Single crystal iron was ramp-compressed along the [100] axis at 4 GPa/ns through the $\alpha -\epsilon$ phase transformation at 13 GPa. Unlike shock experiments, in these ramp compression experiments the bcc lattice is observed to be nearly isotropically relaxed before the phase transition. There was a mixed $\alpha /\epsilon$ phase region starting at $\sim 13$ GPa, which was observed for 0.75 ns until the peak stress of 18 GPa was reached. In situ x-ray diffraction measurements show the formation of new hcp orientations not reported in shock or quasistatic compression experiments. The new hcp orientations appear to be caused by $\left\{10\overline{1}1\right\}\langle 10\overline{1}\overline{2}\rangle \to \left\{\overline{1}011\right\}\langle \overline{1}01\overline{2}\rangle$ sequential twins that occur during the phase transition. This twinning mechanism relieves the shear strain caused by the bcc-hcp phase transition from the isotropically relaxed bcc phase while subject to uniaxial compression. These results show that in iron the induced microstructure through a phase transition and the phase transition mechanism depend on the loading history.

In frustrated magnetic systems, the competition amongst interactions can introduce extremely high degeneracy and prevent the system from readily selecting a unique ground state. In such cases, the magnetic order is often exquisitely sensitive to the balance among the interactions, allowing tuning among novel magnetically ordered phases. In antlerite, ${\mathrm{Cu}}_3{\mathrm{SO}}_4{\left(\mathrm{OH}\right)}_4, {\mathrm{Cu}}^{2+}$ ($S=1/2$) quantum spins populate three-leg zigzag ladders in a highly frustrated quasi-one-dimensional structural motif. We demonstrate that at zero applied field, in addition to its recently reported low-temperature phase of coupled ferromagnetic and antiferromagnetic spin chains, this mineral hosts an incommensurate $\mathrm{helical}+$cycloidal state, an idle-spin state, and a multiple-$q$ phase which is the magnetic analog of misfit crystal structures. The antiferromagnetic order on the central leg is reentrant. The high tunability of the magnetism in antlerite makes it a particularly promising platform for pursuing exotic magnetic order.

The rare-earth nitrides form a series of structurally simple intrinsic ferromagnetic semiconductors, a rare class of both fundamental interest and application potential. Within the series there is a wide range of magnetic properties relating to the spin/orbit contributions to the ferromagnetic ground states. We report an x-ray magnetic circular dichroism investigation of the spin/orbit magnetic dipole alignments of Sm and Gd ions in epitaxial ${\mathrm{Gd}}_x{\mathrm{Sm}}_{1-x}\mathrm{N}$ films. The Sm spin-alignment expectation value $\langle S_z\rangle$ is seen to be strengthened by the Gd/Sm exchange interaction, providing guidance that the composition for an angular momentum compensation point, where the volume-averaged total angular momentum of a film is zero, lies near $x=0.24$.

We followed step by step the transition from an antiferromagnetic (AFM) Mott insulator to a superconducting (SC) metal in the ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ (Bi-2212) cuprate using electronic Raman scattering spectroscopy. This was achieved by tracking the doping dependence of the spin singlet excitation (SSE) originating from the AFM Mott insulator, the normal-state quasiparticle excitation related to the mobile charge carriers, and the Bogoliubov quasiparticles related to the SC gap. We show that the signature of the pseudogap phase which develops during this transition can be interpreted as the blocking of charge carriers by the enhancement of the AFM correlations as the temperature drops. We find that the energy scale of the pseudogap, ${\mathrm{\Delta }}_{\text{pg}}\left(p\right)$, closely follows that of the SSE, ${\mathrm{\Delta }}_{\text{sse}}\left(p\right)$ with doping $p$. The quasiparticle lifetime considerably increases with doping when the pseudogap collapses. We reveal that the maximum amplitude of the SC gap ${\mathrm{\Delta }}_{\text{sc}}^{\text{max}}$ and the SC transition temperature $T_{\mathrm{c}}$ are linked in an extended range of doping, such as ${\mathrm{\Delta }}_{\text{sc}}^{\text{max}}\left(p\right)\propto {\mathrm{\Delta }}_{\text{sse}}\left(p\right)T_{\mathrm{c}}\left(p\right)$. This relation suggests that the AFM correlations play a key role in the mechanism of superconductivity.

