We study the effect of disorder on the London penetration depth in iron-based superconductors. The theory is based on a two-band model with quasi-two-dimensional Fermi surfaces, which allows for the coexistence region in the phase diagram between magnetic and superconducting states in the presence of intraband and interband scattering. Within the quasiclassical approximation we derive and solve Eilenberger's equations, which include a weak external magnetic field, and provide analytical expressions for the penetration depth in the various limiting cases. A complete numerical analysis of the doping and temperature dependence of the London penetration depth reveals the crucial effect of disorder scattering, which is especially pronounced in the coexistence phase. The experimental implications of our results are discussed.

Numerical calculations on a mesoscopic ring of a type-II superconductor in the London limit suggest that an Abrikosov vortex can be trapped in such a structure above a critical magnetic field and generate a phase shift in the magnetoresistance oscillations. We prepared submicron-sized superconducting loops of single-crystal, type-II superconductor $\mathrm{NbSe}_2$ and measured magnetoresistance oscillations resulting from vortices crossing the loops. The free-energy barrier for vortex crossing determines the crossing rate and is periodically modulated by the external magnetic flux threading the loop. We demonstrated experimentally that the crossing of vortices can be directed at a pair of constrictions in the loop, leading to more pronounced magnetoresistance oscillations than those in a uniform ring. The vortex trapping in both a simple ring and a ring featuring two constrictions was found to result in a phase shift in the magnetoresistance oscillations as predicted in the numerical calculations. The controlled crossing and trapping of vortices demonstrated in our $\mathrm{NbSe}_2$ devices provide a starting point for the manipulation of individual Abrikosov vortices, which is useful for future technologies.

In magnetically doped quantum dots, changing the carrier occupancy from open to closed shells leads to qualitatively different forms of carrier-mediated magnetic ordering. While it is common to study such nanoscale magnets within a mean-field approximation, excluding the spin fluctuations can mask important phenomena and lead to spurious thermodynamic phase transitions in small magnetic systems. By employing coarse-grained, variational, and Monte Carlo methods on singly and doubly occupied quantum dots to include spin fluctuations, we evaluate the relevance of the mean-field description and distinguish different finite-size scaling in nanoscale magnets.

Hybrids of graphene and two-dimensional transition-metal dichalcogenides (TMDCs) have the potential to bring graphene spintronics to the next level. As we show here by performing first-principles calculations of graphene on monolayer ${\mathrm{MoS}}_2$, there are several advantages of such hybrids over pristine graphene. First, Dirac electrons in graphene exhibit a giant global proximity spin-orbit coupling, without compromising the semimetallic character of the whole system at zero field. Remarkably, these spin-orbit effects can be very accurately described by a simple effective Hamiltonian. Second, the Fermi level can be tuned by a transverse electric field to cross the ${\mathrm{MoS}}_2$ conduction band, creating a system of coupled massive and massless electron gases. Both charge and spin transport in such systems should be unique. Finally, we propose to use graphene/TMDC structures as a platform for optospintronics, in particular, for optical spin injection into graphene and for studying spin transfer between TMDCs and graphene.

We clarify analytically and numerically the physical origin and the behavior of the Norton field scattered by a narrow slit, at optical frequencies. This apparent surface field, which comes in addition to the surface plasmon-polariton and classic cylindrical light waves, features its own radiation lobe associated with oscillating induced currents that spread over both horizontal metallic parts forming the slit. Theory is given taking into account the finite size of the aperture and is illustrated with materials such as gold and amorphous silicon in different spectral regions.

We investigate the evolution of attosecond to femtosecond screening and emergent potentials that govern the dynamics and energetics of electrons and holes excited in the various stages of multiphoton photoemission processes and control the photoelectron yield in recently reported experiments [X. Cui, C. Wang, A. Argondizzo, S. Garrett-Roe, B. Gumhalter, and H. Petek, Nat. Phys. 10, 505 (2014)]. The study is focused on the dynamical screening of holes created in preexistent quasi-two-dimensional Shockley state bands on Ag(111) and Cu(111) surfaces and of electrons excited to the intermediate and emerging screened states. Using the formalism of self-consistent electronic response, we analyze first the effects of screening on the dynamics of photoexcited electrons and holes and then of the Coulomb correlated photoexcited pair. Special attention is paid to the correlated primary electron-hole states, which commence as transient surface excitons and develop in the course of screening into uncorrelated electrons and holes propagating in the image potential and surface state bands, respectively. The obtained results enable to establish a consistent picture of transient electron dynamics at Ag(111) and Cu(111) surfaces that are becoming accessible by the time-, energy-, and momentum-resolved pump-probe multiphoton photoelectron spectroscopies.

