In this paper, we present a novel route to manipulate the spontaneous valley coherence in two-dimensional valleytronic materials interfaced with other layered materials hosting anisotropic polaritonic modes. We propose two implementations—one, using anisotropic plasmons in phosphorene and another with hyperbolic phonon polaritons in $\alpha \text{−}{\mathrm{MoO}}_3$. In particular, we show the electrostatic tunability of the spontaneous valley coherence achieving robust valley coherence values in the near-infrared wavelengths at room temperature. The tunability of this valley coherence shown in these heterostructures would enable the realization of active valleytronic quantum circuitry.

$\beta \text{−}\mathrm{PdB}{\mathrm{i}}_2$ has attracted much attention for its prospective ability to possess simultaneously topological surface and superconducting states due to its unprecedented spin-orbit interaction (SOC). Whereas most works have focused solely on investigating its topological surface states, the coupling between spin and charge degrees of freedom in this class of quantum material remains unexplored. Here, we report a study of spin-to-charge conversion in a $\beta \text{−}\mathrm{PdB}{\mathrm{i}}_2$ ultrathin film grown by molecular beam epitaxy, utilizing a spin pumping technique to perform inverse spin Hall effect measurements. We find that the room temperature spin Hall angle of $\mathrm{Fe}/\beta \text{−}\mathrm{PdB}{\mathrm{i}}_2$, ${\theta }_{SH}=0.037$. This value is one order of magnitude larger than that of reported conventional superconductors, and is comparable to that of the best SOC metals and topological insulators. Our results provide an avenue for developing superconductor-based spintronic applications.

The high-pressure response of titanium dioxide (${\mathrm{TiO}}_2$) is of interest because of its numerous industrial applications and its structural similarities to silica (${\mathrm{SiO}}_2$). We used three platforms—Sandia's Z machine, Omega Laser Facility, and density-functional theory-based quantum molecular dynamics (QMD) simulations—to study the equation of state (EOS) of ${\mathrm{TiO}}_2$ at extreme conditions. We used magnetically accelerated flyer plates at Sandia to measure Hugoniot of ${\mathrm{TiO}}_2$ up to pressures of 855 GPa. We used a laser-driven shock wave at Omega to measure the shock temperature in ${\mathrm{TiO}}_2$. Our Z data show that rutile ${\mathrm{TiO}}_2$ reaches 2.2-fold compression at a pressure of 855 GPa and Omega data show that ${\mathrm{TiO}}_2$ is a reflecting liquid above 230 GPa. The QMD simulations are in excellent agreement with the experimental Hugoniot in both pressure and temperature. A melt curve for ${\mathrm{TiO}}_2$ is also proposed based on the QMD simulations. The combined experimental results show ${\mathrm{TiO}}_2$ is in a liquid at these explored pressure ranges and is not highly incompressible as suggested by a previous study.

The interfacial Dzyaloshinskii-Moriya interaction (DMI) is of great interest as it can stabilize chiral spin structures in thin films. Experiments verifying the orientation of the interfacial DMI vector remain rare, in part due to the difficulty of separating vector components of DMI. In this study, Fe/Ni bilayers and Co/Ni multilayers were deposited epitaxially onto Cu(001) and Pt(111) substrates, respectively. By tailoring the effective anisotropy, spin reorientation transitions (SRTs) are employed to probe the orientation of the DMI vector by measuring the spin structure of domain walls on both sides of the SRTs. The interfacial DMI is found to be sufficiently strong to stabilize chiral Néel walls in the out-of-plane magnetized regimes, while achiral Néel walls are observed in the in-plane magnetized regimes. These findings experimentally confirm that the out-of-plane component of the DMI vector is insignificant in these fcc(001) and fcc(111) oriented interfaces, even in the presence of atomic steps.

Recent experiments on disordered superconductors find that the superfluid density $n_s\left(T\right)$ decreases dramatically and characteristically with disorder, differently from what is expected for a mean order parameter field or BCS, amplitude only, picture for the superconductor. We describe here a new microscopic free energy functional which explicitly describes its dependence on both the amplitude and phase of superconducting order, in a gauge invariant manner. We use this here in an approximation of noninteracting phase fluctuations (Gaussian or harmonic approximation) to obtain $n_s\left(T\right)$ in the presence of (static) disorder. We compare our results successfully with experiment.

