Motivated by variety of realizations of the compact Lifshitz model, I predict and explore properties of its three phases, study its phase transitions and derive its fractonic gauge dual. The resulting U(1) vector gauge theory efficiently and robustly encodes the restricted mobility of its dipole conserving charged matter and the corresponding topological vortex defects. The gauge theory provides a transparent formulation of the three phases of the Lifshitz model and gives a field theoretic formulation of the associated two-stage Higgs transitions.

The rare-earth monopnictide (REM) family, which hosts magnetic ground states with extreme magnetoresistance, has established itself as a fruitful playground for the discovery of interesting topological phases. Here, by using high-resolution angle-resolved photoemission spectroscopy complemented by first-principles density-functional-theory-based modeling, we examine the evolution of the electronic structure of the candidate REM Dirac semimetal NdSb across the magnetic transition. A complex angel-wing-like band structure near the zone center along with arclike features at the zone center and the zone corner are observed. This dramatic reconstruction of the itinerant bands around the zone center is shown to be driven by a magnetic transition: Specifically, the Nd $5d$ electron band backfolds at the $\overline{\mathrm{\Gamma }}$ point and hybridizes with the Sb $5p$ hole bands in the antiferromagnetic phase. Our study indicates that antiferromagnetism plays an intricate role in the electronic structure of the REM family.

The effects associated with interatomic hoppings of excitons and the excitonic Bose condensate formation in strongly correlated spin crossover systems are considered in the framework of the effective Hamiltonian for the two-band Kanamori model. The appearance of antiferromagnetic ordering due to the exciton order is found even in the absence of interatomic exchange interaction. The spectrum of excitonic excitations is calculated at various points of the temperature vs crystal field phase diagram. Outside the region of exciton ordering, the spectrum has a gap, which vanishes at the boundary of the exciton condensate phase. The nonuniform spectral weight distribution over the Brillouin zone is found. The role of electron-phonon interaction is discussed as well.

We theoretically study the quantum transport in a three-dimensional spin-1 chiral fermion system in the presence of impurity scattering. Within the self-consistent Born approximation, we find peak structure of the density of states and significant suppression of electrical conductivity around the zero energy. The zero-energy conductivity depends less on impurity concentration, unlike a Weyl fermion. These properties originate from the flat-band structure of the spin-1 chiral fermion.

We consider theoretically a system of exciton-polariton micropillars arranged in a honeycomb lattice. The naturally present TE-TM splitting and an alternating Zeeman splitting, where the different sublattices experience opposite Zeeman splitting, shift the Dirac points in energy, giving rise to antichiral behavior. In a strip geometry having zigzag edges, two pairs of edge states exist and propagate in the same direction (including the states at the opposite edges). The edge modes localized at the opposite edges have opposite spins (circular polarizations), which leads to copropagating “$\pm$” spin channels. The antichiral edge states are protected by nonzero winding numbers and can propagate around a ${60}^{\circ }$ bend without being reflected. We further compare the transport properties of these edge states with chiral edge modes and propose a scheme to realize them experimentally.

Two-dimensional antiferromagnetic materials with robust second harmonic generation (SHG) have been attracting significant research interest. In recent experiments, enhanced SHG from layered antiferromagnet (AFM) bilayer chromium trioxide $\left({\mathrm{CrI}}_3\right)$ with $A$-type antiferromagnetic order has been observed. However, bilayer ${\mathrm{CrI}}_3$ with $A$-type, $C$-type, and $G$-type antiferromagnetic order may simultaneously occur in experimental synthesis, as the total free-energy difference between the three AFMs is small. Here, based on symmetry analysis and first-principles calculations, we study the three kinds of bilayer AFMs with and without the spin-orbit coupling (SOC) effect. We find that for all three types, the $i$-type SHG response vanishes due to the centrosymmetric lattice structure. However, significant $c$-type SHG response can arise in $A$-type and $C$-type AFMs but still vanishes for $G$-type AFM. Under normal incidence, both $A$-type and $C$-type AFMs exhibit three independent nonvanishing SHG components. Remarkably, the nonvanishing SHG components of $A$-type and $C$-type AFMs are mutually exclusive, namely, the SHG components that are finite for $A$-type vanish for $C$-type and vice versa. In particular, the SHG of both $A$-type and $C$-type AFMs is sensitive to the SOC effect and it becomes enhanced when the SOC effect is fully considered. Hence, the SHG response would be an efficient method for probing bilayer ${\mathrm{CrI}}_3$ with different antiferromagnetic orders.