The conductivity of graphene on a boron nitride substrate exhibits features in the terahertz (THz) and infrared (IR) frequency regimes that are associated with the periodic moiré pattern formed by weakly coupled two-dimensional materials. The THz and IR features are strongest when the two honeycomb lattices are orientationally aligned, and in this case they are Pauli blocked unless the Fermi level is close to $\pm 150$ meV relative to the graphene sheet Dirac point. Because the transition energies between moiré bands formed above the Dirac point are small, ac conductivity features in $n$-doped graphene tend to be overwhelmed by the Drude peak. The substrate-induced band splitting is larger at energies below the Dirac point, however, and it can lead to sharp features at THz and IR frequencies in $p$-doped graphene. In this paper, we focus on the strongest few THz and IR features, explaining how they arise from critical points in the moiré-band joint density of states, and commenting on the interval of Fermi energy over which they are active.

The monolayer FeSe with a thickness of one unit cell grown on a single-crystal $\mathrm{SrTiO}_3$ substrate (FeSe/STO) exhibits striking high-temperature superconductivity with transition temperature $T_c$ over 65 K reported by recent experimental measurements. In this work, through analyzing the distinctive electronic structure, and providing systematic classification of the pairing symmetry, we find that both $s$- and $p$-wave pairing with odd parity give rise to topological superconducting states in monolayer FeSe, and the exotic properties of $s$-wave topological superconducting states have close relations with the unique nonsymmorphic lattice structure which induces the orbital-momentum locking. Our results indicate that the monolayer FeSe could be in the topological nontrivial $s$-wave superconducting states if the relevant effective pairing interactions are dominant in comparison with other candidates.

Polaritons of defined momentum and energy are excited resonantly on the lower polariton branch of a planar semiconductor microcavity in the strong coupling regime, and the spectrally and momentum resolved emission is analyzed. We observe ghost branches from scattering within the lower polariton branch, as well as from scattering to the middle polariton branch, showing the nonlinear mixing between different branches. Extending the theoretical treatment of spontaneous parametric luminescence developed in Ciuti et al. [Phys. Rev. B 63, 041303 (2001)], the eigenmodes of the driven polariton system and its photoluminescence are modeled. A quantitative agreement with the measured branch positions and a qualitative agreement with the branch intensities is found.

When quartz is compressed at room temperature it retains its crystal structure at pressures well above its stability domain (0–2 GPa), and collapses into denser structures only when pressure reaches 20 GPa. Depending on the experimental conditions, pressure-induced densification can be accompanied by amorphization; by the formation of crystalline, metastable polymorphs; and can be preceded by the appearance of an intermediate phase, quartz II, with unknown structure. Based on molecular dynamic simulations, we show that this rich phenomenology can be rationalized through a unified theoretical framework of the atomistic pathways leading to densification. The model emphasizes the role played by the oxygen sublattice, which transforms from a bcc-like order in quartz into close-packed arrangements in the denser structures, through a ferroelastic instability of martensitic nature.

The dependence of the longitudinal thermoelectric power on the orientation of magnetization in Pt/Co/Pt sandwiches is investigated. In the Co thickness range $\le 6$ nm, where interface scattering is relevant, the thermoelectric power depends on the orientation of magnetization in the plane perpendicular to the temperature gradient. This behavior reveals the thermoelectric analog to the anisotropic interface magnetoresistance. It is shown that this interfacial magnetothermoelectric power fulfills Mott's formula, however, significant deviations from the bulk anisotropic magnetothermoelectric behavior are reported. The dependence of the Seebeck effect on magnetization orientation therefore provides experimental evidence of differences in the electronic states of the bulk and interface. We demonstrate that for the very same system the Seebeck coefficient does not show a one-to-one correspondence to the conductivity.

We study the spin-$\frac{1}{2}$ Heisenberg model on the triangular lattice with antiferromagnetic first- $\left(J_1\right)$ and second- $\left(J_2\right)$ nearest-neighbor interactions using density matrix renormalization group. By studying the spin correlation function, we find a ${120}^{\circ }$ magnetic order phase for $J_2\lesssim 0.07J_1$ and a stripe antiferromagnetic phase for $J_2\gtrsim 0.15J_1$. Between these two phases, we identify a spin-liquid region characterized by exponential decaying spin and dimer correlations, as well as large spin singlet and triplet excitation gaps on finite-size systems. We find two near degenerating ground states with distinct properties in two sectors, which indicates more than one spin-liquid candidate in this region. While the sector with spinons is found to respect time reversal symmetry, the even sector without spinons breaks such a symmetry for finite-size systems. Furthermore, we detect the signature of the fractionalization by following the evolution of different ground states with inserting spin flux into the cylinder system. Moreover, by tuning the anisotropic bond coupling, we explore the nature of the spin-liquid phase and find the optimal parameter region for gapped $Z_2$ spin liquids.

