We apply the efficient two-optimized references thermodynamic integration using Langevin dynamics method [Phys. Rev. B 96, 224202 (2017)] to calculate highly accurate melting properties of Al and magnetic Ni from first principles. For Ni we carefully investigate the impact of magnetism on the liquid and solid free energies including longitudinal spin fluctuations and the reverse influence of atomic vibrations on magnetic properties. We show that magnetic fluctuations are effectively canceling out for both phases and are thus not altering the predicted melting temperature. For both elements, the generalized gradient approximation (GGA) and the local-density approximation (LDA) are used for the exchange-correlation functional revealing a reliable ab initio confidence interval capturing the respective experimental melting point, enthalpy of fusion, and entropy of fusion.

Silicon carbide is a very promising platform for quantum applications because of the extraordinary spin and optical properties of point defects in this technologically friendly material. These properties are strongly influenced by crystal vibrations, but the exact relationship between them and the behavior of spin qubits is not fully investigated. We uncover the local vibrational modes of the Si vacancy spin qubits in as-grown 4H-SiC. We apply microwave-assisted spectroscopy to isolate the contribution from one particular type of defects, the so-called V2 center, and observe the zero-phonon line together with seven equally separated phonon replicas. Furthermore, we present first-principles calculations of the photoluminescence line shape, which are in excellent agreement with our experimental data. To boost up the calculation accuracy and decrease the computation time, we extract the force constants using machine-learning algorithms. This allows us to identify the dominant modes in the lattice vibrations coupled to an excited electron during optical emission in the Si vacancy. A resonance phonon energy of 36 meV and a Debye-Waller factor of about $6\%$ are obtained. We establish experimentally that the activation energy of the optically induced spin polarization is given by the local vibrational energy. Our findings give insight into the coupling of electronic states to vibrational modes in SiC spin qubits, which is essential to predict their spin, optical, mechanical, and thermal properties. The approach described can be applied to a large variety of spin defects with spectrally overlapped contributions in SiC as well as in other three- and two-dimensional materials.

The relaxation of a many-body system is still a challenging problem that has not been well understood. In this work we exactly calculate the dynamics of the quantum $XY$ model with boundary dissipation, in which the density matrix in terms of Majorana operators can be decoupled into independent subspaces ${\mathcal{K}}_i$ represented by different number of Majorana fermions. The relaxation is characterized by multiple timescales, and in the long-time limit it is determined by the single-particle relaxation process in a typical timescale $T^{*}$. For the bulk bands, we find $T^{*}\propto N^3/\gamma n^2$ in the weak dissipation limit and $T^{*}\propto \gamma N^3/n^2$ in the strong dissipation limit, where $N$ is the chain length, $\gamma$ is the dissipation rate, and $n$ is the band index. For the edge modes, $T^{*}\propto 1/\gamma$ indicates the most vulnerable to dissipation in the long-chain limit. These results are counterintuitive because it means any weak dissipation can induce strong relaxation, while strong dissipation can induce weak relaxation. We find that these two limits correspond to two different physics, which are explained based on the first- and second-order perturbation theory in an equivalent non-Hermitian model. Finally, we show that even in the long-chain limit with weak dissipation, the relaxation may exhibit strong odd-even effect, which can be washed out by the strong dissipation. These results shed new insight into the dynamics of topological qubits in the environment, and the mechanism for many-body dissipation may also have universal significance in understanding the many-body dissipation in the other models.

We report the tunable conducting behavior of a series of ${\left(\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4\right)}_4/{\left(\mathrm{S}{\mathrm{r}}_3\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7\right)}_N$ ($N$ = 1, 2, and 4) atomic layer superlattices, in which phases of itinerant electrons, itinerant holes, localized electrons, and an anomalous charge region can be varied dependent on the period and the temperature. Specifically, for the $N$ = 4 superlattice, the electron-to-hole transition occurs at a temperature of 35 K, at which the sample behaves as an intrinsic insulator without either electrons or holes. Upon further reducing the temperature to below 16 K, the superlattice enters an anomalous phase region in which an abrupt zero-to-negative magnetoresistance transition and a Hall resistivity kink are observed at 1.3 and 2.1 T, respectively. Above these fields, polarized neutron reflectivity measurements revealed ferromagnetism confined in the $\mathrm{S}{\mathrm{r}}_3\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$ layers. Moreover, the $N$ = 2 superlattice exhibits a bump feature in the anomalous Hall resistivity near 1.3 T, which is similar to the previously reported topological Hall phenomenon in ultrathin $\mathrm{SrRu}{\mathrm{O}}_3$ heterostructures. We demonstrate that the layered $4d/5d$ oxide superlattice is a powerful platform in engineering exotic types of phases which have not been explored in their thin-film or bulk samples.