We study the superconductor-insulator transition (SIT) in the ground state of the attractive honeycomb Hubbard model in the presence of a staggered potential (a mass term), using a combination of unbiased computational methods, namely, exact diagonalization and quantum Monte Carlo simulations. We probe the nature of the lowest-energy charge excitations across the SIT and show that they are bosonic, as inferred (and shown in the strongly interacting regime) in a previous study of the same model in the square lattice. Increasing the strength of the staggered potential leads to a crossover in which bosonic low-energy excitations give way to fermionic ones within the insulating phase. We also show that the SIT belongs to the $3d$-XY universality class, like in its square lattice counterpart. The robustness of our results in these two lattice geometries supports the expectation that our findings are universal for SITs in clean systems.

The stability of three-dimensional relativistic semimetals to disorder has recently attracted great attention, but the effect of disorder remains elusive for multifold fermions, that are not present in the framework of quantum field theory. In this paper, we investigate one type of multifold fermions, so-called triple-point fermions (TPFs), which have pseudospin-1 degrees of freedom and topological charges $\pm 2$. Specifically, we consider the effect of disorder on a minimal, three-band tight-binding model, which realizes the minimal number of two TPFs. The numerically obtained, disorder-averaged density of states suggests that, within a finite energy window, the TPFs are robust up to a critical strength of disorder. In the strong disorder regime, the inter-TPF scattering is the main mechanism for destroying a single TPF. Moreover, we study the effects of disorder on the distribution of Fermi arcs and surface Berry curvature. We demonstrate that the Fermi arc retains its sharpness at weak disorder, but gradually dissolves into the metallic bulk for stronger disorder. In the clean limit, the surface Berry curvature exhibits a bipolar configuration in the surface Brillouin zone. With increasing disorder, the positive and negative surface Berry curvatures start to merge at the nearby momenta where the Fermi arcs penetrate into bulk.

The infinite projected entangled simplex state (iPESS), a type of tensor network (TN) state, has been used successfully for simulating and characterizing nonchiral spin liquids on the kagome lattice. Here, we demonstrate that iPESS also provides a faithful representation of a chiral spin liquid (CSL) on the same lattice, namely the ground state of the spin-$1/2$ kagome Heisenberg antiferromagnet with a scalar chirality. By classifying local tensors according to $\mathrm{SU}\left(2\right)$ and point group symmetries, we construct a chiral ansatz breaking reflection $P$ and time reversal $T$ symmetries while preserving $PT$. The variational TN states are shown to host, for bond dimension $D\ge 8$, a chiral gapless entanglement spectrum following $\mathrm{SU}{\left(2\right)}_1$ conformal field theory. The correlation function shows a small weight long-range tail complying with the prediction of the TN bulk-edge correspondence. We identify a nonchiral manifold spanned by only a subset of symmetric tensors where a new emergent tensor conservation law is realized. This allows us to both probe the stability of the nonchiral spin liquid and discuss its transition to CSL induced by a scalar chirality term.

In recent years, fermionic topological phases of quantum matter has attracted a lot of attention. In a pioneer work by Gu, Wang, and Wen, the concept of equivalence classes of fermionic local unitary (FLU) transformations was proposed to systematically understand nonchiral topological phases in 2D fermion systems and an incomplete classification was obtained. On the other hand, the physical picture of fermion condensation and its corresponding super pivotal categories give rise to a generic mathematical framework to describe fermionic topological phases of quantum matter. In particular, it has been pointed out that in certain fermionic topological phases, there exists the so-called q-type anyon excitations, which have no analogues in bosonic theories. In this paper, we generalize the Gu, Wang, and Wen construction to include those fermionic topological phases with q-type anyon excitations. We argue that all nonchiral fermionic topological phases in $2+1\mathrm{D}$ are characterized by a set of tensors $(N_k^{ij},F_k^{ij},F_{kln,\chi \delta }^{ijm,\alpha \beta },n_i,d_i)$, which satisfy a set of nonlinear algebraic equations parameterized by phase factors ${\mathrm{\Xi }}_{kl}^{ijm,\alpha \beta }$ and ${\mathrm{\Xi }}_{kln,\chi \delta }^{ij}$. Moreover, consistency conditions among algebraic equations give rise to additional constraints on these phase factors, which allow us to construct a topological invariant partition for an arbitrary triangulation of 3D spin manifold. Finally, several examples with q-type anyon excitations are discussed, including the fermionic topological phase from Tambara-Yamagami category for ${\mathbb{Z}}_{2N}$, which can be regarded as the ${\mathbb{Z}}_{2N}$ parafermion generalization of Ising fermionic topological phase.