We systematically investigate the crystal structure, magneto-optical properties, magnetization, and magnetotransport properties of a new ferromagnetic semiconductor $(\mathrm{G}{\mathrm{a}}_{1-x},\mathrm{F}{\mathrm{e}}_x)\mathrm{Sb}$ grown by low-temperature molecular beam epitaxy. Crystal structure analyses by x-ray diffraction, scanning transmission microscopy, and transmission electron diffraction indicate that the $(\mathrm{G}{\mathrm{a}}_{1-x},\mathrm{F}{\mathrm{e}}_x)\mathrm{Sb}$ thin films maintain the zinc-blende crystal structure up to $x=20\%$. We carried out the characterizations of the magnetic properties of the $(\mathrm{G}{\mathrm{a}}_{1-x},\mathrm{F}{\mathrm{e}}_x)\mathrm{Sb}$ thin films by various methods, including magnetic circular dichroism spectroscopy, anomalous Hall effect, and superconducting quantum interference device magnetometry, and found that $(\mathrm{Ga},\mathrm{Fe})\mathrm{Sb}$ is an intrinsic ferromagnetic semiconductor without any second-phase precipitations. The Curie temperature $T_{\mathrm{C}}$ of $(\mathrm{G}{\mathrm{a}}_{1-x},\mathrm{F}{\mathrm{e}}_x)\mathrm{Sb}$ depends on $x$ and hole concentration, as in the case of hole-induced ferromagnetism. The highest $T_{\mathrm{C}}$ reaches 230 K at $x=20\%$, which is the highest value so far reported in III-V based ferromagnetic semiconductors.

Intrinsic, two-dimensional ferromagnetic semiconductors are an important class of materials for overcoming the limitations of dilute magnetic semiconductors for spintronics applications. ${\text{CrSiTe}}_3$ is a particularly interesting member of this class, since it can likely be exfoliated down to single layers, where $T_C$ is predicted to increase dramatically. Establishing the nature of the magnetism in the bulk is a necessary precursor to understanding the magnetic behavior in thin-film samples and the possible applications of this material. In this work, we use elastic and inelastic neutron scattering to measure the magnetic properties of single-crystalline ${\text{CrSiTe}}_3$. We find that there is a very small single ion anisotropy favoring magnetic ordering along the $c$ axis and that the measured spin waves fit well to a model where the moments are only weakly coupled along that direction. Finally, we find that both static and dynamic correlations persist within the $ab$ plane up to at least 300 K, strong evidence of this material's two-dimensional characteristics that are relevant for future studies on thin-film and monolayer samples.

A theoretical concept is presented for the screening of several magnetic moments locally exchange coupled to conduction electrons in a metallic nanostructure. We consider a quantum confined multi-impurity Kondo model which exhibits the competition between finite-size effects, RKKY interactions, and Kondo physics. In the limit of weak coupling, Kondo correlations are cut by the finite system size; perturbation theory can then be used to derive the low-energy effective model, which is of generalized central-spin form. The theory successfully predicts the degeneracy, total spin, and spin correlations of the ground state, and allows the number of screening channels to be identified. This is demonstrated for a two-impurity model on a finite one-dimensional ring. Density-matrix renormalization-group calculations confirm the physical picture at weak coupling. The nontrivial crossovers to RKKY and strong-coupling regimes are also studied. The numerical renormalization group, tailored to treat finite systems, is used to examine the crossover to the thermodynamic limit.

We give evidence of nanocomposite ZnO-Co thin films displaying room temperature ferromagnetism as well as semiconducting-type behavior. The remanent magnetization may reach up to $70\mathrm{kA}{\mathrm{m}}^{-1}$ and the resistivity at 300 K is ranging from 0.1 to $2\mathrm{\Omega }\mathrm{cm}$. The size (from 2 to 30 nm) and shape of the metallic Co clusters in the ZnO host matrix are determined by the temperature and the pressure conditions used during the pulsed laser deposition process. While the ferromagnetic response is entirely based on the Co clusters, the electrical transport essentially takes place in the oxide matrix. At low temperature $(<100\mathrm{K})$, the resistivity varies according to the variable range hopping model while at high temperature $(>100\mathrm{K})$ a Kondo-type $ln\left(T\right)$ dependence is observed. The magnetoresistance (MR) at high magnetic fields $(>1\mathrm{T})$ is shown to result from the oxide matrix. Large positive MR values $(+20\%)$ are obtained at low temperature (10 K), originating from the $\mathrm{C}{\mathrm{o}}^{2+}$ paramagnetic ions in the ZnO host, while low negative values $(-1\%)$ are systematically found in the high-temperature range $(>80\mathrm{K})$ due to spin scattering at defects. Distinctly, at low magnetic fields $(<1\mathrm{T})$, MR hysteresis curves are measured, in perfect correspondence with the magnetization curves due to the Co clusters. This proves that the ZnO-Co boundaries act as efficient spin filters for the electrons which cross them, making the nanocomposite films interesting for spintronic applications.

