Single crystal synchrotron x-ray diffraction as a function of temperature and pressure has revealed a complex biphase mixture in superconducting FeSe. Based on our experimental results, we construct a phase diagram where structural behavior and superconducting properties of FeSe are found to be correlated. We show that below 6 GPa, where pressure promotes the superconducting critical temperature, the FeSe structure is composed of two-dimensional layers of edge-shared $\mathrm{FeS}{\mathrm{e}}_4$ tetrahedra, while above 6 GPa, the superconductivity is strongly suppressed on formation of a new orthorhombic polymorph characterized by a three-dimensional network of face sharing $\mathrm{FeS}{\mathrm{e}}_6$ octahedra. Therefore, changes in topology and connectivity of the FeSe structure are found to be detrimental to superconductivity. The previously controversial crystal structure of the high-pressure polymorph of FeSe was unambiguously determined in the present paper. High-pressure FeSe adopts an orthorhombic MnP-type structure (Pnma) which corresponds to a slightly distorted hexagonal NiAs-type arrangement ($P{6}_3/mmc$). The structural transformation from the low- to high-pressure FeSe polymorph is first order in nature and is manifested as antiparallel displacements within the Fe and Se sublattices.

The valley has been exploited as a binary degree of freedom to realize valley-selective Hall transport and circular dichroism in two-dimensional layered materials, in which valley-contrasting physics is indispensable in making the valley index an information carrier. In this Rapid Communication, we reveal valley-contrasting physics in all-dielectric valley photonic crystals. The link between the angular momentum of light and the valley state is discussed, and unidirectional excitation of the valley chiral bulk state is realized by sources carrying orbital angular momentum with proper chirality. Characterized by the nonzero valley Chern number, valley-dependent edge states and the resultant broadband robust transport is found in such an all-dielectric system. Our work has potential in the orbital angular momentum assisted light manipulation and the discovery of valley-protected topological states in nanophotonics and on-chip integration.

$\mathrm{Ti}{\mathrm{O}}_2$ in the rutile phase is known to be an incipient ferroelectric. Considering Nb-Cr codoping we examine if ferroelectricity can be induced at the low doping limit in $\mathrm{T}{\mathrm{i}}_{\left(1\text{−}x\right)}{\left(\mathrm{N}{\mathrm{b}}_{0.5}\mathrm{C}{\mathrm{r}}_{0.5}\right)}_x{\mathrm{O}}_2$ ($x=0.05\%$, 1%, 5%, and 10%). A relaxor behavior is found in the temperature range 20–120 K which obeys the Vogel-Fulcher relation while pyrocurrent measurements confirm switching of the electric polarization. The spontaneous net polarization is doping dependent with a maximum at 1% and for doping concentrations above 5% is found to be paraelectric. Ab initio density functional theory based calculations suggest that the Nb-Cr pair behaves like a dipole and polarizes the neighboring $\mathrm{Ti}{\mathrm{O}}_6$ octahedra, stabilizing a ferroelectric ground state akin to magnetic impurities in dilute magnetic semiconductors. At larger doping concentrations one finds that Nb-Cr clusters result in a vanishing polarization.

We investigate the ultrafast dynamics of the atomic angular momentum in ferrimagnets irradiated by laser pulses. Our study is based on a quantum atomistic approach and the particle dynamics is performed using a Monte Carlo technique. We focus on microscopic mechanisms that lead to the dissipation of the total angular momentum in a rare earth–transition metal (RE-TM) alloy in which the two sublattices have opposite spin orientation. We describe the coherent transfer of atomic angular momentum between the spin and the orbital momentum. The orbital momentum quenching induced by the lattice field and the Elliott-Yafet collision mechanism are also included. The simulations show that the observed ultrafast magnetization quenching may be explained at a microscopical level by the combined effects of the coherent spin transfer between the RE and the TM sublattices along with the quenching of the localized orbital angular momentum induced by the lattice field.