We study a two-dimensional heterostructure comprised of a monolayer of the magnetic insulator chromium triiodide (${\mathrm{CrI}}_3$) on a superconducting lead (Pb) substrate. Through first-principles computation and a tight-binding model, we demonstrate that charge transfer from the Pb substrate dopes the ${\mathrm{CrI}}_3$ into an effective half-metal, allowing for the onset of a gapless topological superconductivity phase via the proximity effect. This phase, in which there exists a superconducting gap only in part of the Fermi surface, is shown to occur generically in two-dimensional (2D) half-metal-superconductor heterostructures which lack twofold in-plane rotational symmetry. However, a sufficiently large proximity-induced pairing amplitude can bring such a system into a fully gapped topological superconducting (TSC) phase. As such, these results are expected to better define the optimal 2D component materials for future proposed TSC heterostructures.

The topological classification of matter has been extended to include semimetallic phases characterized by the presence of topologically protected band degeneracies. In Weyl semimetals, the foundational gapless topological phase, chiral degeneracies are isolated near the Fermi level and give rise to the Fermi-arc surface states. However, it is now recognized that chiral degeneracies are ubiquitous in the band structures of systems with broken spatial inversion ($\mathcal{P}$) or time-reversal ($\mathcal{T}$) symmetry. This leads to a broadly defined notion of topological metals, which implies the presence of disconnected Fermi surface sheets characterized by nonzero Chern numbers inherited from the enclosed chiral degeneracies. Here, we address the possibility of experimentally observing surface-related signatures of chiral degeneracies in metals. As a representative system we choose bcc iron, a well-studied archetypal ferromagnetic metal with two nontrivial electron pockets. We find that the (110) surface presents arclike resonances attached to the topologically nontrivial electron pockets. These Fermi-arc resonances are due to two different chiral degeneracies, a type-I elementary Weyl point and a type-II composite (Chern numbers $\pm 2$) Weyl point, located at slightly different energies close to the Fermi level. We further show that these surface resonances can be controlled by changing the orientation of magnetization, eventually being eliminated following a topological phase transition. Our study thus shows that the intricate Fermi-arc features can be observed in materials as simple as ferromagnetic iron and are possibly very common in polar and magnetic materials broadly speaking. Our study also provides methodological guidelines to identifying Fermi-arc surface states and resonances, establishing their topological origin and designing control protocols.

Recent discovery of two-dimensional (2D) magnetic materials has brought magnetism to the flatland and opened up exciting opportunities for the exploration of fundamental physics as well as novel device applications. Here, we predict a thermodynamically stable 2D magnetic material, ${\mathrm{K}}_2{\mathrm{CoS}}_2$, which retains its in-plane bulk antiferromagnetic (AFM) order down to the monolayer and bilayer limits. Magnetic moments $(2.5{\mu }_B/\mathrm{Co})$ are found to form a quasi-one-dimensional antiferromagnetically ordered chain of Co atoms. The nonmagnetic electronic spectrum of the monolayer film is found to host flatbands and van Hove singularities, which play a key role in stabilizing the magnetic ground state. Based on classical Monte Carlo simulations, we estimate the Neel temperature for the AFM monolayer to be $\approx 15\mathrm{K}$. Our study demonstrates that ${\mathrm{K}}_2{\mathrm{CoS}}_2$ hosts a robust AFM state which persists from the monolayer limit to the bulk material.

