Transient absorption following generation of free carriers in the bulk of ${\mathrm{CeBr}}_3$ shows that the ${\mathrm{Ce}}^{3+*}$ excited state responsible for scintillation light emission in this material is populated with a 10% to 90% rise time of 540 fs. The Stokes shift of luminescence from this ${\mathrm{Ce}}^{3+*}(4f$, $5d$) Frenkel exciton establishes that it is self-localized by lattice relaxation. Charge transfer transitions from the valence band to the $4f$ hole component of the ${\mathrm{Ce}}^{3+*}(4f,5d$) lattice-relaxed exciton and from its $5d$ electron component to the conduction band are identified in ${\mathrm{CeBr}}_3$. In ${\mathrm{LaBr}}_3$:Ce, energy transfer from bromine-based relaxed excitons to the ${\mathrm{Ce}}^{3+}$ dopant closest to the point of exciton localization occurred in tens of picoseconds depending on Ce concentration, whereas in ${\mathrm{CeBr}}_3$ the bromine-based relaxed exciton transfers energy to the ${\mathrm{Ce}}^{3+*}(4f,5d$) exciton in 540 fs. We failed to find evidence of any significant signature of ${\mathrm{Ce}}^{4+}$, i.e., holes trapped on ${\mathrm{Ce}}^{3+}$ ions, in excited ${\mathrm{CeBr}}_3$ or ${\mathrm{LaBr}}_3$:Ce in the spectral range 0.41 to 2.16 eV. The accurate time sequencing of population in different excited-state and trapped-charge species involved in energy storage, transport, and emission after ionizing excitation is useful for understanding the fundamental mechanisms at work in scintillation as they may influence light yield, its proportionality to energy deposition, and energy resolution. Isolating the specific rise time of population in the emitting state from the other known contributors to coincidence timing of gamma rays contributes information to the search for novel advances in ultrafast time-of-flight detection.

Machine-learning techniques are evolving into a subsidiary tool for studying phase transitions in many-body systems. However, most studies are tied to situations involving only one phase transition and one order parameter. Systems that accommodate multiple phases of coexisting and competing orders, which are common in condensed matter physics, remain largely unexplored from a machine-learning perspective. In this paper, we investigate multiclassification of phases using support vector machines (SVMs) and apply a recently introduced kernel method for detecting hidden spin and orbital orders to learn multiple phases and their analytical order parameters. Our focus is on multipolar orders and their tensorial order parameters whose identification is difficult with traditional methods. The importance of interpretability is emphasized for physical applications of multiclassification. Furthermore, we discuss an intrinsic parameter of SVM, the bias, which allows for a special interpretation in the classification of phases, and its utility in diagnosing the existence of phase transitions. We show that it can be exploited as an efficient way to explore the topology of unknown phase diagrams where the supervision is entirely delegated to the machine.

Ab initio calculations, the generalized gradient approximations $(\text{GGA}/\text{GGA}+U)$, are used to propose a spin Hamiltonian for the $B$-site ordered double perovskite ${\mathrm{Sr}}_2{\mathrm{NiWO}}_6$. Our results show that the exchange interaction constants between the next-nearest neighbors in both intra- and inter-$ab$ planes $(J_2$ and $J_{2c})$ are an order of magnitude larger than the ones between the nearest neighbors $(J_1$ and $J_{1c})$. Employing the Monte Carlo simulation, we show that the obtained Hamiltonian properly describes the finite-temperature properties of ${\mathrm{Sr}}_2{\mathrm{NiWO}}_6$. Our ab initio calculations also reveal a small magnetic anisotropy and nontrivial biquadratic interaction between the nearest inter-$ab$ plane neighbors, which play essential roles in stabilizing the type-II antiferromagnetic ground state of ${\mathrm{Sr}}_2{\mathrm{NiWO}}_6$.

