either Al, Zn, or Cd) provide yet another platform to study nontrivial electronic states of matter ranging from topological and magnetic orders to unconventional multipolar orders and superconductivity. In this paper, we report measurements of the electronic heat capacity in two Pr-based 1-2-20 materials: ${\mathrm{PrNi}}_2{\mathrm{Cd}}_{20}$ and ${\mathrm{PrPd}}_2{\mathrm{Cd}}_{20}$. We find that the lowest-energy multiplet of the Pr $4f^2$ valence configuration is a ${\mathrm{\Gamma }}_3$ non-Kramers doublet and the first excited triplet is assumed to be a magnetic ${\mathrm{\Gamma }}_5$. By analyzing the dependence of the energy splitting between the ground and first excited singlet states on an external magnetic field, we found that the maximum in the heat capacity corresponding to the Schottky anomaly in ${\mathrm{PrPd}}_2{\mathrm{Cd}}_{20}$, unlike ${\mathrm{PrNi}}_2{\mathrm{Cd}}_{20}$, shows a pronounced linear dependence on an external magnetic field at higher field values. This effect is associated with the exchange interactions between the field-induced magnetic dipole moments.

Topological Weyl semimetals (TWSs) are exotic crystals possessing emergent relativistic Weyl fermions connected by unique surface Fermi arcs (SFAs) in their electronic structures. To realize the TWS state, certain symmetries (such as the inversion or time reversal symmetry) must be broken, leading to a topological phase transition (TPT). Despite the great importance in understanding the formation of TWSs and their unusual properties, direct observation of such a TPT has been challenging. Here, using a recently discovered magnetic TWS ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$, we were able to systematically study its TPT with detailed temperature dependence of the electronic structures by angle-resolved photoemission spectroscopy. The TPT with drastic band structure evolution was clearly observed across the Curie temperature $(T_{\mathrm{C}}=177\mathrm{K})$, including the disappearance of the characteristic SFAs and the recombination of the spin-split bands that leads to the annihilation of Weyl points with opposite chirality. These results not only reveal important insights on the interplay between the magnetism and band topology in TWSs, but also provide a method to control their exotic physical properties.

Our systematic studies on the topological aspects of plasmons of Weyl semimetals in the presence of a magnetic field have uncovered two types of three-dimensional (3D) topological plasmons: the bulk plasmons and the Fermi-arc plasmons over opposite surfaces. As a consequence of the bulk-boundary correspondence, there are unidirectional surface/edge plasmons whose directions and dispersions can be controlled by the external field. These chiral surface plasmons possess momentum-location lock and strong confinement of electromagnetic (EM) fields. The anomalous Hall conductivity brings about abundant anisotropic plasmon dispersions, from linear to parabolic or even hyperbolic bands. In addition, a semiclassical picture of electron motion is proposed to show the formation of Weyl orbits and the consequent effect on plasmons. Our work thus provides instructive insights into the electron dynamics and collective excitations of Weyl semimetals and suggests it is a good seed for 3D topological plasmonics.

Duality places an important constraint on the renormalization group flows and the phase diagrams. For self-dual theories, the self-duality can be promoted as a symmetry, this leads to the multicriticalities. This work investigates a description of the deconfined quantum criticality, the $N_f=2{\mathrm{QED}}_3$, as an example of self-dual theories, and its multicritical behavior under perturbative deformations. The multicriticality is described by the theory with Gross-Neveu couplings and falls in a different universality class than the standard deconfined quantum criticality. We systematically calculate the scaling dimensions of various operators in the 3D quantum electrodynamics with the Chern-Simons term and Gross-Neveu couplings by the large-$N$ renormalization group analysis. Specifically, we find certain Drelativistic four-fermion interactions which correspond to the dimer-dimer interactions in the lattice model will drive the deconfined quantum criticality to the first-order transition, this result is consistent with previous numerical studies.

Capacitance-voltage measurements on self-assembled quantum dot layers exposed to strong electric fields and with large distances to the reservoirs show a marked hysteretic behavior. It is shown that at low temperatures this hysteresis can be explained quantitatively in terms of state-dependent capture and emission rates that are obtained by a rate equation model, applied to the measured capacitance transients. The occupation dynamics and the steady-state configuration can be extracted from these data via a Markov chain model.

The fractional Josephson effect is known to be a characteristic phenomenon of topological Josephson junctions hosting Majorana zero modes (MZMs), where the Josephson current has a $4\pi$ (rather than a $2\pi$) periodicity in the phase difference between the two topological superconductors. We introduce a one-dimensional model of a topological superconductor/normal-metal/superconductor (SNS) junction with the normal-metal (N) region of finite length, which is intermediate regime between the short- and long-junction limits. Assuming weak tunneling at the SN interfaces, we investigate resonance and finite-size effects on the fractional Josephson effect due to the existence of several discrete energy levels in the N region in which wave functions have oscillating nodal structure. Through careful analysis of the sign change in the transmission amplitudes through the junction and the fermion parity of the two MZMs, we find that the fractional Josephson current is proportional to the parity of total fermion numbers including both filled normal levels and two MZMs. Furthermore, we elucidate drastic enhancement of the Josephson current due to the resonance between a discrete level in the N region and MZMs.