Construction of an effective Hamiltonian including crystal field parameters (CFPs) by an accurate ab initio technique can provide a powerful approach for the measurements of tiny magnetic fields. Here, we first calculate the crystal field parameters (CFPs) of trivalent rare-earth magnetic ions ${\mathrm{R}}^{3+}$ in lithium rare-earth tetrafluorides ${\mathrm{LiRF}}_4$ (R $=$ Tb, Dy, Ho, Er, Tm, and Yb) by the density functional theory plus the novel CFP scheme employing open-core treatment and Wannier functions. The behaviors of the real and imaginary parts of the CFPs are studied through the series of compounds. Then, by the calculated CFPs, we find the splittings of the energy levels of the $+3$ rare-earth ions by constructing an effective Hamiltonian for each case. The multiplet splittings of the $+3$ rare-earth ions are found to be consistent with those predicted by group theory and Hund's rules apart from some multiplet splitting of the ${\mathrm{Tm}}^{3+}$ and ${\mathrm{Dy}}^{3+}$ ions. For the former case, we have compared our theoretical results with the available empirical splittings of the multiplets. However, for the latter case due to the lack of experimental splittings, we have first empirically obtained the splittings of the multiplets employing the available experimental CFPs of the ${\mathrm{LiDyF}}^{3+}$:${\mathrm{Dy}}^{3+}$ single crystal and then compared our empirical data with our ab initio theoretical predictions. The deviations of these two ions from the predictions of group theory and Hund's rules are found to be consistent with the experimental data. This validates the results reported and the reliability of the procedures performed to produce them. To simplify the effective Hamiltonian by reducing the number of CFPs, it is sometimes possible to use the ${\mathrm{D}}_{\text{2d}}$ symmetry for some systems having ${\mathrm{S}}_4$ symmetry. However, by evaluating the matrix elements of the Stevens Hamiltonian term by term appeared in the Stevens CF Hamiltonian, it is shown that the actual ${\mathrm{S}}_4$ symmetry may provide more reliable results than its successor ${\mathrm{D}}_{\text{2d}}$ symmetry for the systems under study having ${\mathrm{S}}_4$ symmetry. It can be predicted that this approach can be used for developing and improving sensitive magnetometer devices which, in turn, can play a key role in diverse areas.

Two-dimensional ${\mathrm{Na}}_3\mathrm{Bi}$ is a dual topological insulator protected by time-reversal and mirror symmetry, resulting in a promising platform for device design. However, in reality, the design of topological devices is hindered by a sensitivity against disorder and temperature. We study the topological properties of ${\mathrm{Na}}_3\mathrm{Bi}$ in the presence of intrinsic defects, investigating the robustness of the edge states and the resulting transport properties. We apply a recursive Green's function technique enabling the study of disordered systems with lengths comparable to experimentally synthesized materials, in the order of micrometers. We combine our findings to propose a topological insulator device, where intrinsic defects are used to filter the response of trivial bulk states. This results in a stable conductance throughout a large range of electronic temperatures, and controllable by a perpendicular electric field. Our proposal is general, enabling the design of various dual topological insulator devices.

Isostructural transitions in layered $M{X}_2$ compounds are governed by competing van der Waals (vdW) and Coulomb interactions. While an isostructural transition (at $P\sim$ 20 GPa) has been observed before metallization in ${\mathrm{MoS}}_2$ when subjected to pressure, it is surprisingly missing in layered ${\mathrm{MoSe}}_2$ and ${\mathrm{MoTe}}_2$. Using synchrotron x-ray diffraction and Raman spectroscopic measurements of structural and vibrational properties of layered MoSSe crystals subjected to pressures up to 30 GPa and first-principles density functional theoretical analysis, we demonstrate a layer sliding isostructural transition from its $2H_c^{\prime }$ structure (space group ${P6}_{3}mc$) to a mixed-phase of $2H_a^{\prime }+2H_c^{\prime }$ at $P\sim$ 10.8 GPa, marked by discontinuity in lattice parameters, pressure coefficients of Raman modes, and accompanying changes in electronic structure. The origin of the unusually lower transition pressure of MoSSe compared with ${\mathrm{MoS}}_2$ is shown to be linked to chemical ordering of S and Se atoms on the anionic sublattice, possible because of moderate lattice mismatch between the parent compounds ${\mathrm{MoS}}_2$ and ${\mathrm{MoSe}}_2$ and large interlayer space in the vdW-bonded structure. Notably, we also report a lower-pressure transition observed at $P\sim 3$ GPa and not reported earlier in the isostructural Mo-based chalcogenides, marked by a discontinuity in the pressure coefficient of the $c/a$ ratio and indirect band gap. The transition observed at $P\sim 10.8$ GPa appears due to the change in the sign of the pressure coefficient of the direct band gap originating from inversion of the lowest-energy conduction bands. Our theoretical analysis shows that the phase transition at $P\sim 18$ GPa marked by sharp changes in pressure coefficients of $A_1$ Raman modes is associated with the metallization of the $2H_a^{\prime }$ phase.