Three independent components of critical current density, one for the $H//c$ axis and the other two for the $H//ab$ plane, have been studied in ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ single crystals. When the magnetic field is applied along the $c$ axis, we observed fish-tail-like peaks in the $M-H$ hysteresis loop, and the magnetization at higher temperatures exceeds that at lower temperatures at high fields. When the field is applied parallel to the $ab$ plane, a dip structure is observed in the $M-H$ hysteresis loop near the self-field. In addition, for the $H//ab$ plane, we succeeded in separately evaluating the large and significantly anisotropic in-plane and out-of-plane $J_{\mathrm{c}}$. Transmission electron microscopy revealed the presence of planar defects parallel to the $ab$ plane in ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$, which have not been observed in any other iron-based superconductors. We discuss the possible relationship between the anomalous $J_{\mathrm{c}}$ behavior and the planar defects.

We propose an approach describing correlated electronic systems within the two-particle irreducible functional renormalization-group (2PI-fRG) approach, which uses initial self-energy and the two-particle irreducible vertices, obtained from (extended) dynamical mean-field theory [E(DMFT)]. Using the 2PI-fRG approach allows us to treat both local and nonlocal interactions. In the case of purely local interaction the corresponding equations have similar (although not identical) structure to the earlier developed DMFT+fRG $\left({\mathrm{DMF}}^2\mathrm{RG}\right)$ approach. In a simplest truncation, neglecting scale dependence of the two-particle irreducible vertices, we reproduce the results for the two-particle vertices/susceptibilities in the ladder approximation of the dual-boson (DB) or the dynamic vertex approximation approach; in more sophisticated truncations the method allows us to consider nonlocal corrections to the self-energy as well as the interplay of charge and spin correlations. The proposed scheme is tested on the two-dimensional standard and extended $U\text{−}V$ half-filled Hubbard models. For the standard Hubbard model we obtain nonlocal self-energy, which is in agreement with numerical studies; for the extended Hubbard model we find the boundary of charge instability, which agrees well with the results of the DB approach. We also ascertain that the effect of spin correlations on electron interaction in the charge channel, not considered previously in the DB approach, only slightly reduces critical next-nearest-neighbor interaction of charge instability of the extended Hubbard model at the considered finite low temperature, yielding better agreement with dynamic cluster approximation. The considered method is rather general and can be applied to study various phenomena in strongly correlated electronic systems.

Within a Kubo formalism we calculate the absorptive part of the dynamic longitudinal conductivity $\sigma \left(\mathrm{\Omega }\right)$ of a two-dimensional (2D) semi-Dirac material. In the clean limit we provide separate analytic formulas for intraband (Drude) and interband contributions for $\sigma \left(\mathrm{\Omega }\right)$ in both the relativistic and nonrelativistic directions. At finite doping, in the relativistic direction, a sum rule holds between the increase in optical spectral weight in the Drude component and that lost in the interband optical transitions. For the nonrelativistic direction, no such sum rule applies. Results are also presented when an energy gap opens in the energy dispersion. Numerical results due to finite residual scattering are provided and analytic results for the dc limit are derived. Energy dependence and possible anisotropy in the impurity scattering rate is considered. Throughout, we provide comparison of our results for $\sqrt{{\sigma }^{xx}{\sigma }^{yy}}$ with the corresponding results for graphene. A generalization of the 2D Hamiltonian to include powers of higher order than quadratic (nonrelativistic) and linear (relativistic) is considered. We also discuss the modifications introduced when an additional flat band is included via a semi-Dirac version of the $\alpha \text{−}{\mathcal{T}}_3$ model, for which an $\alpha$ parameter tunes between the 2D semi-Dirac (graphenelike) limit and the semi-Dirac version of the dice or ${\mathcal{T}}_3$ lattice.

Surface and core are two essential but distinct structural parts of a nanowire—but their individual effects on overall thermal expansion coefficient of a nanowire have never been quantified. Here we present an average bond-length based framework to determine the effects of the surface and core regimes of $3C$-SiC nanowires on their effective volumetric thermal expansion coefficient over a wide range of temperatures. Our results suggest that the surface and core atoms exhibit differential anharmonic response at finite temperatures, which makes the surface regime exhibit disparate expansion behavior compared to the core. While at lower temperatures the differential anharmonicity is negligible, at temperatures higher than the room temperature there is a pronounced differential anharmonicity in the nanowire. Furthermore, temperature-dependent expansion coefficients of the nanowire and the surface and core regimes qualitatively follow the behavior of the bulk—but they vary substantially quantitatively, with the maximum coefficient exhibited by the surface at higher temperatures. The diameter-dependent expansion coefficients follow inverse power laws with their exponents varying from 0.95 to 2.5. In thinner nanowires the expansion coefficient is controlled by an intricate combination of mass inertia and bond stiffness at the surface and core, whereas the expansion of thicker nanowires is dominated by the anharmonic motion of the core atoms alone. The surface effects saturate with increasing diameter, but the core effects decay nonlinearly with increasing diameter and approaches the bulk value as $d\to \infty$.

