We investigate theoretically magnetism and magnetic phase transitions induced by electrostatic gating of two-dimensional square metal-organic framework compounds. We find that electrostatic gating can induce phase transitions between homogeneous ferromagnetic and various spin-textured antiferromagnetic states. Electronic structure and Wannier function analysis can reveal hybridizations between transition-metal $d$ orbitals and conjugated $\pi$ orbitals in the organic framework. Mn-containing compounds exhibit a strong $d-\pi$ hybridization that leads to partially occupied spin-minority bands, in contrast to compounds containing transition-metal ions other than Mn, for which electronic structure around the Fermi energy is only slightly spin split due to weak $d-\pi$ hybridization and the magnetic interaction is of the Ruderman-Kittel-Kasuya-Yosida type. We use a ferromagnetic Kondo lattice model to understand the phase transition in Mn-containing compounds in terms of carrier density and illuminate the complexity and the potential to control two-dimensional magnetization.

Thin topological insulator (TI) films support optical and acoustic plasmonic modes characterized by effective net charge or net spin density, respectively. We combine many-body and electromagnetic calculations to study how these modes can be selectively excited at films and nanodisks at infrared and THz frequencies. We first discuss the excitation of propagating plasmons in a thin film by a point dipolar source. We emphasize how changing the distance between the dipolar source and the film allows us to control the relative strength of the acoustic and optical plasmons and thus to excite net-spin or net-charge waves on demand. The acoustic and optical modes in a nanodisk structure can be efficiently tuned by changing the size of the disk or by applying electrostatic gating. Furthermore, these modes can be confined to regions of dimensions much smaller than the wavelength. The control of the excitation of acoustic and optical modes indicates that thin topological insulators are a promising system to manipulate the spin and charge properties of the plasmonic response, with potential applications in fast, compact, and electrically-controlled spintronic devices.

The successful realization of metamaterials and metasurfaces requires the judicious choice of constituent elements. In this paper, we demonstrate the implementation of time-varying metamaterials in the terahertz frequency regime by utilizing graphene-wrapped microwires as building blocks and modulation of graphene conductivity through exterior electrical gating. These elements enable enhancement of light-graphene interaction by utilizing optical resonances associated with Mie scattering, yielding a large tunability and modulation depth. We develop a semianalytical framework based on transition-matrix formulation for modeling and analysis of periodic and aperiodic arrays of such time-varying building blocks. The proposed method is validated against full-wave numerical results obtained using the finite-difference time-domain method. It provides an ideal tool for mathematical synthesis and analysis of space-time gradient metamaterials, eliminating the need for computationally expensive numerical models. Moreover, it allows for a wider exploration of exotic space-time scattering phenomena in time-modulated metamaterials. We apply the method to explore the role of modulation parameters in the generation of frequency harmonics and their emerging wavefronts. Several potential applications of such platforms are demonstrated, including frequency conversion, holographic generation of frequency harmonics, and spatiotemporal manipulation of light. The presented results provide key physical insights to design time-modulated functional metadevices using various building blocks and open up new directions in the emerging paradigm of time-modulated metamaterials.

We have measured the electronic heat capacity of thin film nanowires of copper and silver at temperatures 0.1–0.3 K; the films were deposited by standard electron-beam evaporation. The specific heat of the Ag films of sub-100-nm thickness agrees with the bulk value and the free-electron estimate, whereas that of similar Cu films exceeds the corresponding reference values by one order of magnitude. The origin of the anomalously high heat capacity of copper films remains unknown for the moment. Based on the small heat capacity at low temperatures and the possibility to devise a tunnel probe thermometer on it, metal films form a promising absorber material, e.g., for microwave photon calorimetry.

Interactions between charge carriers in graphene lead to logarithmic renormalization of observables mimicking the behavior known in (3+1)-dimensional quantum electrodynamics (QED). Here we analyze soft electron-hole $\left(e\text{−}h\right)$ excitations generated as a result of fast charge dynamics, a direct analog of the signature QED effect—multiple soft photons produced by the QED vacuum shakeup. We show that such excitations are generated in photon absorption, when a photogenerated high-energy $e\text{−}h$ pair cascades down in energy and gives rise to multiple soft $e\text{−}h$ excitations. This fundamental process is manifested in a double-log divergence in the emission rate of soft pairs and a characteristic power-law divergence in their energy spectrum of the form $\frac{1}{\omega }\text{ln}\left(\frac{\omega }{\mathrm{\Delta }}\right)$. Strong carrier-carrier interactions make pair production a prominent pathway in the photoexcitation cascade.