The discovery of superconductivity in layered vanadium-based kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A$: K, Rb, Cs) has added a new family of materials to the growing class of possible unconventional superconductors. However, the nature of the superconducting pairing in these materials remains elusive. We present a microscopic theoretical study of the leading superconducting instabilities on the kagome lattice based on spin- and charge-fluctuation mediated Cooper pairing. The applied methodology includes effects of both on-site and nearest-neighbor repulsive Coulomb interactions. Near the upper van Hove filling—relevant for the $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ materials—we find a rich phase diagram with several pairing symmetries being nearly degenerate. In particular, while a substantial fraction of the phase diagram is occupied by a spin-singlet order parameter transforming as a two-dimensional irreducible representation of the point group, several nodal spin-triplet pairing states remain competitive. We compute the band and interaction parameter dependence of the hierarchy of the leading superconducting instabilities and determine the detailed momentum dependence of the resulting preferred gap structures. Crucially, for moderate values of the interaction parameters, the individual pairing states depend strongly on momentum and exhibit multiple nodes on the Fermi surface. We discuss the properties of these superconducting gap structures in light of recent experimental developments of the $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ materials.

Further to the structure of the intrinsic magnetic topological insulators $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4·n\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)$ with $n<4$, where index $n$ is the number of quintuple Te-Bi-Te-Bi-Te building blocks inserted between the neighboring septuple Te-Bi-Te-Mn-Te-Bi-Te building blocks, the structure of the members with $n=4$, 5, and 6 was studied using x-ray powder diffraction. The unit-cell parameters and atomic positions were obtained. The obtained and available structural data were summarized to show that the crystal structure of all members of $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4·n\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)$ follows the cubic close-packing principle, independently of the space group of the given member. Confocal Raman spectroscopy was then applied. Comparative analysis of the number, frequency, symmetry, and broadening of the vibration modes responsible for the lines in the Raman spectra of the systems with $n=1$,…,6, as well as $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4\left(n=0\right)$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ ($n=\infty$) has shown that lattice dynamics of $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4·n\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)$ with $n>0$ overwhelmingly dominates by the cooperative atomic displacements in the quintuple building blocks.

It is described how the heat generation by an electrical current in a ferromagnetic tunnel contact to a nonmagnetic material depends on the spin accumulation in the nonmagnetic electrode for both Joule and Peltier heating. This enables the control of the heating in the tunnel contact by an external applied magnetic field that induces spin precession of the spin accumulation. Thereby, a Hanle type of spin signal is imprinted in the heating power. We derive expressions for the magnitude and sign of the magnetic-field-dependent heating power for Joule and Peltier heating, and discuss the important parameters. For contacts with small resistance-area product, the spin accumulation provides the dominant contribution to the Joule heating power, which can therefore be modulated by more than a factor of 2 with a magnetic field. The described phenomenon can also produce genuine spin signals in various devices, including nonlocal lateral spin valves.

The kagome antiferromagnetic metal FeSn has become an attractive platform for the exploration of novel electronic states such as topological Dirac states and the formation of flat bands by localized electrons. Apart from the electronic properties, Dirac magnons and flat magnon bands have also been proposed by applying simplified Heisenberg models to kagome magnetic systems. Inelastic neutron scattering studies on FeSn found well-defined magnon dispersions at low energies, but magnons at high energies are strongly damped, which cannot be explained by localized spin models. In this paper, we utilize both linear spin-wave theory and time-dependent density functional perturbation theory to investigate spin fluctuations of FeSn. Through the comparison of calculated spin-wave spectra and the Stoner continuum, we explicitly show that the damping of magnons at high energies is due to the Landau damping, and the appearance of high-energy optical-magnon-like branches at the $M$ and $K$ points is the result of the relatively low Stoner excitation intensity at those regions.

