Recently, the celebrated Rytova-Keldysh potential has been widely used to describe the Coulomb interaction of few-body complexes in monolayer transition-metal dichalcogenides. Using this potential to model charged excitons (trions), one finds a strong dependence of the binding energy on whether the monolayer is suspended in air, supported on ${\mathrm{SiO}}_2$, or encapsulated in hexagonal boron-nitride. However, empirical values of the trion binding energies show weak dependence on the monolayer configuration. This deficiency indicates that the description of the Coulomb potential is still lacking in this important class of materials. We address this problem and derive a new potential form, which takes into account the three atomic sheets that compose a monolayer of transition-metal dichalcogenides. The new potential self-consistently supports (i) the nonhydrogenic Rydberg series of neutral excitons and (ii) the weak dependence of the trion binding energy on the environment. Furthermore, we identify an important trion-lattice coupling due to the phonon cloud in the vicinity of charged complexes. Neutral excitons in their ground state, on the other hand, have weaker coupling to the lattice due to the confluence of their charge neutrality and small Bohr radius.

We study the competition of hole capture between an In(Ga)As quantum dot and a directly adjacent ionized impurity in view of spin-photon interfaces. The Kerr rotation noise spectroscopy at 4.2 K shows that the hole-capture probability of the In(Ga)As quantum dot is about one order of magnitude higher compared to the hole-capture probability of the ionized impurity and suggests that a simultaneous occupation of quantum dot and impurity by a hole is efficiently suppressed due to Coulomb interaction. A theoretical model of interconnected spin and charge noise allows the quantitative specification of all relevant time scales.

Spin chains with open boundaries, such as the transverse field Ising model, can display coherence times for edge spins that diverge with the system size as a consequence of almost conserved operators, the so-called strong zero modes. Here we discuss the fate of these coherence times when the system is perturbed in two different ways. First, we consider the effects of a unitary coupling connecting the ends of the chain; when the coupling is weak and noninteracting, we observe stable long-lived harmonic oscillations between the strong zero modes. Second, and more interestingly, we consider the case when dynamics becomes dissipative. While in general dissipation induces decoherence and loss of information, here we show that particularly simple environments can actually enhance correlation times beyond those of the original unitary case. This allows us to generalize the notion of strong zero modes to irreversible Markovian time evolutions, thus defining conditions for dissipative strong zero maps. Our results show how dissipation could, in principle, play a useful role in protocols for storing information in quantum devices.

Magnetic behavior of a $3d$ solute in a ferromagnetic lattice can be very sensitive to local environment, which is the case of manganese in bcc Fe. Body-centered cubic iron-manganese alloys are studied by means of density functional theory in order to elucidate properties of the lowest-energy magnetic states. Multiple magnetic minima are determined even for the simplest case of an isolated Mn and a Mn dimer in bcc iron. The magnetoenergetic landscape is analyzed within and beyond the collinear magnetic approximation. A direct correlation is identified between the local electronic charge and the local magnetic moment of a Mn solute, being either isolated or forming a small cluster. In particular, the presence of a vacancy near the Mn atom, inducing a local charge decrease, tends to favor the antiferromagnetic Fe-Mn interaction while the presence of an interstitial impurity with a strong electronic hybridization with Mn can favor a ferromagnetic Fe-Mn interaction. Energetic and magnetic properties of Fe-Mn alloys are systematically investigated for a large range of Mn concentrations. An unmixing tendency is clearly noted. A detailed magnetic analysis suggests the Mn-Mn magnetic interactions to be generally dominant over the Fe-Mn interactions, both exhibiting an antiferromagnetic tendency. The average magnetic moment of the Mn atoms in locally random alloys tends to be antiparallel (parallel) to lattice Fe moments for Mn concentrations smaller (larger) than approximately 6 at. %. The transition concentration is shown to be lowered if considering Mn clustering which is energetically favorable. The unsolved discrepancies between experimental and theoretical predictions on the critical concentration for the Mn magnetic behavior change in Fe-Mn solid solution are discussed in the light of the obtained results.