Using the local density approximation plus dynamical mean-field theory (LDA+DMFT), we have computed the valence-band photoelectron spectra and magnetic excitation spectra of ${\mathrm{BiFeO}}_3$, one of the most studied multiferroics. Within the DMFT approach, the local impurity problem is tackled by the exact diagonalization solver. The solution of the impurity problem within the LDA+DMFT method for the paramagnetic and magnetically ordered phases produces result in agreement with the experimental data on electronic and magnetic structures. For comparison, we also present results obtained by the $\mathrm{LDA}+U$ approach which is commonly used to compute the physical properties of this compound. Our LDA+DMFT derived electronic spectra match adequately with the experimental hard x-ray photoelectron spectroscopy and resonant photoelectron spectroscopy for Fe $3d$ states, whereas the $\mathrm{LDA}+U$ method fails to capture the general features of the measured spectra. This indicates the importance of accurately incorporating the dynamical aspect of electronic correlation among Fe $3d$ orbitals to reproduce the experimental excitation spectra. Specifically, the LDA+DMFT derived density of states exhibits a significant amount of Fe $3d$ states at the position of Bi lone pairs, implying that the latter are not alone in the spectral scenario. This fact might modify our interpretation about the origin of ferroelectric polarization in this material. Our study demonstrates that the combination of orbital cross sections for the constituent elements and broadening schemes for the spectral functions are crucial to explain the detailed structures of the experimental electronic spectra. Our magnetic excitation spectra computed from the LDA+DMFT result conform well with the inelastic neutron scattering data.

Coulomb interactions famously drive three-dimensional quadratic band crossing semimetals into a non-Fermi liquid phase of matter. In a previous work [Nandkishore and Parameswaran, Phys. Rev. B 95, 205106 (2017)], the effect of disorder on this non-Fermi liquid phase was investigated, assuming that the band structure was isotropic, assuming that the conduction and valence bands had the same band mass, and assuming that the disorder preserved exact time-reversal symmetry and statistical isotropy. It was shown that the non-Fermi liquid fixed point is unstable to disorder and that a runaway flow to strong disorder occurs. In this paper, we extend that analysis by relaxing the assumption of time-reversal symmetry and allowing the electron and hole masses to differ (but continuing to assume isotropy of the low energy band structure). We first incorporate time-reversal symmetry breaking disorder and demonstrate that there do not appear any new fixed points. Moreover, while the system continues to flow to strong disorder, time-reversal-symmetry-breaking disorder grows asymptotically more slowly than time-reversal-symmetry-preserving disorder, which we therefore expect should dominate the strong-coupling phase. We then allow for unequal electron and hole masses. We show that whereas asymmetry in the two masses is irrelevant in the clean system, it is relevant in the presence of disorder, such that the ‘effective masses’ of the conduction and valence bands should become sharply distinct in the low-energy limit. We calculate the RG flow equations for the disordered interacting system with unequal band masses and demonstrate that the problem exhibits a runaway flow to strong disorder. Along the runaway flow, time-reversal-symmetry-preserving disorder grows asymptotically more rapidly than both time-reversal-symmetry-breaking disorder and the Coulomb interaction.