Structural and physical properties of the high-entropy alloy ${\left(\mathrm{NbTa}\right)}_{0.67}{\left(\mathrm{MoHfW}\right)}_{0.33}$ were studied by x-ray powder diffraction, energy dispersive x-ray spectroscopy, magnetization, electrical resistivity, and specific heat measurements. The experimental results were supported by theoretical calculations using two complementary approaches for electronic structure calculations: the Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA) and projector augmented wave (PAW) within the density functional theory. It was found that the alloy forms with a cubic, body-centered structure (space group $Im\overline{3}m$, W-type structure) with a lattice parameter $a=3.287\left(1\right)$ Å and a random (but microscopically homogeneous) distribution of the constituent elements. At high temperature the alloy exhibits simple metallic behavior, while at low temperature it becomes a type-II superconductor with the critical temperature $T_{\mathrm{c}}\approx 4.3$ K and the upper critical field ${\mu }_0{H}_{\mathrm{c}2}\approx 1.45$ T. The electron-phonon coupling constant calculated from the PAW data, ${\lambda }_{\mathrm{el}-\mathrm{ph}}^{\mathrm{th}-\mathrm{PAW}}=0.63$, is in perfect agreement with the experimental results.

Axion insulators are generally understood as magnetic topological insulators whose Chern-Simons axion coupling term is quantized and equal to $\pi$. Inversion and time reversal, or the composition of either one with a rotation or a translation, are symmetries which protect this invariant. In this work, we focus our attention on the composition of a twofold rotation with time reversal, and we show that insulators with this symmetry possess a ${\mathbb{Z}}_2$ invariant arising from Atiyah's real $K$ theory. We call this invariant the $K$-theory Kane-Mele invariant due to the similarities it has with the Kane-Mele invariant for systems with time reversal symmetry. Whenever all Chern numbers vanish, we demonstrate that this invariant is equivalent to the Chern-Simons axion coupling, and in the presence of the inversion symmetry, we show how this invariant could be obtained from the eigenvalues of the inversion operator on its fixed points in momentum space. For the general case of nontrivial Chern numbers, the Chern-Simons axion coupling term incorporates information of the $K$-theory Kane-Mele invariant as well as information regarding bands with a nontrival Chern number. An explicit formula in terms of $K$-theory generators is presented for the Chern-Simons axion coupling term, the relation with the $K$-theory Kane-Mele invariant is explained, and a formula in terms of eigenvalues of the inversion operator is obtained. Using an effective Hamiltonian model and first-principles calculations, we also show that the occurrence of bulk-band inversion and nontrivial $K$-theory Kane-Mele invariant index can be observed in axion insulators of the pnictides family. In particular, we demonstrate that NpBi can be classified as an axion insulator due to the detection of additional topological indicators such as the quantum spin Hall effect, gapped surface states, surface quantized anomalous Hall effect, and chiral hinge modes.

A study of the influence of strong electron-electron interactions and Wigner-molecule formation on the spectra of $2e$ singlet-triplet double-dot Si qubits is presented based on a full configuration-interaction (FCI) approach that incorporates the valley degree of freedom (VDOF) in the context of the continuous (effective mass) description of semiconductor materials. Our FCI solutions correspond to treating the VDOF as an isospin in addition to the regular spin. A major advantage of our treatment is its capability to assign to each energy curve in the qubit's spectrum a complete set of good quantum numbers for both the spin and the valley isospin. This allows for the interpretation of the Si double-dot spectra according to an underlying $\mathrm{SU}\left(4\right)\supset \mathrm{SU}\left(2\right)\times \mathrm{SU}\left(2\right)$ group-chain organization. Considering parameters in the range of actual experimental situations, we uncover and demonstrate for a double-quantum dot that, in the (2,0) charge configuration and compared to the expected large, and dot-size determined, single-particle (orbital) energy gap, the strong $e\text{−}e$ interactions drastically quench the spin-singlet–spin-triplet energy gap $E_{\mathrm{ST}}^{\oplus }$ within the same valley, making it competitive to the small energy gap $E_V$ between the two valleys. We present results for both the $E_{\mathrm{ST}}^{\oplus }<E_V$ and $E_{\mathrm{ST}}^{\oplus }>E_V$ cases, which have been reported to occur in different experimental qubit devices. In particular, we investigate the spectra as a function of detuning and demonstrate the strengthening of the all-important avoided crossings due to a lowering of the interdot barrier and/or the influence of valley-orbit coupling. We further demonstrate, as a function of an applied magnetic field, the emergence of avoided crossings in the (1,1) charge configuration due to the more general spin-valley coupling, in agreement with experiments. The valleytronic FCI method formulated and implemented in this paper, and demonstrated for the case of two electrons confined in a tunable double quantum dot, offers also a most effective tool for analyzing the spectra of Si qubits with more than two wells and/or more than two electrons, in field-free conditions, as well as under the influence of an applied magnetic field. Furthermore, it can also be straightforwardly extended to the case of bilayer graphene quantum dots.