We study the interplay between disorder and interactions for emergent bosonic degrees of freedom induced by an external magnetic field in the Br-doped spin-gapped antiferromagnetic material $\mathrm{Ni}{\left({\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x\right)}_2-4\mathrm{SC}{\left({\mathrm{NH}}_2\right)}_2$ ($\mathrm{DTN}X$). Building on nuclear magnetic resonance experiments at high magnetic field [A. Orlova et al., Phys. Rev. Lett. 118, 067203 (2017)], we describe the localization of isolated impurity states, providing a realistic theoretical modeling for $\mathrm{DTN}X$. Going beyond single impurity localization we use quantum Monte Carlo simulations to explore many-body effects from which pairwise effective interactions lead to a (impurity-induced) Bose-Einstein condensation (BEC) revival [M. Dupont, S. Capponi, and N. Laflorencie, Phys. Rev. Lett. 118, 067204 (2017)]. We further address the question of the existence of a many-body localized Bose-glass (BG) phase in $\mathrm{DTN}X$, which is found to compete with a series of a new kind of BEC regimes made out of the multi-impurity states. The global magnetic field–temperature phase diagram of $\mathrm{DTN}X$ reveals a very rich structure for low impurity concentration, with consecutive disorder-induced BEC minidomes separated by intervening many-body localized BG regimes. Upon increasing the impurity level, multiple mini-BEC phases start to overlap, while intermediate BG regions vanish.

We study the Josephson effect in a trijunction formed by two topological superconductor (TS) wires and a conventional $s$-wave superconductor. Using a boundary Green's function formalism, analytical results for the current-phase relation are obtained in various limiting cases by modeling the TS wires via the low-energy limit of a Kitaev chain. We show that Josephson transport critically depends on the spin canting angle $\theta$ between the boundary spin polarizations of the TS wires, which in turn suggests that the spin structure of Majorana states can be accessed through supercurrent measurements. We also extend the boundary Green's function approach to a more microscopic spinful wire model and thereby compute the dependence of $\theta$ on experimentally accessible parameters such as the Zeeman field and/or the chemical potential. Furthermore, we show that the equilibrium current-phase relation between both TS wires exhibits a robust $4\pi$ periodicity since the conventional superconducting lead effectively locks the fermion parity of the trijunction.

We theoretically study static and dynamic properties of short Josephson junctions (JJs) with singlet and triplet Josephson coupling. In singlet Josephson weak links, two singlet superconductors S are connected with each other by a normal film (N) or wire. Triplet JJs, which we denote ${\mathrm{S}}_{\text{m}}$-N(F)-${\mathrm{S}}_{\text{m}}$, are formed by two singlet BCS superconductors covered by a thin layer of a weak ferromagnet ${\mathrm{F}}_{\text{w}}$. These superconductors ${\mathrm{S}}_{\text{m}}$ are separated from the N (or F) layer by spin filters, which pass electrons with only one spin orientation. The triplet Cooper pairs propagating from the left (right) superconductors ${\mathrm{S}}_{\text{m}}$ differ from each other not only in polarizations, but also in chiralities. The latter is determined by the magnetization orientation in weak ferromagnets ${\mathrm{F}}_{\text{w}}$. We obtain analytical formulas for the critical Josephson current in both types of JJs. If the chiralities of the triplet Cooper pairs penetrating into the N film in ${\mathrm{S}}_{\text{m}}$-N(F)-${\mathrm{S}}_{\text{m}}$ JJs from the left and right ${\mathrm{S}}_{\text{m}}$ are different, the Josephson current is not 0 in the absence of the phase difference (spontaneous Josephson current). We also calculate the admittance $Y\left(\mathrm{\Omega }\right)$ for arbitrary frequencies $\mathrm{\Omega }$ in the case of singlet JJs and for low frequencies in the case of triplet JJs. At low temperatures $T$, the real part of the admittance $Y^{\prime }\left(\mathrm{\Omega }\right)$ in singlet JJs starts to increase from 0 at $\hslash \mathrm{\Omega }\ge {\mathrm{\Delta }}_{\text{sg}}$, but at $T\ge {\mathrm{\Delta }}_{\text{sg}}$, it has a peak at low frequencies the magnitude of which is determined by inelastic processes. The subgap ${\mathrm{\Delta }}_{\text{sg}}$ depends on the transparencies of the S/N interfaces and on the phase difference $2{\chi }_0$. The low-frequency peak in $Y^{\prime }\left(\mathrm{\Omega }\right)$ in triplet JJs disappears.

We calculate the coefficient of thermal expansion (CTE) in tungsten-rhenium random alloys for Re concentrations between 0% and 50% and for temperatures up to 2400 K by employing the quasiharmonic approximation within the ab initio framework of density functional theory. We treat chemical disorder by the virtual crystal approximation and compute the phonon density of states at two levels of sophistication. While the traditional Debye-Grüneisen (DG) model fails to account for the experimentally observed increase in CTE upon Re addition for concentrations above 10% Re, explicit phonon calculations within density functional perturbation theory lead to an overall good agreement with experiment. Thereby we identify the pronounced phonon softening and anisotropy between transversal and longitudinal modes in W-Re to be responsible for the breakdown of the DG model.