We investigated the orbital hybridization mechanism in $3d-5d$ double perovskites (DPs) of ${\mathrm{La}}_2{\mathrm{CoIrO}}_6$ and ${\mathrm{La}}_2{\mathrm{CoPtO}}_6$ using x-ray absorption spectroscopy. It is clearly evidenced by O $K$-edge and Co $K$-edge x-ray absorption spectra that the Co $3d$ orbitals hybridize not only with the half-filled Ir/Pt $j_{\mathrm{eff}}$ states but also with the fully empty (unpolarized) Ir/Pt $e_g$ states in both DPs. The Co $3d$ $e_g$-Ir $5d$ $e_g$ hybridization cannot contribute to the ferrimagnetic long-range order in ${\mathrm{La}}_2{\mathrm{CoIrO}}_6$ established by spin-selective Co $3d$ $t_{2g}$-Ir $5d$ $j_{\mathrm{eff}}$ hybridization through the intermediate oxygen $p$ state but could serve as an origin of paramagnetism. The strengths of such orbital hybridizations were found to be almost invariant to temperature, even far above the Curie temperature, implying persistent paramagnetism against the antiferromagnetic ordering in the spin-orbit entangled $3d-5d$ DPs.

We propose a machine-learning method for evaluating the potential barrier governing atomic transport based on the preferential selection of dominant points for atomic transport. The proposed method generates numerous random samples of the entire potential energy surface (PES) from a probabilistic Gaussian process model of the PES, which enables defining the likelihood of the dominant points. The robustness and efficiency of the method are demonstrated on a dozen model cases for proton diffusion in oxides, in comparison with a conventional nudge elastic band method.

We investigate the ${\mathrm{CaF}}_1/\mathrm{Si}\left(111\right)$ interface using a combination of high-resolution scanning tunneling and noncontact atomic force microscopy operated at cryogenic temperature as well as x-ray photoelectron spectroscopy. Submonolayer ${\mathrm{CaF}}_1$ films grown at substrate temperatures between 550 and 600 ${}^{\circ }\mathrm{C}$ on $\mathrm{Si}\left(111\right)$ surfaces reveal the existence of two island types that are distinguished by their edge topology, nucleation position, measured height, and inner defect structure. Our data suggest a growth model where the two island types are the result of two reaction pathways during ${\mathrm{CaF}}_1$ interface formation. A key difference between these two pathways is identified to arise from the excess species during the growth process, which can be either fluorine or silicon. Structural details as a result of this difference are identified by means of high-resolution noncontact atomic force microscopy and add insights into the growth mode of this heteroepitaxial insulator-on-semiconductor system.

Raman spectroscopy is employed to probe directly the soft rotation and tilting modes, which are two primary order parameters predicted in the hybrid improper ferroelectric material ${\mathrm{Ca}}_3{\mathrm{Mn}}_2{\mathrm{O}}_7$. We observe a giant softening of the $107-{\mathrm{cm}}^{-1}$ octahedron tilting mode by $26{\mathrm{cm}}^{-1}$ on heating through the structural transition from a ferroelectric to paraelectric orthorhombic phase. This is contrasted by a small softening of the $150-{\mathrm{cm}}^{-1}$ rotational mode by $6{\mathrm{cm}}^{-1}$. In the intermediate phase, the competing soft modes with different symmetries coexist, bringing about many-faceted anomalies in spin excitations and lattice vibrations. Our work demonstrates that the soft rotation and tilt patterns, relying on a phase-transition path, are a key factor in determining ferroelectric, magnetic, and lattice properties of ${\mathrm{Ca}}_3{\mathrm{Mn}}_2{\mathrm{O}}_7$.

A series of iron arsenides (e.g., ${\text{CaRbFe}}_4{\text{As}}_4, {\text{SrCsFe}}_4{\text{As}}_4$) have been discovered recently, and have provoked a rise in superconductor searches in a different phase, known as the 1144 phase. For the presence of various chemical substitutions, it is believed that more 1144 compounds remain to be discovered. In this work, we perform general model analysis as well as scenario calculation on a basis of density functional theory to investigate phase stability in a variety of compounds. We predict that the 1144-type phase could be stabilized in ${\text{EuKFe}}_4{\text{As}}_4, {\text{EuRbFe}}_4{\text{As}}_4, {\text{EuCsFe}}_4{\text{As}}_4, {\text{CaCsFe}}_4{\text{P}}_4, {\text{SrCsFe}}_4{\text{P}}_4, {\text{BaCsFe}}_4{\text{P}}_4, {\text{InCaFe}}_4{\text{As}}_4, {\text{InSrFe}}_4{\text{As}}_4$, etc. Remarkably, it involves rare earths, trivalence elements (e.g., indium) and iron phosphides, which greatly expands the range of its existence and suggests a promising prospect for experimental synthesis. In addition, we find that the formation of many random doping compounds (e.g., ${\text{Ba}}_{0.5}{\text{Cs}}_{0.5}{\text{Fe}}_2{\text{As}}_2, {\text{Ba}}_{0.5}{\text{Rb}}_{0.5}{\text{Fe}}_2{\text{As}}_2$) is driven by entropy and could be annealed to a 1144-type phase. Eventually, we plot a phase diagram about two structural factors $\mathrm{\Delta }a$ and $\mathrm{\Delta }c$, giving a bird's-eye view of stability of various 1144 compounds.