The origin of the steplike shoulder on the high-energy side of the low-temperature photoluminescence spectrum of heavily $p$-doped GaAs is studied experimentally. It is shown that it is controlled by both the Fermi-Dirac distribution of the holes and the energy distribution of the photoexcited electrons exhibiting a sharp steplike dependence. The latter results from abrupt changes in the energy relaxation rate at the percolation threshold separating localized from delocalized electron states. A comprehensive set of optical techniques based on spin orientation of electrons, namely, the Hanle effect, time- and polarization-resolved photoluminescence, as well as transient pump-probe Faraday rotation, are used for these studies. Two different electron ensembles with substantially different lifetimes of 20 and 280 ps are identified. Their spin relaxation times are longer than 2 ns, so that the spin lifetime is limited by the electron lifetime. The relative contribution of short- and long-lived photoexcited electrons to the emission spectrum changes abruptly at the step in the high-energy photoluminescence tail. For energies above the percolation threshold, the electron states are empty due to fast energy relaxation, while for lower energies the relaxation is suppressed and the majority of photoelectrons populates the states located there.

Unidirectional reflectionless resonance observed in parity-time-symmetric systems suggests a promising strategy to produce extremely asymmetric scattering. Inspired by such a unique feature, we propose a single-sided acoustic beam splitter that splits sound incident from a specific side but is totally transparent for sound incident from the opposite side. The unidirectional response is due to a parity-time-symmetric refractive index distribution. At the exceptional point when the real-part index and gain/loss modulations are balanced, it interacts with obliquely incident waves in a single-sided manner. In addition, by engineering the sidewall boundaries within an acoustic waveguide, we provide a general approach for obtaining the required complex refractive index, in which we show that the resistive or reactive component of the sidewall impedance can independently modulate the gain/loss or the real-part index. Based on this, a planar waveguide implementation of the single-sided acoustic beam splitter is demonstrated. Our study presents opportunities enabled by exploiting parity-time-symmetric systems in a higher-dimensional space and could find applications such as sensing and communication in a wide range of wave systems including but not limited to acoustics.

We discuss the circularly polarized light (of amplitude $A_0$ and frequency $\omega$) driven thermoelectric transport properties of type-I and type-II multi-Weyl semimetals (mWSMs) in the high-frequency limit. Considering the low-energy model, we employ the Floquet-Kubo formalism to compute the thermal Hall and Nernst conductivities for both types of mWSMs. We show that the anisotropic nature of the dispersion for arbitrary integer monopole charge $n>1$ plays an important role in determining the effective Fermi surface behavior; interestingly, one can observe momentum-dependent corrections in Floquet mWSMs in addition to the momentum-independent contribution as observed for Floquet single WSMs. Apart from the nontrivial tuning of the Weyl node position $\pm Q\to \pm Q-A_0^{2n}/\omega$, our study reveals that the momentum-independent terms result in leading order contributions in the conductivity tensor. This has the form of $n$ times the single-WSM results with effective chemical potential $\mu \to \mu -A_0^{2n}/\omega$. On the other hand, momentum-dependent corrections lead to subleading order terms which are an algebraic function of $\mu$ and are present for $n>1$. Remarkably, this analysis further allows us to distinguish type-I mWSMs from their type-II counterparts. For type-II mWSMs, we find that the transport coefficients for $n\ge 2$ exhibit an algebraic dependence on the momentum cutoff in addition to the weak logarithmic dependence as noticed for $n=1$ WSMs. We demonstrate the variation and qualitative differences of transport coefficients between type-I and type-II mWSM as a function of external driving parameter $\omega$.

