We investigate the origin of exoticity in Fe-based systems via studying the fermiology of ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ employing angle-resolved photoemission spectroscopy. While the Fermi surfaces (FSs) at 200 K and 31 K are observed to exhibit two-dimensional and three-dimensional (3D) topology, respectively, the FSs at intermediate temperatures reveal the emergence of the 3D topology at a temperature much lower than the structural and magnetic phase transition temperature (170 K, for the sample under scrutiny). This leads to the conclusion that the evolution of FS topology is not directly driven by the structural transition. In addition, we discover the existence in ambient conditions of energy bands related to the cT phase. These bands are distinctly resolved in the high-photon energy spectra exhibiting strong Fe $3d$ character. They gradually move to higher binding energies due to thermal compression with cooling, leading to the emergence of 3D topology in the Fermi surface. These results reveal the so-far hidden existence of a cT phase under ambient conditions, which is argued to lead to quantum fluctuations responsible for the exotic electronic properties in Fe-pnictide superconductors.

The structure of the layered transition-metal borides $A{\mathrm{B}}_2(A=\text{Os},\mathrm{Ru})$ is built up by alternating $T$ and B layers with the B layers forming a puckered honeycomb. Here we report superconducting properties of ${\mathrm{RuB}}_2$ with a $T_c\approx 1.5$ K using measurements of the magnetic susceptibility versus temperature $T$, magnetization $M$ versus magnetic field $H$, resistivity versus $T$, and heat capacity versus $T$ at various $H$. We observe a reduced heat capacity anomaly at $T_c$ given by $\mathrm{\Delta }C/\gamma T_c\approx 1.1$ suggesting multigap superconductivity. Strong support for this is obtained by the successful fitting of the electronic specific heat data to a two-gap model with gap values ${\mathrm{\Delta }}_1/k_B{T}_c\approx 1.88$ and ${\mathrm{\Delta }}_2/k_B{T}_c\approx 1.13$. Additionally, $M$ versus $H$ measurements reveal a behavior consistent with type-I superconductivity. This is confirmed by comparing the experimental critical field $\approx 122$ Oe obtained from extrapolation to $T=0$ of the $H\text{−}T$ phase diagram, with an estimate of the $T=0$ thermodynamic critical field $\approx 114$ Oe. Additionally, the Ginzburg-Landau parameter was estimated to be $\kappa \approx 0.1\text{–}0.66$. These results strongly suggest multigap type-I superconductivity in ${\mathrm{RuB}}_2$. We also calculate the band structure and obtain the Fermi surface for ${\mathrm{RuB}}_2$. The Fermi surface consists of one quasi-two-dimensional sheet and two concentric ellipsoidal sheets very similar to ${\mathrm{OsB}}_2$. An additional small fourth sheet is also found for ${\mathrm{RuB}}_2$. ${\mathrm{RuB}}_2$ could thus be an example of a multigap type-I superconductor.

The interplay between interactions and quenched disorder can result in rich dynamical quantum phenomena far from equilibrium, particularly when many-body localization prevents the system from full thermalization. With the aim of tackling this interesting regime, here we develop a semianalytical flow equation approach to study the time evolution of strongly disordered interacting quantum systems. We apply this technique to a prototype model of interacting spinless fermions in a random on-site potential in both one and two dimensions. Key results include (i) an explicit construction of the local integrals of motion that characterize the many-body localized phase in one dimension, ultimately connecting the microscopic model to phenomenological descriptions, (ii) calculation of these quantities in two dimensions, and (iii) an investigation of the real-time dynamics in the localized phase which reveals the crucial role of $l$-bit interactions for enhancing dephasing and relaxation.

Understanding the physical properties of magnetic skyrmions is important for fundamental research with the aim to develop new spintronic device paradigms where both logic and memory can be integrated at the same level. Here, we show a universal model based on the micromagnetic formalism that can be used to study skyrmion stability as a function of magnetic field and temperature. We consider ultrathin, circular ferromagnetic magnetic dots. Our results show that magnetic skyrmions with a small radius—compared to the dot radius—are always metastable, while large radius skyrmions form a stable ground state. The change of energy profile determines the weak (strong) size dependence of the metastable (stable) skyrmion as a function of temperature and/or field.

We analyze junctions between noninteracting fermionic topological phases. A closed-form condition for the existence of localized modes at the interface is derived using the transfer matrix approach. These analytical conditions as well as numerical exact diagonalization are used to study interfaces between Weyl semimetals. We observe a Fermi arc reconstruction at the interface, leading to closed curves of zero energy modes in the interface Brillouin zone. These are stable even in certain cases where the two Weyl semimetals differ only in their Fermi arc connectivities.