We implement and characterize a numerical algorithm inspired by the $s$ source framework [B. Swingle and J. McGreevy, Phys. Rev. B 93, 045127 (2016)] for building a quantum many-body ground state wave function on a lattice of size $2L$ by applying adiabatic evolution to the corresponding ground state at size $L$, along with $L$ interleaved ancillae. The procedure can in principle be iterated to repeatedly double the size of the system. We implement the algorithm for several one-dimensional (1D) spin model Hamiltonians, and find that the construction works particularly well when the gap is large and, interestingly, at scale-invariant critical points. We explain this feature as a natural consequence of the lattice expansion procedure. This behavior holds for both the integrable transverse-field Ising model and nonintegrable variations. We also develop an analytic perturbative understanding of the errors deep in either phase of the transverse-field Ising model, and suggest how the circuit could be modified to parametrically reduce errors. In addition to sharpening our perspective on entanglement renormalization in 1D, the algorithm could also potentially be used to build states experimentally, enabling the realization of certain long-range correlated states with low-depth quantum circuits.

In ${\mathrm{Zn}}_{1-x}{\mathrm{Mn}}_x\mathrm{Se}$/(Zn,Be)Se quantum wells with $x<0.035$, nonthermalized ${\mathrm{Mn}}^{2+}$ ions demonstrate in spin-flip scattering spectra multiple Stokes and anti-Stokes transitions whose absolute energies deviate by up to 20% from each other. This asymmetry is tuned significantly by the optical power density, magnetic field direction, and Mn ion concentration. The nonequidistant ${\mathrm{Mn}}^{2+}$ spin transitions are modeled by the Zeeman splitting and quadrupolar crystal-field components taking values of up to 7 GHz. We suggest that nonequilibrium carriers dynamically polarize the Mn-ion spins so that they occupy levels with positive and negative spin projection numbers giving rise to asymmetric spin transitions.

We report an investigation of the combined structural and electronic properties of the bronze ${\mathrm{Na}}_{1.5}{\left({\mathrm{PO}}_2\right)}_4{\left({\mathrm{WO}}_3\right)}_{20}$. Its low-dimensional structure and possible large reconstruction of the Fermi surface due to charge density wave instability make this bulk material a natural superlattice with a reduced number of carriers and Fermi energy. Signatures of multilayered two-dimensional (2D) electron weak localization are consequently reported, with an enhanced influence of quantum oscillations. A crossover between these two antagonistic entities, previously observed only in genuine low-dimensional materials and devices, is shown to occur in a bulk crystal due to its hidden 2D nature.

The Lieb-Schultz-Mattis (LSM) theorem and its descendants impose strong constraints on the low-energy behavior of interacting quantum systems. In this paper, we formulate LSM-type constraints for lattice translation invariant systems with generalized U(1) symmetries which have recently appeared in the context of fracton phases: U(1) polynomial shift and subsystem symmetries. Starting with a generic interacting system with conserved dipole moment, we examine the conditions under which it supports a symmetric, gapped, and nondegenerate ground state, which we find requires that both the filling fraction and the bulk charge polarization take integer-values. Similar constraints are derived for systems with higher moment conservation laws or subsystem symmetries, in addition to lower bounds on the ground-state degeneracy when certain conditions are violated. Finally, we discuss the mapping between LSM-type constraints for subsystem symmetries and the anomalous symmetry action at boundaries of subsystem symmetric topological (SSPT) states.

Dyakonov surface waves are an example of surface electromagnetic waves propagating along a plane interface between isotropic and anisotropic dielectric media. Here we investigate the spectrum and radiative losses of Dyakonov waves at the curved interface considering both cases of “positive” and “negative” curvature of anisotropic medium. We demonstrate how Dyakonov waves at a plane interface continuously transform to the leaky or guided modes of an anisotropic cylindrical waveguide and derive asymptotic equation for radiative losses. All analytical results are confirmed by solving the exact dispersion equation numerically. We believe that our work extends the potential practical application of Dyakonov surface waves at a curved interfaces, e.g., anisotropic cylindrical waveguides, in optics and photonics devices.

We introduce a local radiative heat-pumping effect between two bodies in a many-body system, obtained by periodically modulating both the temperature and the position of an intermediate object using an external source of energy. We show that the magnitude and the sign of energy flow can be tuned by changing the oscillation amplitude and dephasing of the two parameters. This many-body effect paves the way for an efficient and active control of heat fluxes at the nanoscale.