The lattice dynamics in ${\mathrm{MnFe}}_{0.95}{\mathrm{Si}}_{0.50}{\mathrm{P}}_{0.50}$ were investigated experimentally using ${}^{57}\mathrm{Fe}$ nuclear inelastic scattering and inelastic x-ray scattering across the first-order magnetic transition which occurs close to room temperature. The lattice dynamics characterization was supported by a macroscopic magnetic characterization, an x-ray diffraction study, and a hyperfine interactions characterization using Mössbauer spectroscopy. The Fe specific and the x-ray generalized density of phonon states were obtained both in the ferromagnetic and in the paramagnetic state. A prominent shift, $2\text{meV}$ at $20\text{meV}$, in the x-ray generalized density of phonon states across the first-order magnetic transition, that involves vibrations with essentially Fe character, is revealed corroborated by a change in the local environment quantified in the isomer shift and the quadrupole splitting. Above $35\text{meV}$ the vibrational modes are practically insensitive to the magnetic transition. The entropy change induced by a $1\text{T}$ magnetic field across the magnetic transition, $\sim 10\mathrm{J}/\mathrm{K}/\mathrm{kg}$, is only a fraction of the Fe vibrational entropy change, $62\left(21\right)\mathrm{J}/\mathrm{K}/\mathrm{kg}$.

The magnetism of a dense assembly of ultrasmall ferrimagnetic nanoparticles exhibits unique features due to the combination of intraparticle and strong interparticle interactions. To model such system we need to account for the internal particle structure and the short- and long-range interparticle interactions. We have developed a mesoscopic model for the particle assembly that includes three spins (two for the surface and one for the core) for the description of each nanoparticle, interparticle dipolar interactions and the interparticle exchange interactions for the nanoparticles in contact. The temperature dependence of the observed exchange bias effect, due to exchange coupling at the interface between core/surface spins and the interparticle exchange coupling, and the zero-field-cooled–field-cooled magnetization vs temperature curves have been investigated using the Monte Carlo simulation technique with the implementation of the Metropolis algorithm. Our simulations reproduce well the experimental data of ultrasmall $\sim 2$-nm $\mathrm{Mn}{\mathrm{Fe}}_2{\mathrm{O}}_4$ nanoparticles, confirming the close relationship between the superspin glass state and the exchange-bias effect in dense nanoparticle systems, owing to the interplay between the intraparticle structure and the interparticle effects.

We investigate the quantum dynamics of many-body systems subject to local (i.e., restricted to a limited space region) time-dependent perturbations. If the system crosses a quantum phase transition, an off-equilibrium behavior is observed, even for a very slow driving. We show that, close to the transition, time-dependent quantities obey scaling laws. In first-order transitions, the scaling behavior is universal, and some scaling functions can be computed exactly. For continuous transitions, the scaling laws are controlled by the standard critical exponents and by the renormalization-group dimension of the perturbation at the transition. Our protocol can be implemented in existing relatively small quantum simulators, paving the way for a quantitative probe of the universal off-equilibrium scaling behavior, without the need to manipulate systems close to the thermodynamic limit.