We study electron scattering in graphene in hybrid Bose-Fermi systems. We calculate the energy-dependent electron relaxation time, accounting for the processes of emission and absorption of a Bogoliubov excitation (a bogolon). Then, using the Bloch-Grüneisen approach, we find the finite-temperature resistivity of graphene and show that its principal behavior is $\sim T^4$ in the limit of low temperatures and linear at high temperatures. We show that bogolon-mediated scattering can surpass the acoustic-phonon-assisted relaxation. It can be controlled by the distance between the layers and the condensate density, giving us additional degrees of freedom and a useful tool to render electron mobility by the sample design and external pump.

We report ${}^{59}\mathrm{Co}$ NMR and transport measurements on $n$-type filled skutterudites ${\mathrm{Ba}}_x{\mathrm{Yb}}_y{\mathrm{Co}}_4{\mathrm{Sb}}_{12}$ and $A_x{\mathrm{Co}}_4{\mathrm{Sb}}_{12}(A=\mathrm{Ba}$ or Sr), promising thermoelectric materials. The results demonstrate consistently that a shallow defect level near the conduction-band minimum dominates the electronic behavior, in contrast to the behavior of unfilled ${\mathrm{CoSb}}_3$. To analyze the results, we modeled the defect as having a single peak in the density of states, occupied at low temperatures due to donated charges from filler atoms. We fitted the NMR shifts and spin-lattice relaxation rates allowing for arbitrary carrier densities and degeneracies. The results provide a consistent picture for the Hall data, explaining the temperature dependence of the carrier concentration. Furthermore, without adjusting model parameters, we calculated Seebeck coefficient curves, which also provide good consistency. In agreement with recently reported computational results, it appears that composite native defects induced by the presence of filler atoms can explain this behavior. These results provide a better understanding of the balance of charge carriers, which is of crucial importance for designing improved thermoelectric materials.

We report measurements on the high-temperature ionic and low-temperature electronic properties of the three-dimensional topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$ using ion-implanted ${}^8\mathrm{Li}\beta$-detected nuclear magnetic relaxation and resonance. With implantation energies in the range $5–28\mathrm{keV}$, the probes penetrate beyond the expected range of the topological surface state, but are still within $250\mathrm{nm}$ of the surface. At temperatures above $\sim 150\mathrm{K}$, spin-lattice relaxation measurements reveal isolated ${}^8\mathrm{Li}^{+}$ diffusion with an activation energy $E_A=0.185\left(8\right)\mathrm{eV}$ and attempt frequency ${\tau }_0^{-1}=8\pm 3\times {10}^{11}{\mathrm{s}}^{-1}$ for atomic site-to-site hopping. At lower temperature, we find a linear Korringa-type relaxation mechanism with a field-dependent slope and intercept, which is accompanied by an anomalous field dependence to the resonance shift. We suggest that these may be related to a strong contribution from orbital currents or the magnetic freeze-out of charge carriers in this heavily compensated semiconductor, but that conventional theories are unable to account for the extent of the field dependence. Conventional NMR of the stable host nuclei may help elucidate their origin.

Carbon (C) -doped hexagonal boron nitride (hBN) has been experimentally reported to be ferromagnetic at room temperature. Substitution by C in hBN has also been reported to form islands of graphene. In this work, we derive a mechanistic understanding of ferromagnetism with graphene islands in hBN from first principles and the mean-field Hubbard model. We find a general property that in bipartite lattices where the sublattices differ in on-site energies, as in hBN, the ordering between local magnetic moments can be substantial and predominantly antiferromagnetic (AFM) if they are embedded in the same sublattice, unless dominated by Mott-like intersublattice spin separation due to strong localization. The dominant AFM order is rooted at spin-resolved spatial separation of lone pairs of nitrogen (N) and back-transferred electrons on boron (B) due to Coulomb repulsion, thus essentially implying a superexchange pathway. Subsequently, we propose a class of ferrimagnetically ordered interpenetrating superlattices of magnetic graphene islands in hBN, which can be chosen to be a ferromagnetic semiconductor or a half-metal, and we retain a net nonzero magnetic moment at room temperature.