The energy spectrum of the valence band in $\mathrm{HgTe}/{\mathrm{Cd}}_x{\mathrm{Hg}}_{1-x}\mathrm{Te}$ quantum wells of a width ($8–20$) nm has been studied experimentally by magnetotransport effects and theoretically in the framework of a four-band $kP$ method. Comparison of the Hall density with the density found from a period of the Shubnikov–de Haas (SdH) oscillations clearly shows that the degeneracy of states of the top of the valence band is equal to 2 at the hole density $p<5.5\times {10}^{11}{\mathrm{cm}}^{-2}$. Such degeneracy does not agree with the calculations of the spectrum performed within the framework of the four-band $kP$ method for symmetric quantum wells. These calculations show that the top of the valence band consists of four spin-degenerate extremes located at $k\ne 0$ (valleys) which gives the total degeneracy $K=8$. It is shown that taking into account the “mixing of states” at the interfaces leads to the removal of the spin degeneracy that reduces the degeneracy to $K=4$. Accounting for any additional asymmetry, for example, due to the difference in the mixing parameters at the interfaces, the different broadening of the boundaries of the well, etc., leads to reduction of the valleys degeneracy, making $K=2$. It is noteworthy that for our case twofold degeneracy occurs due to degeneracy of two single-spin valleys. The hole effective mass ($m_h$) determined from analysis of the temperature dependence of the amplitude of the SdH oscillations shows that $m_h$ is equal to $(0.25\pm 0.02)m_0$ and weakly increases with the hole density. Such a value of $m_h$ and its dependence on the hole density are in a good agreement with the calculated effective mass.

The electronic properties of epitaxial oxide thin films grown on compound semiconductors are largely determined by the interfacial atomic structure, as well as the thermodynamic conditions during synthesis. Ferroelectric polarization and Fermi-level pinning in ${\mathrm{SrTiO}}_3$ films have been attributed to the presence of oxygen vacancies at the oxide/semiconductor interface. Here, we present scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy analyses of GaAs films grown on ${\mathrm{SrTiO}}_3$ combined with first-principles calculations to determine the atomic and electronic structures of the ${\mathrm{SrTiO}}_3/\mathrm{GaAs}$ interfaces. An atomically abrupt SrO/As interface is observed and the interfacial SrO layer is found to be O-deficient. First-principles density functional theory (DFT) calculations show SrO/Ga and Sr/As interfaces are favorable under O-rich and O-poor conditions, respectively. The SrO/Ga interface is reconstructed via the formation of Ga-Ga dimers while the Sr/As interface is abrupt and consistent with the experiment. DFT calculations further reveal that intrinsic two-dimensional electron gas (2DEG) forms in both SrO/Ga and Sr/As interfaces, and the Fermi level is pinned to the localized 2DEG states. Interfacial O vacancies can enhance the 2DEG density while it is possible for Ga/As vacancies to unpin the Fermi level from the 2DEG states.

Recently, single-layer group III monochalcogenides have attracted both theoretical and experimental interest at their potential applications in photonic devices, electronic devices, and solar energy conversion. Excited by this, we theoretically design two kinds of highly stable single-layer group IV-V ($\mathrm{IV}=\mathrm{Si},\mathrm{Ge}$, and Sn; $\mathrm{V}=\mathrm{N}$ and P) and group V-IV-III-VI ($\mathrm{IV}=\mathrm{Si},\mathrm{Ge}$, and Sn; $\mathrm{V}=\mathrm{N}$ and P; $\mathrm{III}=\mathrm{Al},\mathrm{Ga}$, and In; $\mathrm{VI}=\mathrm{O}$ and S) compounds with the same structures with single-layer group III monochalcogenides via first-principles simulations. By using accurate hybrid functional and quasiparticle methods, we show the single-layer group IV-V and group V-IV-III-VI are indirect bandgap semiconductors with their bandgaps and band edge positions conforming to the criteria of photocatalysts for water splitting. By applying a biaxial strain on single-layer group IV-V, single-layer group IV nitrides show a potential on mechanical sensors due to their bandgaps showing an almost linear response for strain. Furthermore, our calculations show that both single-layer group IV-V and group V-IV-III-VI have absorption from the visible light region to far-ultraviolet region, especially for single-layer SiN-AlO and SnN-InO, which have strong absorption in the visible light region, resulting in excellent potential for solar energy conversion and visible light photocatalytic water splitting. Our research provides valuable insight for finding more potential functional two-dimensional semiconductors applied in optoelectronics, solar energy conversion, and photocatalytic water splitting.