We studied the effect of strain engineering on the electronic, structural, mechanical, and optical properties of orthorhombic diboron dinitride (o-${\mathrm{B}}_2{\mathrm{N}}_2$) through first-principles calculations. The 1.7-eV direct band gap observed in the unstrained o-${\mathrm{B}}_2{\mathrm{N}}_2$ can be tuned up to 3 eV or down to 1 eV by applying 12% tensile strain in armchair and zigzag directions, respectively. Ultimate strain values of o-${\mathrm{B}}_2{\mathrm{N}}_2$ were found to be comparable with that of graphene. Our calculations revealed that the partial alignment of the band edges with the redox potentials of water in pristine o-${\mathrm{B}}_2{\mathrm{N}}_2$ can be tuned into a full alignment under the armchair and biaxial tensile strains. The anisotropic charge carrier mobility found in o-${\mathrm{B}}_2{\mathrm{N}}_2$ prolongs the average lifetime of the carrier drift, creating a suitable condition for photoinduced catalytic reactions on its surface. Finally, we found that even in extreme straining regimes, the highly anisotropic optical absorption of o-${\mathrm{B}}_2{\mathrm{N}}_2$ with strong absorption in the visible range is preserved. Having strong visible light absorption and prolonged carrier migration time, we propose that strain engineering is an effective route to tune the band gap energy and band alignment of o-${\mathrm{B}}_2{\mathrm{N}}_2$ and turn this two-dimensional material into a promising photocatalyst for efficient hydrogen production from water splitting.

In this paper, we employ a fully microscopic approach to the study of interlayer excitons in heterobilayers. We use Fowler's and Karplus' method to access the dynamical polarizability of noninteracting interlayer excitons in a ${\mathrm{WSe}}_2/{\mathrm{WS}}_2$-based van der Waals heterostructure. Following from the calculation of the linear polarizability, we consider Svendsen's variational method to the calculation of the dynamic third-order polarizability. With this variational method, we study both two-photon absorption and third-harmonic generation processes for interlayer excitons in a ${\mathrm{WSe}}_2/{\mathrm{WS}}_2$ heterobilayer, discussing the various selection rules of intraexcitonic energy level transitions.

The discovery of ferroelectric ${\mathrm{HfO}}_2$ in thin films and more recently in bulk is an important breakthrough because of its silicon compatibility and unexpectedly persistent polarization at low dimensions, but the origin of its ferroelectricity is still under debate. The stabilization of the metastable polar orthorhombic phase was often considered as the cumulative result of various extrinsic factors such as stress, grain boundary, and oxygen vacancies as well as phase transition kinetics during the annealing process. We propose a mechanism to stabilize the polar orthorhombic phase over the nonpolar monoclinic phase that is the bulk ground state. Our first-principles calculations demonstrate that the doubly positively charged oxygen vacancy, an overlooked defect but commonly presenting in binary oxides, is critical for the stabilization of the ferroelectric phase. The charge state of the oxygen vacancy serves as a degree of freedom to control the thermodynamic stability of competing phases of wide band gap oxides.

For applications making use of magnonic spin currents damping effects, which decrease the spin conductivity, have to be minimized. We here investigate the magnon transport in a yttrium iron garnet thin film with strongly reduced effective magnetization. We show that in a three-terminal device the effective magnon conductivity can be increased by a factor of up to six by a current applied to a modulator electrode, which generates damping compensation above a threshold current. Moreover, we find a linear dependence of this threshold current on the applied magnetic field. We can explain this behavior by the reduced effective magnetization and the associated nearly circular magnetization precession.

We numerically demonstrate that, although it is critical, the two-box symmetric $\mathrm{SU}\left(3\right)$ chain possesses edge states in the adjoint representation whose excitation energy scales with the number of sites $N_s$ as $1/(N_{s}log{N}_s)$, in close analogy to those found in half-integer $\mathrm{SU}\left(2\right)$ chains with spin $S\ge 3/2$. We further show that these edge states dominate the entanglement entropy of finite chains, explaining why it has been impossible so far to verify with density-matrix renormalization group simulations the field theory prediction that this model is in the $\mathrm{SU}{\left(3\right)}_1$ universality class. Finally, we show that these edge states are very efficiently screened by attaching adjoint representations at the ends of the chain, leading to an estimate of the central charge consistent within 1% with the prediction $c=2$ for $\mathrm{SU}{\left(3\right)}_1$.

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.