Charge and energy transport across organic/metal interfaces play a decisive role for the functionality of organic semiconductor devices. As well-defined model systems for such heterointerfaces, we consider thin films of the organic semiconductor perylene-tetracarboxylicacid-dianhydride (PTCDA) deposited on the single-crystalline (111)-surfaces of the noble metals silver and gold. By means of time-resolved photoluminescence we investigate the exciton dynamics of these systems in the energy and time domain. Systematic variation of the PTCDA film thickness enables us to follow the exciton relaxation rates as a function of the molecule–metal separation from several nanometers down to a few angstroms. Spatially localized excitations, such as excimers, are found to relax by nonradiative energy transfer to the metal. In contrast, the relaxation of charge-transfer (CT) excitons can be explained by exciton diffusion and subsequent annihilation at the organic/metal interface. Both mechanisms are found to be much more efficient on Ag(111) than on Au(111). For excimers, the faster relaxation on the silver substrate presumably involves the excitation of intraband transitions inside the metal. The higher relaxation rate of CT excitons is explained in terms of enhanced charge transfer across the PTCDA/Ag(111) interface, which is mediated by the electronic interface state inherent to this organic/metal interface.

We show how Floquet Majorana fermions may be experimentally realized by periodic driving of a solid-state platform. The system comprises a planar Josephson junction made of a proximitized heterostructure containing a two-dimensional electron gas with Rashba spin-orbit coupling and Zeeman field. We map the subgap Andreev bound states of the junction to an effective one-dimensional Kitaev model with long-range hopping and pairing terms. Using this effective model, we study the response of the system to periodic driving of the chemical potential, as applied via microwaves through a top gate. We use the bulk Floquet topological invariants to characterize the system in terms of the number of zero and $\pi$ Majorana modes for experimentally realistic parameters. We present signatures of these modes on differential conductance and local density of states. A notable feature is subharmonic response to the drive when both Majorana zero and $\pi$ modes are present, thus opening up the possibility of using this feature to clearly identify Majorana modes. We highlight the robustness of these features to disorder, and show that despite hybridization due to long localization lengths, the main features of the subharmonic response are still identifiable in an experimental system that is readily accessible.

We report terahertz (THz) electrodynamics of a moderately clean A15 superconductor (SC) following ultrafast excitation to manipulate quasiparticle (QP) transport. In the Martensitic normal state, we observe a photo enhancement in the THz conductivity using optical pulses, while the opposite is observed for the THz pump. This demonstrates wavelength-selective nonthermal control of conductivity distinct from sample heating. The photo enhancement persists up to an additional critical temperature, above the SC one, from a competing electronic order. In the SC state, the fluence dependence of pair-breaking kinetics together with an analytic model provides an implication for a “one photon to one Cooper pair” nonresonant energy transfer during the 35-fs laser pulse; i.e., the fitted photon energy $\hslash {\omega }_{}$ absorption to create QPs set by $2{\mathrm{\Delta }}_{SC}/\hslash \omega =0.33\%$. This is more than one order of magnitude smaller than in previously studied BCS SCs, which we attribute to strong electron-phonon coupling and possible influence of phonon condensation.