Here, we present a comprehensive analysis of the effective medium approximation (EMA) breakdown in all-dielectric deep-subwavelength multilayers made of alternating layers by means of the transfer matrix method. We demonstrated that the approximation is invalid at the vicinity of the effective medium's critical angle for total internal reflection and obtained an analytical criterion for the breakdown of the EMA, which depends on the layer thickness, the incident angle, and the permittivity difference between the alternate layers. We rebuilt the EMA by adding higher-order correction onto the traditional effective permittivity. Furthermore, we found that the EMA breakdown that arises from the boundary effect cannot be repaired in the traditional homogenization strategy with only one layer of effective medium. By adding an artificial matched layer after the conventional effective layer, the boundary effect breakdown was neatly removed.

We employ two-dimensional (2D) coherent, nonlinear spectroscopy to investigate couplings within individual InAs quantum dots (QDs) and QD molecules. Swapping pulse ordering in a two-beam sequence permits one to distinguish between rephasing and nonrephasing four-wave mixing (FWM) configurations. We emphasize the nonrephasing case, allowing one to monitor two-photon coherence dynamics. Respective Fourier transform yields a double quantum 2D FWM map, which is corroborated with its single quantum counterpart, originating from the rephasing sequence. We introduce referencing of the FWM phase with the one carried by the driving pulses, overcoming the necessity of its active stabilization, as required in 2D spectroscopy. Combining single and double quantum 2D FWM provides a pertinent tool in detecting and ascertaining coherent coupling mechanisms between individual quantum systems, as exemplified experimentally.

Despite its extremely weak intrinsic spin-orbit coupling (SOC), graphene has been shown to acquire considerable SOC by proximity coupling with exfoliated transition metal dichalcogenides (TMDs). Here we demonstrate strong induced Rashba SOC in graphene that is proximity coupled to a monolayer TMD film, $\mathrm{Mo}{\mathrm{S}}_2$ or $\mathrm{WS}{\mathrm{e}}_2$, grown by chemical-vapor deposition with drastically different Fermi level positions. Graphene/TMD heterostructures are fabricated with a pickup-transfer technique utilizing hexagonal boron nitride, which serves as a flat template to promote intimate contact and therefore a strong interfacial interaction between TMD and graphene as evidenced by quenching of the TMD photoluminescence. We observe strong induced graphene SOC that manifests itself in a pronounced weak-antilocalization (WAL) effect in the graphene magnetoconductance. The spin-relaxation rate extracted from the WAL analysis varies linearly with the momentum scattering time and is independent of the carrier type. This indicates a dominantly Dyakonov-Perel spin-relaxation mechanism caused by the induced Rashba SOC. Our analysis yields a Rashba SOC energy of ∼1.5 meV in graphene/$\mathrm{WS}{\mathrm{e}}_2$ and ∼0.9 meV in graphene/$\mathrm{Mo}{\mathrm{S}}_2$. The nearly electron-hole symmetric nature of the induced Rashba SOC provides a clue to possible underlying SOC mechanisms.

Materials with large magnetocrystalline anisotropy and strong electric field effects are highly needed to develop new types of memory devices based on electric field control of spin orientations. Instead of using modified transition metal films, we propose that certain monolayer transition metal dichalcogenides are the ideal candidate materials for this purpose. Using density functional calculations, we show that they exhibit not only a large magnetocrystalline anisotropy (MCA), but also colossal voltage modulation under an external field. Notably, in some materials such as ${\mathrm{CrSe}}_2$ and ${\mathrm{FeSe}}_2$, where spins show a strong preference for in-plane orientation, they can be switched to an out-of-plane direction. This effect is attributed to the large band character alteration that the transition metal $d$ states undergo around the Fermi energy due to the electric field. We further demonstrate that strain can also greatly change MCA, and can help to improve the modulation efficiency when combined with an electric field.