The hydrodynamic regime of charge transport has been recently realized in high-quality conductors. In the hydrodynamic as well as in the Ohmic regimes the main part of the Hall resistance of a long sample is determined by the balance between the Lorentz force and the electric force, acting on conduction electrons. Experimentally observed deviations of the Hall resistance in hydrodynamic samples from such “standard” value are usually associated with the Hall viscosity term in the Navier-Stokes equation. In this work we theoretically study the Hall effect in a Poiseuille flow of a two-dimensional electron fluid. We show that the near-edge semiballistic layers with the width of the order of the interparticle mean-free path, which inevitably appear near sample edges, give the contribution to the Hall resistance which is comparable with the bulk contribution from the Hall viscosity. In this way, the measured deviations of the Hall resistance from the “standard” one in hydrodynamic samples by the usual contact techniques should be associated with both the Hall viscosity in the bulk and the semiballistic effects in the near-edge layers.

In this work we present the melting curves of hafnium and zirconium obtained using quantum molecular dynamics calculations. The mean-square displacements computed during ab initio simulations of a crystal phase are used to reconstruct the melting curve according to the Lindemann criterion. The resulting Zr melting curve shows a steeper slope in the low-pressure region compared to some recent diamond-anvil cell experiments but agrees with our previous estimate via the Clausius-Clapeyron relation. The slope for higher pressures is consistent with the experimental one. As for Zr, the Hf melting curve also has a similar steep initial slope being in agreement with our estimate from the Clausius-Clapeyron relation. Currently there are no data on the melting of Hf above atmospheric pressure, so we demonstrate the first ab initio estimate of the melting curve of Hf up to 225 GPa.

Hybrid organic-inorganic halide perovskites represent a promising next-generation photovoltaic material with drawbacks for structure stability and composition concerns. Demonstrations of ion migration and molecular dynamics suggest room for structural contraction and subsequent property adjustments. Here, we deploy dielectric and infrared spectroscopy under external pressure to probe the full structural phase diagram and dielectric response of methylammonium lead halide perovskites ${\mathrm{CH}}_3{\mathrm{NH}}_3\mathrm{Pb}{X}_3$ ($X=$ I, Br, or Cl). Ion migration can be fully suppressed by pressure beyond 4 GPa. The low-temperature orthorhombic phase transition can be gradually enhanced and stabilized at ambient conditions with increasing pressure. A slow relaxation mode, presumably the motion of the ${\mathrm{CH}}_3{\mathrm{NH}}_3^{+}$ cation, is observed at lower pressure and is absent in the orthorhombic phase for every halide.

The recently synthesized hydrogen boride monolayer in the $Cmmm$ phase is a promising superconductor due to its similarity to ${\mathrm{MgB}}_2$ and the large hydrogen content in its structure. Making use of first-principles calculations based on density functional theory, we study its electronic, vibrational, and superconducting properties and conclude that despite the expectations, hydrogen boride does not have a sizable superconducting critical temperature. The presence of hydrogen in the system alters the boron-boron bonding, weakening the electron-phonon interaction. We have studied the effect of enhancing the critical temperature by doping the system, but the inclusion of electrons or holes reveals this to be ineffective. We attribute the small critical temperature of this system to the vanishing hydrogen character of the states at the Fermi level, which are dominated by boron $p$ states. Our results hint at a possible relation between the presence of a large proportion of hydrogenlike states at the Fermi level and a large superconducting critical temperature in hydrogenated monolayers.

The resonant frequencies of three-dimensional (3D) microwave cavities are explicitly dependent on the dielectric constant of the material filling the cavity, making them an ideal system for probing material properties. In particular, dielectric constant measurements allow one to extract the helium density through the Clausius-Mossotti relation. By filling a cylindrical aluminum cavity with superfluid helium, we make precision measurements of the dielectric constant of liquid ${}^4\mathrm{He}$ at saturated vapor pressure for range of temperatures 30 to 300 mK and at pressures of 0 to 25.0 bar at 30 mK, essentially the zero temperature limit for the properties of ${}^4\mathrm{He}$. After reviewing previous measurements, we find systematic discrepancy between the low and high frequency determination of the dielectric constant in the zero-temperature limit and moderate discrepancy with previously reported values of pressure-dependent density. Our precision measurements suggest 3D microwave cavities are a promising choice for refining previously measured values in helium, with potential applications in metrology.