The perovskite ${\mathrm{TbFe}}_{0.5}{\mathrm{Cr}}_{0.5}{\mathrm{O}}_3$ shows two anomalies in its magnetic susceptibility at $T_N=257$ K and $T_{\mathrm{SR}}=190$ K which are, respectively, the antiferromagnetic and spin-reorientation transition that occur in the Fe/Cr sublattice. Magnetic susceptibility of this compound reveals canonical signatures of a Griffiths-like phase: a negative deviation from the ideal Curie-Weiss law and in less-than-unity power-law susceptibility exponents. Neutron-diffraction data analysis confirms two spin-reorientation transitions in this compound. The first one from ${\mathrm{\Gamma }}_2$ (${\mathrm{C}}_x, {\mathrm{G}}_y, {\mathrm{F}}_z$) to ${\mathrm{\Gamma }}_4$ (${\mathrm{A}}_x, {\mathrm{F}}_y, {\mathrm{G}}_z$) occurs at $T_N=257$ K and a second one from ${\mathrm{\Gamma }}_4$ (${\mathrm{A}}_x, {\mathrm{F}}_y, {\mathrm{G}}_z$) to ${\mathrm{\Gamma }}_2$ (${\mathrm{C}}_x, {\mathrm{G}}_y, {\mathrm{F}}_z$) at $T_{\mathrm{SR}}=190$ K in the $Pnma$ space-group setting. The ${\mathrm{\Gamma }}_2$ (${\mathrm{C}}_x, {\mathrm{G}}_y, {\mathrm{F}}_z$) structure is stable down to 7.7 K, leading to an ordered moment of 3.34(1) ${\mu }_{\mathrm{B}}/{\mathrm{Fe}}^{3+}({\mathrm{Cr}}^{3+}$). In addition to the long-range magnetic order, experimental indication of diffuse magnetism is observed in neutron-diffraction data at 7.7 K. Tb develops a ferromagnetic component along the $z$ axis at 20 K. Thermal conductivity and spin-phonon coupling of ${\mathrm{TbFe}}_{0.5}{\mathrm{Cr}}_{0.5}{\mathrm{O}}_3$ studied through Raman spectroscopy are also presented in the paper. The magnetic anomalies at $T_N$ and $T_{\mathrm{SR}}$ do not appear in the thermal conductivity of ${\mathrm{TbFe}}_{0.5}{\mathrm{Cr}}_{0.5}{\mathrm{O}}_3$, which appears to be robust up to 9 T. On the other hand, they are revealed in the temperature dependence of full-width-at-half-maximum curves derived from Raman intensities. An antiferromagnetic structure with $\uparrow \downarrow \uparrow \downarrow$ arrangement of Fe/Cr spins is found as the ground state through first-principles energy calculations, supporting the experimentally determined magnetic structure at 7.7 K. The spin-resolved total and partial density of states show that ${\mathrm{TbFe}}_{0.5}{\mathrm{Cr}}_{0.5}{\mathrm{O}}_3$ is insulating with a band gap of $\sim 0.12$ (2.4) eV within GGA ($\mathrm{GGA}+U$) functionals.

Voltage-controlled magnetic anisotropy (VCMA) in an atomically thin Ni film placed at an epitaxial Fe-MgO interface has been studied. We characterize the VCMA by employing Fe/Ni/MgO-based magnetic tunnel junction. Moreover, we conduct x-ray magnetic circular dichroism (XMCD) spectroscopy around Ni absorption edges under an external voltage application to the tunnel junction device. This operando XMCD spectroscopy demonstrates voltage-induced changes of orbital and effective spin magnetic moments of Ni. As compared with the previous study, where operando XMCD spectroscopy was performed on an atomically thin Co film, we find that the magnitude of the VCMA effect in Ni and Co is proportional to the voltage-induced change of the orbital magnetic moment. This study would lead to a comprehensive understanding of how an electric field affects interfacial magnetism in ferromagnetic metals.