Calcium vanadate ${\mathrm{CaV}}_2{\mathrm{O}}_4$ has a crystal structure of quasi-one-dimensional zigzag chains composed of orbital-active ${\mathrm{V}}^{3+}$ ions and undergoes successive structural and antiferromagnetic phase transitions at $T_s\sim 140$ K and $T_N\sim 70$ K, respectively. We perform ultrasound velocity measurements on a single crystal of ${\mathrm{CaV}}_2{\mathrm{O}}_4$. The temperature dependence of its shear elastic moduli exhibits huge Curie-type softening upon cooling that emerges above and below $T_s$ depending on the elastic mode. The softening above $T_s$ suggests the presence of either onsite Jahn-Teller-type or intersite ferrotype orbital fluctuations in the two inequivalent ${\mathrm{V}}^{3+}$ zigzag chains. The softening below $T_s$ suggests the occurrence of a dimensional spin-state crossover, from quasi-one to three, that is driven by the spin-lattice coupling along the inter-zigzag-chain orthogonal direction. The successive emergence of the orbital- and spin-driven lattice instabilities above and below $T_s$, respectively, is unique to the orbital-spin zigzag chain system of ${\mathrm{CaV}}_2{\mathrm{O}}_4$.

We report thermoelectric properties of ${\mathrm{Ir}}_{1-x}{\mathrm{Rh}}_x{\mathrm{Te}}_2$ ($0\le x\le 0.3$) alloy series where superconductivity at low temperatures emerges as the high-temperature structural transition ($T_s$) is suppressed. The isovalent ionic substitution of Rh into Ir has different effects on physical properties when compared to the anionic substitution of Se into Te, in which the structural transition is more stable with Se substitution. Rh substitution results in a slight reduction of lattice parameters and in an increase of number of carriers per unit cell. Weak-coupled BCS superconductivity in ${\mathrm{Ir}}_{0.8}{\mathrm{Rh}}_{0.2}{\mathrm{Te}}_2$ that emerges at low temperature ($T_c^{\mathrm{zero}}=2.45\mathrm{K}$) is most likely driven by electron-phonon coupling rather than dimer fluctuations mediated pairing.

Formation of vortex rings around moving spherical objects in superfluid ${}^4\mathrm{He}$ at 0 K is modeled by time-dependent density functional theory. The simulations provide detailed information of the microscopic events that lead to vortex ring emission through characteristic observables, such as liquid current circulation, drag force, and hydrodynamic mass. A series of simulations was performed to determine velocity thresholds for the onset of dissipation as a function of the sphere radius up to 1.8 nm and at external pressures of zero and 1 bar. The threshold was observed to decrease with the sphere radius and increase with pressure thus showing that the onset of dissipation does not involve roton emission events (Landau critical velocity) but rather vortex emission (Feynman critical velocity), which is also confirmed by the observed periodic response of the hydrodynamic observables as well as visualization of the liquid current circulation. An empirical model, which considers the ratio between the boundary layer kinetic and vortex ring formation energies, is presented for extrapolating the current results to larger length scales. The calculated critical velocity value at zero pressure for a sphere that mimics an electron bubble is in good agreement with the previous experimental observations at low temperatures. The stability of the system against symmetry breaking was linked to its ability to excite quantized Kelvin waves around the vortex rings during the vortex shedding process. At high vortex ring emission rates, the downstream dynamics showed complex vortex ring fission and reconnection events that appear similar to those seen in previous Gross-Pitaevskii theory-based calculations and which mark the onset of turbulent behavior.

Recent computational studies have predicted that rare-earth superhydrides are promising high-temperature superconductors. Of these phases having very high hydrogen content ($\mathrm{X}{\mathrm{H}}_n,n>6$, where X is the rare-earth atom) a cubic phase of lanthanum hydride, recently synthesized at 170 GPa and identified as $\mathrm{La}{{\mathrm{H}}_{10}}_{\pm x}$, is in good agreement with theoretical predictions. The experiments found that the stability of the phase extended to lower pressure and a distorted form was found on decompression. Here we examine the nuclear quantum effects and anharmonic dynamics on $\mathrm{La}{\mathrm{H}}_{10}$, including the behavior of the hydrogen sublattice in comparison with the predicted atomic metallic hydrogen at higher pressure. We also examine the vibrational dynamics and electronic properties of a distorted lower pressure phase of $\mathrm{La}{\mathrm{H}}_{10}$ and find that superconducting $T_c$ decreases relative to cubic but remains relatively high (i.e., 229–245 K).