We analyze the topological properties of the possible superconducting states emerging from a ${\mathrm{Cd}}_3{\mathrm{As}}_2$-like, ${\mathcal{C}}_4$-symmetric Dirac semimetal, with two fourfold-degenerate Dirac points separated in the $k_z$ direction. Unlike the simplest Weyl semimetal for which all pairing orders are topologically obstructed and nodal, we show that the topological obstruction for pairing in Dirac semimetals crucially only exists for certain pairing symmetries. In particular, we focus on odd-parity $B_{1u}$ and $B_{2u}$ pairing states, both of which can be induced by Ising ferromagnetic fluctuations. The $B_{1u}$ and $B_{2u}$ pairing states inherit the topological obstruction from the normal state, which dictates that these states necessarily host four Bogoliubov–de Gennes (BdG) Dirac point nodes protected by a ${\mathbb{Z}}_2$ monopole charge. By a Wannier state analysis, we show that the topological obstruction in the superconducting states is of higher-order nature. As a result, in a rod geometry with gapped surfaces, arcs of higher-order Majorana zero modes exist in certain $k_z$ regions of the hinges between the BdG Dirac points. Unlike Fermi arcs in Weyl semimetals, the higher-order Majorana arcs are stable against self-annihilation due to an additional $\mathbb{Z}$-valued monopole charge of the BdG Dirac points protected by ${\mathcal{C}}_4$ symmetry. We find that the same $\mathbb{Z}$-valued charge is also carried by $B_{1g}$ and $B_{2g}$ channels, where the BdG spectrum hosts bulk “nodal cages,” i.e., cages formed by nodal lines, that are stable against symmetry-preserving perturbations.

We present the detection of the spin-flop transition in the antiferromagnetic van der Waals material ${\mathrm{MnPS}}_3$ via thermally generated nonlocal magnon transport using permalloy (Py) detector strips. Py detector strips possess the inverse anomalous spin Hall effect, which has the unique power to detect an out-of-plane spin accumulation [Das et al., Nano Lett. 18, 5633 (2018)]. This enables us to detect magnons with an out-of-plane spin polarization, in contrast to strips of high-spin-orbit materials such as Pt which possess only the spin Hall effect and are sensitive to only an in-plane spin polarization of the spin accumulation. We show that nonlocal magnon transport is able to measure the spin-flop transition in the absence of spurious magnetoresistance effects. Our measurements show the detection of magnons generated by the spin Seebeck effect before and after the spin-flop transition. We observe a signal reversal of the magnon spin accumulation which agrees with the out-of-plane spin polarization carried by magnon modes before and after the spin-flop transition.

The results of a muon spin rotation experiment on the layered oxyselenide ${\mathrm{Sr}}_2{\mathrm{CoO}}_2{\mathrm{Ag}}_2{\mathrm{Se}}_2$ are presented. The compound contains square-planar ${\mathrm{CoO}}_2$ layers and is found to exhibit a phase transition at 160.4(1) K to an antiferromagnetic configuration of the ${\mathrm{Co}}^{2+}$ ions. Density functional theory calculations were performed in order to determine the stopping site of the muon within the unit cell. The calculated magnetic dipole field at the muon stopping site was shown to be consistent with the proposed magnetic structure.

A theoretical formalism of Andreev reflection is employed to provide theoretical support for distinguishing between the singlet pairing and the triplet pairing by the point contact Andreev reflection (PCAR) experiments. We utilize our theoretical curves to fit the data of the PCAR experiment on unconventional superconductivity in the Bi/Ni bilayer [G. J. Zhao, X. X. Gong, J. C. He, J. A. Gifford, H. X. Zhou, Y. Chen, X. F. Jin, C. L. Chien, and T. Y. Chen, Triplet p-wave superconductivity with ABM state in epitaxial Bi/Ni bilayers, arXiv:1810.10403] and find the Anderson-Brinkman-Morel (ABM) state satisfies the main characteristics of the experimental data. The chiral cross section of the ABM state might explain well the broken time-reversal symmetry determined by the polar Kerr effect measurements and the time-domain terahertz spectroscopy in Bi/Ni bilayers. Moreover, the Andreev reflection spectra of the Balian-Werthamer state and the chiral $p$-wave state are also presented.

It is well known that the ground state of homogeneous superconducting systems with spin-orbit coupling in the presence of the Zeeman field is the so-called helical state, which is characterized by the phase modulation of the order parameter, but zero supercurrent density. In this paper we investigate the realization of the helical state in a hybrid system with spatially separated superconductivity and exchange field by considering a superconductor/ferromagnet ($S/F$) bilayer on top of a three-dimensional topological insulator. This system is characterized by strong spin-momentum locking and, consequently, provides the most favorable conditions for the helical state generation. The analysis is based on the microscopic theory in terms of the quasiclassical Green's functions. We demonstrate that in the bilayer the helical state survives if the exchange field has a nonzero component perpendicular to the $S/F$ interface even in spite of the fact that the superconducting order parameter and the exchange field are spatially separated. At the same time, in this spatially inhomogeneous situation the helical state is accompanied by the spontaneous currents distributed over the bilayer in such a way that the net current vanishes. Further, we show that this hybrid helical state gives rise to nonreciprocity in the system. We demonstrate the realization of the nonreciprocity in the form of the superconducting diode effect and investigate its dependence on the parameters of the bilayer.