We investigate the electronic dispersion and transport properties of graphene/${\mathrm{WSe}}_2$ heterostructures in the presence of a proximity induced spin-orbit coupling (SOC) using a low-energy Hamiltonian, with different types of symmetry breaking terms, obtained from a four-band, first and second nearest-neighbour tight-binding (TB) one. The competition between different perturbation terms leads to inverted SOC bands. Further, we study the effect of symmetry breaking terms on ac and dc transport by evaluating the corresponding conductivities within linear response theory. The scattering-independent part of the valley-Hall conductivity, as a function of the Fermi energy $E_F$, is mostly negative in the ranges $-{\lambda }_R⩽E_F$ and $E_F⩾{\lambda }_R$ when the strength ${\lambda }_R$ of the Rashba SOC increases except for a very narrow region around $E_F=0$ in which it peaks sharply upward. The scattering-dependent diffusive conductivity increases linearly with electron density, is directly proportional to ${\lambda }_R$ in the low- and high-density regimes, but weakens for ${\lambda }_R=0$. We investigate the optical response in the presence of a SOC-tunable band gap for variable $E_F$. An interesting feature of this SOC tuning is that it can be used to switch on and off the Drude-type intraband response. Furthermore, the ac conductivity exhibits interband responses due to the Rashba SOC. We also show that the valley-Hall conductivity changes sign when $E_F$ is comparable to ${\lambda }_R$ and vanishes at higher values of $E_F$. It also exhibits a strong dependence on temperature and a considerable structure as a function of the frequency.

Multilayer graphene with rhombohedral and Bernal stacking is supposed to be metallic, as predicted by density functional theory calculations using semilocal functionals. However, recent angular resolved photoemission and transport data have questioned this point of view. In particular, rhombohedral flakes are suggested to be magnetic insulators, a view supported also by hybrid-functional calculations. Bernal flakes composed of an even number of layers are insulating (for $N\le 6$), while those composed of an odd number of layers are pseudogapped (for $N\le 7$). Here, by systematically benchmarking with plane-waves codes, we develop very accurate all-electron Gaussian basis sets for graphene multilayers, allowing a precise description of the electronic structure in the 100 meV energy range from the Fermi energy at the hybrid-functional level. We find, in agreement with our previous calculations, that rhombohedral stacked multilayers are gapped and magnetic. However, the valence band curvature and the details of the electronic structure at the $\sim 10\mathrm{meV}$ scale show a dependence on the basis set. A substantially extended basis set is needed to describe the long-range interlayer interactions and, consequently, to correctly reproduce the effective mass of the valence band top at the $K$ point. In the case of Bernal stacking, we show that exact exchange gaps the flakes composed by four layers and opens pseudogaps for $N=3$, 6, 7, 8. However, the gap or pseudogap size and its behavior as a function of thickness are not compatible with experimental data. Moreover, hybrid functionals lead to a metallic solution for five layers and a magnetic ground state for five, six, and eight layers. Magnetism is very weak with practically no effect on the electronic structure and the magnetic moments are mostly concentrated in the central layers. Our hybrid-functional calculations on trilayer Bernal graphene are in excellent agreement with $GW$ results. For thicker multilayers, our calculations are a benchmark for many-body theoretical modeling of the low energy electronic structure.

The binary H-O system almost exclusively exists in the form of water ice with stoichiometry of ${\mathrm{H}}_2\mathrm{O}$ in a wide range of pressure (<∼5 TPa) that is one of the most abundant substances in the solar system. Hydrogen peroxide (${\mathrm{H}}_2{\mathrm{O}}_2$) is only metastable at ambient condition. We herein report a stable ${\mathrm{H}}_2{\mathrm{O}}_2$ phase at high pressure identified by first-principles calculations in combination with a swarm-intelligence structure search. The predicted ${\mathrm{H}}_2{\mathrm{O}}_2$ compound is formed by stacking of an intriguing planar ${\mathrm{H}}_2{\mathrm{O}}_2$ molecule different from the metastable three-dimensional isomer at ambient condition. The planarity of hydrogen peroxide molecules is expected to be caused by the steric repulsion between molecules at high pressure, which is responsible for the stability of the crystal. The phase is energetically stable in the pressure range of ∼423–600 GPa. It is thus a low-pressure phase of binary H-O system with H:O stoichiometry other than 2:1 and promises to be accessible in static compression experiment.