The Kitaev model of spin-1/2 on a honeycomb lattice supports degenerate topological ground states and may be useful in topological quantum computation. ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ with a honeycomb lattice of Ir ions has been extensively studied as a candidate for the realization of this model, due to the effective $J_{\text{eff}}=1/2$ low-energy excitations produced by the spin-orbit and crystal-field effect. As the eventual realization of the Kitaev model has remained evasive, it is highly desirable and challenging to tune the candidate materials toward such an end. It is well known that external pressure often leads to dramatic changes in the geometric and electronic structure of materials. In this Rapid Communication, the high-pressure phase diagram of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ is examined by first-principles calculations. It is found that ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ undergoes a sequence of structural and magnetic phase transitions, from a magnetically ordered phase with space group $C2/m$ to two bond-ordered nonmagnetic phases. The low-energy excitations in these high-pressure phases can be well described by the $J_{\text{eff}}=1/2$ states.

Using time-resolved second harmonic generation (TRSHG) and optical transmission, we demonstrate simultaneous optical access to both magnetic and ferroelectric order parameters in a magnetoelectric multiferroic ${\mathrm{GdFeO}}_3$ below the transition temperature 2.2 K. In magnetoelectric phase, the pump pulses impulsively generate polar phonon modes of $B_2$ and $A_1$ symmetry and a quasiferromagnetic resonance mode, creating coherent lattice and spin motions. While collective motions associated with the ferroelectric and antiferromagnetic ordering are simultaneously excited, only the polar motion is detected in TRSHG, resulting in a direct subpicosecond excitation of the improper ferroelectric order. Our result also experimentally indicates that the soft mode in improper ferroelectrics is IR inactive above the phase transition, unlike proper ferroelectrics. Furthermore, we investigate the crystal polar symmetry and potential coupling between the polar phonons and the quasiantiferromagnetic resonance mode, which can shed light on the studies of subpicosecond magnetoelectric coupling based on collective quasiparticle excitations.

We study nonequilibrium dynamics of integrable and nonintegrable closed quantum systems whose unitary evolution is interrupted with stochastic resets, characterized by a reset rate $r$, that project the system to its initial state. We show that the steady-state density matrix of a nonintegrable system, averaged over the reset distribution, retains its off-diagonal elements for any finite $r$. Consequently a generic observable $\widehat{O}$, whose expectation value receives a contribution from these off-diagonal elements, never thermalizes under such dynamics for any finite $r$. We demonstrate this phenomenon by exact numerical studies of experimentally realizable models of ultracold bosonic atoms in a tilted optical lattice. For integrable Dirac-like fermionic models driven periodically between such resets, the reset-averaged steady state is found to be described by a family of generalized Gibbs ensembles characterized by $r$. We also study the spread of particle density of a noninteracting one-dimensional fermionic chain, starting from an initial state where all fermions occupy the left half of the sample, while the right half is empty. When driven by resetting dynamics, the density profile approaches at long times to a nonequilibrium stationary profile that we compute exactly. We suggest concrete experiments that can possibly test our theory.

The rich interaction phenomena at antiferromagnet (AFM)/ferromagnet (FM) interfaces are key ingredients in AFM spintronics, where many underlying mechanisms remain unclear. Here we report on a correlation observed between the interfacial Dzyaloshinskii-Moriya interaction (DMI) $D_{\mathrm{S}}$ and effective spin-mixing conductance $g_{\mathrm{eff}}^{\uparrow \downarrow }$ at the IrMn/CoFeB interface. Both $D_{\mathrm{S}}$ and $g_{\mathrm{eff}}^{\uparrow \downarrow }$ are quantitatively determined with Brillouin light-scattering measurements, and increase with IrMn thickness in the range of $2.5\sim 7.5$ nm. Such correlation likely originates from the AFM-states-mediated spin-flip transitions in the FM, which promote both the interfacial DMI and spin-pumping effect. Our findings provide deeper insight into the AFM-FM interfacial coupling for future spintronic design.