We study the inverse proximity effect in a bilayer consisting of a thin $s$- or $d$-wave superconductor (S) and a topological insulator (TI). Integrating out the topological fermions of the TI, we find that spin-orbit coupling is induced in the S, which leads to spin-triplet $p$-wave $(f$-wave) correlations in the anomalous Green's function for an $s$-wave $(d$-wave) superconductor. Solving the self-consistency equation for the superconducting order parameter, we find that the inverse proximity effect can be strong for parameters for which the Fermi momenta of the S and TI coincide. The suppression of the gap is approximately proportional to $e^{-1/\lambda }$, where $\lambda$ is the dimensionless superconducting coupling constant. This is consistent with the fact that a higher $\lambda$ gives a more robust superconducting state. For an $s$-wave S, the interval of TI chemical potentials for which the suppression of the gap is strong is centered at ${\mu }_{\mathrm{TI}}=\pm \sqrt{2m{v}_{\mathrm{F}}^2\mu \phantom{A^2}}$, and increases quadratically with the hopping parameter $t$. Since the S chemical potential $\mu$ typically is high for conventional superconductors, the inverse proximity effect is negligible except for $t$ above a critical value. For sufficiently low $t$, however, the inverse proximity effect is negligible, in agreement with what has thus far been assumed in most works studying the proximity effect in S-TI structures. In superconductors with low Fermi energies, such as high-$T_c$ cuprates with $d$-wave symmetry, we again find a suppression of the order parameter. However, since $\mu$ is much smaller in this case, a strong inverse proximity effect can occur at ${\mu }_{\mathrm{TI}}=0$ for much lower values of $t$. Moreover, the onset of a strong inverse proximity effect is preceded by an increase in the order parameter, allowing the gap to be tuned by several orders of magnitude by small variations in ${\mu }_{\mathrm{TI}}$.

The lacunar spinel ${\mathrm{GaV}}_4{\mathrm{S}}_8$ was recently suggested to be a prototype multiferroic material hosting skyrmion lattice states with a sizable polarization $\mathit{P}$ coupled to magnetic order. We explain this phenomenon on the microscopic level. On the basis of density functional theory, we construct an effective model describing the behavior of magnetically active electrons in a weakly coupled lattice formed by molecular orbitals of the ${\left({\mathrm{V}}_4{\mathrm{S}}_4\right)}^{5+}$ clusters. By applying superexchange theory combined with the Berry-phase theory for $\mathit{P}$, we derive a compass model relating the energy and polarization change with the directions of spins ${\mathit{e}}_i$ in magnetic bonds. We argue that, although each skyrmion layer is mainly formed by superexchange interactions in the same plane, the spin dependence of $\mathit{P}$ arises from the stacking misalignment of such planes in the perpendicular direction, which is inherent to the lacunar spinel structure. We predict a strong competition of isotropic, $\sim {\mathit{e}}_i{\mathit{e}}_j$, and antisymmetric, $\sim {\mathit{e}}_i\times {\mathit{e}}_j$, contributions to $\mathit{P}$ that explains the experimentally observed effect.

We have found that magnetically ordered noncentrosymmetric ${\mathrm{CeNiC}}_2$ exhibits superconductivity under very high pressures of near 11 GPa. The transition temperature $T_{\mathrm{c}}$ is 3.5 K, the highest in all Ce heavy-fermion superconductors, implying quite strong electron pairings with a high-energy scale. Several physical quantities show diverging features of a quantum phase transition, however, its criticality appears in a rather narrow range of pressure. The upper critical field ${\mu }_0{H}_{\mathrm{c}2}(T=0)$ is estimated to be 18 T, much higher than the Pauli paramagnetic limiting field of 6.5 T, indicating spin-triplet electron pairings correlated with its noncentrosymmetric structure.

The atomic structure of a superconducting atomic sheet of indium grown on Si(111) surface, i.e., $\mathrm{In}/\mathrm{Si}\left(111\right)\text{−}\sqrt{7}\times \sqrt{3}$, is determined by using surface x-ray diffraction (SXRD) and low-energy electron diffraction (LEED). The structure consists of double layers of In atoms with a rectangular arrangement, verifying a theoretical prediction [J. W. Park and M. H. Kang, Phys. Rev. Lett. 109, 166102 (2012)]. For the so-called hexagonal phase, which has been considered to be a different $\sqrt{7}\times \sqrt{3}$ structure and to undergo a transition to the $\sqrt{7}\times \sqrt{7}$ structure below 265 K, LEED data indicate that the structural basis is $\sqrt{7}\times \sqrt{7}$ even at room temperature. The SXRD data are well reproduced by single-layer In with the In coverage of 10/7 In atoms per Si surface atom, proposed by a theory [J. W. Park and M. H. Kang, Phys. Rev. Lett. 117, 116102 (2016)].