Two-dimensional van der Waals vanadium selenide compounds are expected to be a key platform for searching the new emergent phenomena by forming the artificial heterostructures. Here, we systematically investigated the electronic properties of monolayer and multilayer vanadium selenide films grown with different conditions using in situ angle-resolved photoemission spectroscopy and first-principles calculations. We observed the band dispersions of pristine ${\mathrm{VSe}}_2$ in a sample grown at substrate temperature ($T_{\mathrm{sub}}$) of $250{}^{\circ }\mathrm{C}$, whereas the significant modulations of the Se-derived bands can be seen at $T_{\mathrm{sub}}$ = $380{}^{\circ }\mathrm{C}$. For $T_{\mathrm{sub}}=420{}^{\circ }\mathrm{C}$, we found that the V self-intercalated ${\mathrm{V}}_5{\mathrm{Se}}_8$ was formed. As a simple and convenient way, we also demonstrate that the monolayer ${\mathrm{V}}_5{\mathrm{Se}}_8$ can be fabricated by annealing from the multilayer ${\mathrm{VSe}}_2$ film, and the possible formation mechanism is discussed. The establishment of the growth methods of the ${\mathrm{V}}_5{\mathrm{Se}}_8$ film and verifying its electronic properties provide the substantial step for the creation of the two-dimensional van der Waals heterostructures.

We investigate the localization of low-energy single quasiparticle states in 7/9- and 13/15-hybrid nanoribbon systems in the presence of strong interactions and within a finite volume. We consider two scenarios, the first being the Hubbard model at half filling, and perform quantum Monte Carlo simulations for a range $U$ that includes the strongly correlated regime. In the second case, we add a nearest-neighbor superconducting pairing $\mathrm{\Delta }$ and take the symmetric line limit, where $\mathrm{\Delta }$ is equal in magnitude to the hopping parameter $t$. In this limit, the quasiparticle spectrum and wave functions can be directly solved for general on-site interaction $U$. In both cases, we extract the site-dependent quasiparticle wave-function densities and demonstrate that localization persists in these nonperturbative regimes under particular scenarios. Our findings suggest that such localization under the presence of interactions and within a finite volume is a generic feature of hybrid nanoribbons composed of topologically distinct regions.

Satellites in core level spectra of photoelectron spectroscopy (PES) can provide crucial information on the electronic structure and chemical bonding in materials, particularly in transition metal oxides. This paper explores satellites of the Ti $1s$ and $2p$ core level spectra of ${\mathrm{SrTiO}}_3$ and ${\mathrm{TiO}}_2$. Conventionally, soft x-ray PES (SXPS) probes the Ti $2p$ core level; however, it is not ideal to fully capture satellite features due to its inherent spin-orbit splitting (SOS). Here, hard x-ray PES (HAXPES) provides access to the Ti $1s$ spectrum instead, which allows us to study intrinsic charge responses upon core-hole creation without the complication from SOS and with favorable intrinsic linewidths. The experimental spectra are theoretically analyzed by two impurity models, including an Anderson impurity model (AIM) built on local density approximation (LDA) and dynamical mean-field theory (DMFT), and a conventional ${\mathrm{TiO}}_6$ cluster model. The theoretical results emphasize the importance of explicit inclusion of higher-order Ti-O charge-transfer processes beyond the nearest-neighboring Ti-O bond to simulate the core level spectra of ${\mathrm{SrTiO}}_3$ and ${\mathrm{TiO}}_2$. The AIM approach with continuous bath orbitals provided by $\mathrm{LDA}+\mathrm{DMFT}$ represents the experimental spectra well. Crucially, with the aid of the $\mathrm{LDA}+\mathrm{DMFT}$ method, this paper provides a robust prescription of how to use the computationally cheap cluster model in fitting analyses of core level spectra.