Motivated by the experimental report of unusual low-temperature magnetism in the quasi-one-dimensional magnet ${\mathrm{CuInVO}}_5$, we present results of a cluster mean-field study on a spin-$1/2$ Heisenberg model with alternating ferromagnetic and antiferromagnetic nearest-neighbor coupling. We map out the ground-state phase diagrams with varying model parameters, including the effect of an external magnetic field. An unexpected competition between different spin-spin correlations is uncovered. Multiple spin-flop transitions are identified with the help of component-resolved correlation functions. For the material-specific choice of model parameters we discuss the temperature dependence of specific heat and magnetic susceptibility and compare our results with the available experimental data. A detailed account of spin-spin correlations allows us to present a microscopic understanding of the low-temperature magnetic ordering in ${\mathrm{CuInVO}}_5$. Most notably, we identify the origin of an extra peak in the low-temperature specific heat data of ${\mathrm{CuInVO}}_5$ reported by Hase et al. [Phys. Rev. B 94, 174421 (2016)].

We present temperature-dependent infrared spectroscopic studies of ${\mathrm{CaMnSb}}_2$, a proposed three-dimensional topological material. The low plasma edge in the reflectivity spectrum and small Drude component in the optical conductivity indicate a very low carrier density. The low-frequency optical conductivity is well described by the superposition of narrow and broad Drude terms. Several linear components have been observed in the low-temperature optical conductivity, but none of them extrapolate to the origin, at odds with the optical response expected for three-dimensional Dirac fermions. A series of absorption peaks have been resolved in the high-frequency optical conductivity. The energy of these peaks agrees well with the interband transitions expected for the band structures from first-principles calculations. Intriguingly, the lowest band gap increases with decreasing temperature, mimicking the temperature evolution of inverted bands. Furthermore, our theoretical calculations demonstrate the existence of weak coupling between two Sb-chain layers results in the topological trivial surface states in ${\mathrm{CaMnSb}}_2$.

It is known that compressive or tensile planar strain determines the band structure topology of the diamond cubic allotrope of tin ($\alpha$-Sn) to be either a Dirac semimetal (DSM) or topological insulator (TI), respectively. Using analytical techniques, we show that quantum confinement in negatively strained CdTe/$\alpha$-Sn quantum wells induces a phase transition from the DSM state to a quasi-three-dimensional TI as $\alpha$-Sn thickness is decreased. The critical thickness of the transition is found to depend strongly on the orientation of the CdTe, and magnitude of the epitaxial strain, which can be experimentally tuned. This result provides a new rationale for why $\alpha$-Sn has been observed in a three-dimensional (3D) TI state when pseudomorphically grown on zincblende substrates possessing smaller lattice constants. Additional impacts of strain and orientation on the electronic properties and spin texture of $\alpha$-Sn are explored. In particular, we examine the impact of orientation on the critical length of the Bernevig-Hughes-Zhang transition from a 2D TI to trivial insulator, as well as on the Rashba spin orbit coupling effects of an external electric field.

We have carried out the imaging of the electron momentum density of Co $3d$ electrons in ${\mathrm{LaCoO}}_3$ using x-ray Compton scattering, in order to investigate the electron-orbital states relevant to the spin-crossover phenomenon at around 500 K, where an insulator-metal transition takes place. The electron momentum density reconstructed from the Compton profiles indicates the symmetry change in the Co $3d$ electron-orbital states between below and above 500 K, which reveals the electron transfer from $t_{2g}$ to $e_g$ orbitals similar to that of the spin crossover around 100 K. The magnitude of the anisotropy change in the Compton profiles exhibits a steep increase at around 500 K, implying a cooperative characteristics of the 500-K spin-crossover phenomenon. The Compton profiles also show an increment of the molecular orbital feature by the development of the hybridization between Co $3d$ and O $2p$ orbitals with increasing temperature.