We study the intrinsic fully gapped odd-parity superconducting order in doped nodal-loop materials with a torus-shaped Fermi surface. We show that the mirror symmetry, which protects the nodal loop in the normal state, also protects the superconducting state as a topological crystalline superconductor. As a result, the surfaces preserving the mirror symmetry host gapless Majorana cones. Moreover, for a Weyl-loop system (twofold degenerate at the nodal loop), the surfaces that break the mirror symmetry (those parallel to the bulk nodal loop) contribute a Chern (winding) number to the quasi-two-dimensional system in a slab geometry, which leads to a quantized thermal Hall effect and a single Majorana zero mode bound at a vortex line penetrating the system. This Chern number can be viewed as a higher-order topological invariant, which supports hinge modes in a cubic sample when mirror symmetry is broken. For a Dirac-loop system (fourfold degenerate at the nodal loop), the fully gapped odd-parity state can be either time-reversal symmetry-breaking or symmetric, similar to the $A$ and $B$ phases of ${}^3\mathrm{He}$. In a slab geometry, the $A$ phase has a Chern number two, while the $B$ phase carries a nontrivial ${\mathbb{Z}}_2$ invariant. We discuss the experimental relevance of our results to nodal-loop materials such as CaAgAs.

We have investigated the Se isotope effect in the layered bismuth chalcogenide ($\mathrm{BiC}{\mathrm{h}}_2$-based) superconductor $\mathrm{La}{\mathrm{O}}_{0.6}{\mathrm{F}}_{0.4}\mathrm{Bi}{\left(\mathrm{S},\mathrm{Se}\right)}_2$ with ${}^{76}\mathrm{Se}$ and ${}^{80}\mathrm{Se}$. For all examined samples, the Se concentration, which is linked to the superconducting properties, is successfully controlled within $x=1.09–1.14$ in $\mathrm{La}{\mathrm{O}}_{0.6}{\mathrm{F}}_{0.4}\mathrm{Bi}{\mathrm{S}}_{2-x}\mathrm{S}{\mathrm{e}}_x$. From the magnetization and electrical resistivity measurements, changes in $T_{\mathrm{c}}$ are not observed for the $\mathrm{La}{\mathrm{O}}_{0.6}{\mathrm{F}}_{0.4}\mathrm{Bi}{\left(\mathrm{S},\mathrm{Se}\right)}_2$ samples with ${}^{76}\mathrm{Se}$ and ${}^{80}\mathrm{Se}$ isotopes. Our result may indicate that the pairing in $\mathrm{La}{\mathrm{O}}_{0.6}{\mathrm{F}}_{0.4}\mathrm{Bi}{\left(\mathrm{S},\mathrm{Se}\right)}_2$ is not mediated by phonons, which is consistent with recent theoretical and experimental studies suggesting unconventional superconductivity in the $\mathrm{BiC}{\mathrm{h}}_2$-based superconductors.

To verify the first-order liquid-liquid transition (LLT) in liquid Cs reported by Falconi et al. [Phys. Rev. Lett. 94, 125507 (2005).], the pressure dependence of liquid density was measured up to 5 GPa by an x-ray absorption method. The results showed that the density continuously increases over the entire pressure region investigated, without showing the previously reported large volume jump ($\mathrm{\Delta }V/V\sim 17\%$) at 3.9 GPa. No existence of the volume jump is also suggested from the slopes of the melting curve in the phase diagram. Although the electronic transition is one of the most promising candidates that may cause LLT, it is unlikely to occur in liquid Cs because of the gradual nature of the $6s\text{−}5d$ electron transfer.

In-plane longitudinal and transverse thermoelectric phenomena in two parent compounds of iron-based superconductors are studied. Namely, the Seebeck ($S$) and Nernst (ν) coefficients were measured in the temperature range 10–300 K for $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ and $\mathrm{CaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ single crystals that were detwinned in situ. The thermoelectric response shows sizable anisotropy in the spin density wave (SDW) state for both compounds, while some dissimilarities in the vicinity of the SDW transition can be attributed to the different nature of the phase change in $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ and $\mathrm{CaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$. Temperature dependences of $S$ and ν can be described within a two-band model that contains a contribution from highly mobile, probably Dirac, electrons. The Dirac band seems to be rather isotropic, whereas most of the anisotropy in the transport phenomena could be attributed to “regular” holelike charge carriers. We also observe that the off-diagonal element of the Peltier tensor ${\alpha }_{xy}$ is not the same for the $a$ and $b$ orthorhombic axes, which indicates that the widely used Mott formula is not applicable to the SDW state of iron-based superconductors.