The interplay between Kondo screening, indirect magnetic interaction, and geometrical frustration is studied in the two-dimensional Kondo-necklace model on the triangular lattice. Using infinite projected entangled pair states (iPEPS), we compute the ground state as a function of the antiferromagnetic local Kondo interaction $J_K$ and the Ising-type direct spin-spin interaction $I_z$. As opposed to previous studies, we do not find partial Kondo screening (PKS) in the isotropic limit $I_z=0$ but the same behavior as in the unfrustrated case, i.e., a direct phase transition between the paradigmatic phases of the Doniach competition: (i) a disordered phase consisting of local spin singlets at strong $J_K$ and (ii) a magnetically ordered phase at weak $J_K$. For $I_z>0$, we find a PKS ground state but again oppositely to previous studies, we find that the PKS ground state is in strong competition with a second ground state candidate not found before. This state is characterized by a strongly polarized central spin in each hexagon and its antiparallel, weakly polarized (i.e., partially screened) neighbors. We name it the central spin phase.

Among the two materials families used in nanophotonics, the fundamental mode for metal nanostructures is electric, while that for dielectric nanostructures is magnetic. Consequently, the optical properties of hybrid dimers that incorporate both materials have prompted significant research. Here, we study the optical response of such hybrid dimers with the coupled electric and magnetic dipoles method, and demonstrate how their electromagnetic interactions can be used to modulate the electric and magnetic responses. The conditions for the complete suppression of the magnetic dipole in the visible range for high refractive index dielectric nanoparticles are described. The electric and magnetic responses can be enhanced or reduced by positioning the dielectric particle in the nodes of the standing wave formed by the metallic particle. This controlled near-field interaction provides a handle on the near- and far-field responses of the system, with possible applications such as sensing and optical switches.

We theoretically investigate the scenario of a semiconductor quantum well in a microcavity, where the band structure is arranged such that optically excited electron-hole pairs cannot form Coulomb-bound excitonic states. However, it is still possible to form exciton polaritons (part-light, part-matter quasiparticles), where the excitons are bound via the exchange of microcavity photons rather than via Coulomb interactions. Using a diagrammatic theory, we determine the spectral response of the semiconductor microcavity, which includes exciton-polariton resonances as well as a continuum of unbound electron-hole pairs. Our method also gives us access to the photon fraction and the electron-hole wave function of the exciton polariton. In particular, we obtain the conditions under which an exciton is bound by photon exchange and we show that Coulomb interactions can enhance binding at large cavity photon frequencies. Our results for the spectral response are in good agreement with a recent experiment on doped quantum wells [E. Cortese et al., Nat. Phys. 17, 31 (2021)].

Monolayer $1T^{\prime }\text{−}{\mathrm{WTe}}_2$ has been tuned to be superconducting by electrostatic gating in recent experiments, which has been explained by soft-mode-phonon-mediated superconductivity in theory. In addition to the phonon branch softening, the charge doping would also affect the Raman vibration properties of monolayer $1T^{\prime }\text{−}{\mathrm{WTe}}_2$. In this paper, by using the first-principles calculation, we investigate the response of Raman-active modes in monolayer $1T^{\prime }\text{−}{\mathrm{WTe}}_2$ to charge doping systematically. It is found that, when doped with electrons, most of the Raman modes are blueshifted, and when doped with holes, most of the Raman modes are redshifted. Anomalously, $A_g^2, A_g^3$, and $B_g^3$ modes decrease in frequency with the increasing carrier concentration, while $A_g^3$ and $A_g^6$ modes have frequency changes with multiple turning points. The frequency of the $A_g^3$ mode, especially, varies in an “M” type with increasing carrier concentration, which is correlated with the out-of-plane vibrational properties of the Te atoms. The other two vibration modes correspond to the Raman spectral peaks, the $A_g^1$ and $A_g^4$ modes, whose frequencies vary linearly with the doped charge density. This is important for understanding the Raman spectra of doped ${\mathrm{WTe}}_2$ as well as for monitoring the doping levels in ${\mathrm{WTe}}_2$-based electronic devices with a frequency shift for Raman spectral peaks.