The generic phase diagram of lightly hole doped high-$T_c$ cuprates hosts an antiferromagnetic insulating phase with well-defined spin-wave excitations. Contrary to the weak-coupling prediction, these modes persist up to the overdoped metallic regime as intense and dispersive paramagnons. Here we report on our study of the low-energy magnetic and charge excitations within the extended Hubbard model at strong coupling, using a modified $1/N$ expansion method with a variational state serving as the saddle point solution. Despite clear separation of magnetic and Hubbard-$U$ energy scales, we find that incipient Mottness affects qualitatively dispersions and widths of magnetic modes throughout the entire phase diagram. The obtained magnetic and charge dynamical structure factors agree semiquantitatively with recent resonant x-ray and neutron scattering data for ${\mathrm{La}}_{2-\mathit{x}}{\mathrm{Sr}}_{\mathit{x}}{\mathrm{CuO}}_4$ and ${(\mathrm{Bi},\mathrm{Pb})}_2{(\mathrm{Sr},\mathrm{La})}_2{\mathrm{CuO}}_{6+\delta }$ at all available doping levels. The weak-coupling random-phase approximation fails already for underdoped samples, pointing to the nontrivial intertwining of distinct energy scales in cuprate superconductors. The existence of a discrete charge mode which splits off the electron-hole continuum is also predicted.

The specific heat $C$ of the single-layer cuprate superconductor ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ was measured in an underdoped crystal with $T_{\mathrm{c}}=72$ K at temperatures down to 2 K in magnetic fields up to 35 T, a field large enough to suppress superconductivity at that doping ($p\simeq 0.09$). In the normal state at $H=35$ T, a residual linear term of magnitude $\gamma =12\pm 2$ mJ/${\mathrm{K}}^2\mathrm{mol}$ is observed in $C/T$ as $T\to 0$, a direct measure of the electronic density of states. This high value of $\gamma$ has two major implications. First, it is significantly larger than the value measured in overdoped cuprates outside the pseudogap phase ($p>p^{★}$), such as ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ and ${\mathrm{Tl}}_2{\mathrm{Ba}}_2{\mathrm{CuO}}_{6+\delta }$ at $p\simeq 0.3$, where $\gamma \simeq 7$ mJ/${\mathrm{K}}^2\mathrm{mol}$. Given that the pseudogap causes a loss of density of states and assuming that ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ has the same $\gamma$ value as other cuprates at $p\simeq 0.3$, this implies that $\gamma$ in ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ must peak between $p\simeq 0.09$ and $p\simeq 0.3$, namely, at (or near) the critical doping $p^{★}$ where the pseudogap phase is expected to end ($p^{★}\simeq 0.2$). Second, the high $\gamma$ value implies that the Fermi surface must consist of more than the single electronlike pocket detected by quantum oscillations in ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ at $p\simeq 0.09$, whose effective mass $m^{★}=2.7m_0$ yields only $\gamma =4.0$ mJ/${\mathrm{K}}^2\mathrm{mol}$. This missing mass imposes a revision of the current scenario for how pseudogap and charge order, respectively, transform and reconstruct the Fermi surface of cuprates.

We study the phenomenon of many-body localization (MBL) in an interacting system subjected to a combined dc as well as square-wave ac electric field. First, the condition for the dynamic localization and coherent destruction of Wannier-Stark localization in the noninteracting limit is obtained semiclassically. In the presence of interactions (and a confining/disordered potential), a static field alone leads to “Stark-MBL” for sufficiently large field strengths. We find that in the presence of an additional high-frequency ac field, there are two ways of maintaining the MBL intact: either by resonant drive where the ratio of amplitude to the frequency of the drive $(A/\omega )$ is tuned at the dynamic localization point of the noninteracting limit, or by off-resonant drive. Remarkably, resonant drive with $A/\omega$ tuned away from the dynamic localization point leads to a coherent destruction of Stark-MBL. Moreover, a pure (high-frequency) ac field can also give rise to the MBL phase if $A/\omega$ is tuned at the dynamic localization point of the zero dc field problem.