Motivated by the recent inelastic neutron scattering (INS) measurements in the iron pnictides which show a strong anisotropy of spin excitations even above the magnetic transition temperature $T_N$, we study the spin dynamics within the frustrated Heisenberg model with biquadratic spin-spin exchange interactions. Using the Dyson-Maleev (DM) representation, which proves appropriate for all temperature regimes, we find that the spin-spin dynamical structure factors are in excellent agreement with experiment, exhibiting breaking of the $C_4$ symmetry even into the paramagnetic region $T_N<T<T_{\sigma }$, which we refer to as the Ising-nematic phase. In addition to the Heisenberg spin interaction, we include the biquadratic coupling $-K{({\mathbf{S}}_i·{\mathbf{S}}_j)}^2$ and study its effect on the dynamical temperature range $T_{\sigma }-T_N$ of the Ising-nematic phase. We find that this range reduces dramatically when even small values of the interlayer exchange $J_c$ and biquadratic coupling $K$ are included. To supplement our analysis, we benchmark the results obtained using full decoupling in the DM method against those from different nonlinear spin-wave theories, including the recently developed generalized spin-wave theory (GSWT), and find good qualitative agreement among the different theoretical approaches as well as experiment for both the spin-wave dispersions and the dynamical structure factors.

Theoretical sorting of stable and synthesizable “missing compounds” from those that are unstable is a crucial step in the discovery of previously unknown functional materials. This active research area often involves high-throughput (HT) examination of the total energy of a given compound in a list of candidate formal structure types (FSTs), searching for those with the lowest energy within that list. While it is well appreciated that local relaxation methods based on a fixed list of structure types can lead to inaccurate geometries, this approach is widely used in HT studies because it produces answers faster than global optimization methods (that vary lattice vectors and atomic positions without local restrictions). We find, however, a different failure mode of the HT protocol: specific crystallographic classes of formal structure types each correspond to a series of chemically distinct “daughter structure types” (DSTs) that have the same space group but possess totally different local bonding configurations, including coordination types. Failure to include such DSTs in the fixed list of examined candidate structures used in contemporary high-throughput approaches can lead to qualitative misidentification of the stable bonding pattern, not just quantitative inaccuracies. In this work, we (i) clarify the understanding of the general DST-FST relationship, thus improving current discovery HT approaches, (ii) illustrate this failure mode for RbCuS and RbCuSe (the latter being a yet unreported compound and is predicted here) by developing a synthesis method and accelerated crystal-structure determination, and (iii) apply the genetic-algorithm-based global space-group optimization (GSGO) approach which is not vulnerable to the failure mode of HT searches of fixed lists, demonstrating a correct identification of the stable DST. The broad impact of items (i)–(iii) lies in the demonstrated predictive ability of a more comprehensive search strategy than what is currently used—use HT calculations as the preliminary broad screening followed by unbiased GSGO of the final candidates.

We report on the effect of noise on the characteristics of the bistable polariton emission system. The present experiment provides a time-resolved access to the polariton emission intensity. We evidence the noise-induced transitions between the two stable states of the bistable polaritons. It is shown that the external noise specifications, intensity and correlation time, can efficiently modify the polariton Kramers time and residence time. We find that there is a threshold noise strength that provokes the collapse of the hysteresis loop. The experimental results are reproduced by numerical simulations using Gross-Pitaevskii equation driven by a stochastic excitation.

Ion-gated transistors have enabled us to electrically control electronic phase transitions such as superconductivity in a variety of materials. On the other hand, the carrier doping mechanism, particularly in oxide semiconductors, still remains elusive. Here, we report on the low-temperature thermopower of two-dimensional electron systems in ion-gated ${\mathrm{SrTiO}}_3$. We found that the metallic states induced by ionic gating exhibit a large peak in thermoelectric power to over 1 mV/K at around 20 K. This enhancement of cryogenic thermoelectric power is ascribed to a phonon-drag effect, which is in general dramatically suppressed by conventional chemical doping because the phonon mean free path is suppressed by chemical disorder. The large sustained peak, even in the high carrier density regime of $2\times {10}^{14}{\mathrm{cm}}^{-2}$, strongly indicates that a less disordered process, i.e., electrostatic charge accumulation, dominates over electrochemical carrier doping in the ionic gating in ${\mathrm{SrTiO}}_3$.