Ferroelectric tungsten-based Aurivillius oxides are naturally stable superlattice structures, in which $A$-site deficient perovskite blocks ${\left[{\mathrm{W}}_n{\mathrm{O}}_{3n+1}\right]}^{-2}$ ($n=1,2,3,\cdots$) interleave with fluorite-like bismuth oxide layers ${\left[{\mathrm{Bi}}_2{\mathrm{O}}_2\right]}^{+2}$ along the $c$-axis. In the $n=2{\mathrm{Bi}}_2{\mathrm{W}}_2{\mathrm{O}}_9$ phase, an in-plane antipolar distortion dominates but there has been controversy as to the ground-state symmetry. Here we show, using a combination of first-principles density functional theory calculations and experiments, that the ground state is a nonpolar phase of $Pnab$ symmetry. We explore the energetics of metastable phases and the potential for antiferroelectricity in this $n=2$ Aurivillius phase.

The competition between charge density wave (CDW) and superconductivity is a central theme in condensed-matter physics with ramifications to correlated electron systems and high-temperature superconductivity. While the emergence of superconductivity is often observed upon suppressing the CDW transition by tuning the chemical composition or the external pressure, their coexistence has been reported in only a handful of materials, with the cuprates being the most prominent example. Here, we demonstrate that both cooperative electronic phenomena coexist in the cluster compound ${\mathrm{K}}_2\mathrm{M}{\mathrm{o}}_{15}\mathrm{S}{\mathrm{e}}_{19}$. The CDW transition sets in at $T_{\mathrm{CDW}}=114\mathrm{K}$, accompanied by a commensurate periodic modulation of the crystal lattice along the $c$ axis evidenced by electron diffraction. Bulk type-II superconductivity develops upon further cooling below $T_c=2.8\mathrm{K}$. The presence of similar signatures of CDW ordering in other ${\mathrm{A}}_2\mathrm{M}{\mathrm{o}}_{15}\mathrm{S}{\mathrm{e}}_{19}$ compounds shows that this electronic instability may be ubiquitous in these compounds, providing a novel family where the interplay between CDW and superconductivity may be systematically investigated.

Quasicrystals remain among the most intriguing materials in physics and chemistry. Their structure results in many unusual properties, including anomalously low friction as well as poor electrical and thermal conductivity, but it also supports superconductivity, which shows that quantum effects in quasicrystals can be quite unique. We theoretically study superfluidity in a model quantum cluster quasicrystal. Using path-integral Monte Carlo simulations, we explore a two-dimensional ensemble of bosons with the Lifshitz-Petrich-Gaussian pair potential, finding that moderate quantum fluctuations do not destroy the dodecagonal quasicrystalline order. This quasicrystal is characterized by a small yet finite superfluidity, demonstrating that particle clustering combined with the local cogwheel structure can underpin superfluidity even in the almost classical regime. This type of distributed superfluidity may also be expected in certain open crystalline lattices. Large quantum fluctuations are shown to induce transitions to cluster solids, supersolids, and superfluids, which we characterize fully quantum mechanically.

We present the results of a study of the vortex lattice (VL) of the nickel chalcogenide superconductor ${\mathrm{TlNi}}_2{\mathrm{Se}}_2$, using small angle neutron scattering. This superconductor has the same crystal symmetry as the iron arsenide materials. Previous work points to it being a two-gap superconductor, with an unknown pairing mechanism. No structural transitions in the vortex lattice are seen in the phase diagram, arguing against $d$-wave gap symmetry. Empirical fits of the temperature dependence of the form factor and penetration depth rule out a simple $s$-wave model, supporting the presence of nodes in the gap function. The variation of the VL opening angle with field is consistent with earlier reports of multiple gaps.

In this paper, with the help of Hermitian/non-Hermitian dynamic strings, a theory of non-Hermitian topological orders is developed based on a non-Hermitian Wen-plaquette model. The effective models for bosonic topological excitations ($e$ particles and $m$ particles) are Hermitian tight-binding lattice models; the effective model for fermionic topological excitations ($f$ particles) becomes a non-Hermitian tight-binding lattice model. In addition, the effective pseudospin model for topologically degenerate ground states is derived by calculating the expectation values of Hermitian/non-Hermitian topologically closed dynamic strings. For the topologically degenerate ground states of the non-Hermitian Wen-plaquette model on even-by-odd, odd-by-even, and odd-by-odd lattices, anomalous topological degeneracy occurs, i.e., the number of topologically protected ground states may be reduced from 2 to 1. Now, the effective pseudospin model turns into the typical $\mathcal{PT}$-symmetric non-Hermitian Hamiltonian with spontaneous $\mathcal{PT}$ symmetry breaking. At exceptional points, the topologically degenerate ground states merge with each other and the topological degeneracy turns into non-Hermitian degeneracy. In the end, the application of the non-Hermitian $Z_2$ topological order and its possible physics realization are discussed.