In this work, we report the pressure dependence of the effective Coulomb interaction parameters (Hubbard $U$) in paramagnetic NiO within the constrained random phase approximation (cRPA). We consider five different low-energy models starting from the most expensive one that treats both Ni-$d$ and O-$p$ states as correlated orbitals ($dp-dp$ model) to the smallest possible two-orbital model comprising the $e_g$ states only ($e_g-e_g$ model). We find that in all the considered models, the bare interactions are not very sensitive to the compression. However, the partially screened interaction parameters show an almost linear increment as a function of compression, resulting from the substantial weakening of screening effects upon compression. This counterintuitive trend is explained from the specific characteristic changes of the basic electronic structure of this system. We further calculate the nearest-neighbor intersite $d-d$ interaction terms, which also show substantial enhancement due to compression. The computed interaction parameters for antiferromagnetic NiO are almost identical to their paramagnetic counterparts. Results for other prototypical $3d$ transition metal monoxides, FeO, CoO, and CuO, further demonstrate that the monotonic increase of the partially screened interaction parameters under the application of pressure is a generic characteristic of transition metal monoxides. Finally, the effective parameters of $3d$ states grow as a function of the atomic number of the transition metal ion.

We present exact solutions for some eigenstates of hopping models on one- and two-dimensional quasiperiodic tilings and show that they are “critical” states, by explicitly computing their multifractal spectra. These eigenstates are shown to be generically present in 1D quasiperiodic chains, of which the Fibonacci chain is a special case. We then describe properties of the ground states for a class of tight-binding Hamiltonians on the 2D Penrose and Ammann-Beenker tilings. Exact and numerical solutions are seen to be in good agreement.

CeRhSn with a quasikagome lattice of Ce atoms in the hexagonal $c$ plane has been expected to be in close vicinity to a zero-field quantum criticality derived from magnetic frustration. We have studied how the ground state changes with substitution of Pd for Rh in $\mathrm{CeR}{\mathrm{h}}_{1-x}\mathrm{P}{\mathrm{d}}_x\mathrm{Sn}$ $\left(x\le 0.75\right)$ by measuring the specific heat $C$, magnetic susceptibilities ${\chi }_{\mathrm{dc}}$ and ${\chi }_{\mathrm{ac}}$, magnetization $M$, electrical resistivity ρ, and magnetoresistance. For $x=0$, the field dependence of ${\chi }_{\mathrm{ac}}$ at $T=0.03\mathrm{K}$ shows a peak at $B\parallel a=3.5\mathrm{T}$, confirming the spin-flop crossover in the field applied along the hard axis. The temperature dependence of ${\chi }_{\mathrm{ac}}$ shows a broad maximum at 0.1 K whereas $C/T$ continues to increase down to 0.08 K. For $x\geqq 0.1,\rho \left(T\right)$ is dominated by incoherent Kondo scattering and both $C/T$ and ${\chi }_{\mathrm{ac}}\left(T\right)$ exhibit peaks, indicating the development of an antiferromagnetic order. The ordering temperature rises to 2.5 K as $x$ is increased to 0.75. Our results indicate that the ground state in the quasikagome Kondo lattice $\mathrm{CeR}{\mathrm{h}}_{1-x}\mathrm{P}{\mathrm{d}}_x\mathrm{Sn}$ leaves the quantum critical point at $x=0$ with increasing $x$ as a consequence of suppression of both the magnetic frustration and Kondo effect.

Topological semimetals have been at the forefront of experimental and theoretical attention in condensed-matter physics. Among these, recently discovered Weyl semimetals have a dispersion described by a three-dimensional Dirac cone, which is at the root of exotic physics such as the chiral anomaly in magnetotransport. In a time-reversal symmetric (TRS) Weyl semimetal film, the confinement gap gives the quasiparticles a mass, while TRS is preserved by having an even number of valleys with opposite masses. The film can be tuned through a topological phase transition by a gate electric field. In this work, we present a theoretical study of the quantum corrections to the conductivity of a topological semimetal thin film, which is governed by the complex interplay of the chiral band structure, mass term, and scalar and spin-orbit scattering. We study scalar and spin-orbit scattering mechanisms on the same footing, demonstrating that they have a strong qualitative and quantitative impact on the conductivity correction. We show that, due to the spin structure of the matrix Green's functions, terms linear in the extrinsic spin-orbit scattering are present in the Bloch and momentum relaxation times, whereas previous works had identified corrections starting from the second order. In the limit of small quasiparticle mass, the terms linear in the impurity spin-orbit coupling lead to a potentially observable density dependence in the weak antilocalization correction. Moreover, when the mass term is around 30% of the linear Dirac terms, the system is in the unitary symmetry class with zero quantum correction, and switching the extrinsic spin-orbit scattering drives the system to the weak antilocalization. We discuss the crossover between the weak localization and weak antilocalization regimes in terms of the singlet and triplet Cooperon channels, and analyze this transition as a function of the mass and spin-orbit scattering strength. Experimental schemes to detect this transition are discussed.