We present a study that combines polarized and unpolarized neutrons to derive the magnetic structure of the swedenborgite compound $\mathrm{CaBa}\left({\mathrm{Co}}_3\mathrm{Fe}\right){\mathrm{O}}_7$. Integrated intensities from a standard neutron diffraction experiment and polarization matrices from spherical neutron polarimetry have been simultaneously analyzed revealing a complex order, which differs from the usual spin configurations on a kagome lattice. We find that the magnetic structure is well described by a combination of two one-dimensional representations corresponding to the magnetic superspace symmetry $P{2}_1^{\prime }$, and it consists of spins rotating around an axis close to the [110] direction. Due to the propagation vector $\mathbf{q}=\left(\frac{1}{3}00\right)$, this modulation has cycloidal and helicoidal character rendering this system a potential multiferroic. The resulting spin configuration can be mapped onto the classical $\sqrt{3}\times \sqrt{3}$ structure of a kagome lattice, and it indicates an important interplay between the kagome and the triangular layers of the crystal structure.

We present high-resolution measurements of the relative length change as a function of temperature of the organic charge-transfer salt $\kappa -{\left(\mathrm{BEDT}-\mathrm{TTF}\right)}_2{\mathrm{Hg}\left(\mathrm{SCN}\right)}_2\mathrm{Cl}$. We identify anomalous features at $T_g\approx 63$ K which can be assigned to a kinetic glasslike ordering transition. By determining the activation energy $E_A$, this glasslike transition can be related to conformational degrees of freedom of the ethylene endgroups of the organic building block BEDT-TTF. As opposed to other $\kappa$-(BEDT-TTF)${}_{2}X$ salts, we identify a peculiar ethylene endgroup ordering in the present material in which only one of the two crystallographically inequivalent ethylene endgroups is subject to glasslike ordering. This experimental finding is fully consistent with our predictions from ab initio calculations from which we estimate the energy differences $\mathrm{\Delta }E$ and the activation energies $E_A$ between different conformations. The present results indicate that the specific interaction between the ethylene endgroups and the nearby anion layers leads to different energetics of the inequivalent ethylene endgroups, as evidenced by different ratios $E_A/\mathrm{\Delta }E$. We infer that the ratio $E_A/\mathrm{\Delta }E$ is a suitable parameter to identify the tendency of ethylene endgroups toward glasslike freezing.

We investigate the doping-induced changes in the electronic structure of ${\mathrm{CeB}}_6$ on a series of substituted ${\mathrm{Ce}}_{1-x}{R}_x{\mathrm{B}}_6$ samples ($R=\mathrm{La}$, Nd) using diffuse neutron scattering. We observe a redistribution of magnetic spectral weight across the Brillouin zone, which we associate with the changes in the Fermi-surface nesting properties related to the modified charge carrier concentration. In particular, a strong diffuse peak at the corner of the Brillouin zone ($R$ point), which coincides with the propagation vector of the elusive antiferroquadrupolar (AFQ) order in ${\mathrm{CeB}}_6$, is rapidly suppressed by both La and Nd doping, like the AFQ order itself. The corresponding spectral weight is transferred to the $X\left(00\frac{1}{2}\right)$ point, ultimately stabilizing a long-range AFM order at this wave vector at the Nd-rich side of the phase diagram. At an intermediate Nd concentration, a broad diffuse peak with multiple local maxima of intensity is observed around the $X$ point, evidencing itinerant frustration that gives rise to multiple ordered phases for which ${\mathrm{Ce}}_{1-x}{\mathrm{Nd}}_x{\mathrm{B}}_6$ is known. On the La-rich side of the phase diagram, however, dilution of the magnetic moments prevents the formation of a similar $\left(00\frac{1}{2}\right)$-type order despite the presence of nesting. Our results demonstrate how diffuse neutron scattering can be used to probe the nesting vectors in complex $f$-electron systems directly, without reference to the single-particle band structure, and emphasize the role of Fermi surface geometry in stabilizing magnetic order in rare-earth hexaborides.

We present and analyze spin models with long-range interactions whose ground state features a so-called devil's staircase and where plateaus of the staircase are accessed by varying two-body interactions. This is in contrast to the canonical devil's staircase, for example, occurring in the one-dimensional Ising model with long-range interactions, where typically a single-body chemical potential is varied to scan through the plateaus. These systems, moreover, typically feature a particle-hole symmetry which trivially connects the hole part of the staircase (filling fraction $f\ge 1/2$) to its particle part ($f\le 1/2$). Such symmetry is absent in our models and hence the particle sector and the hole sector can be separately controlled, resulting in exotic hybrid staircases.

We present a unified derivation of the dynamical correlation functions including density-density, density-current and current-current, of three-dimensional Weyl/Dirac semimetals by use of the Passarino-Veltman reduction scheme at zero temperature. The generalized Kramers-Kronig relations with arbitrary order of subtraction are established to verify these correlation functions. Our results lead to the exact chiral magnetic conductivity and directly recover the previous ones in several limits. We also investigate the magnetic susceptibilities, the orbital magnetization, and briefly discuss the impact of electron interactions on these physical quantities within the random phase approximation. Our work could provide a starting point for the investigation of the nonlocal transport and optical properties due to the higher-order spatial dispersion in three-dimensional Weyl/Dirac semimetals.

The transfer Hamiltonian tunneling current is derived in a time-dependent density matrix formulation and is used to examine photon-assisted tunneling. Bardeen's tunneling expression arises as the result of first-order perturbation theory in a mean-field expansion of the density matrix. Photon-assisted tunneling from confined electromagnetic fields in the forbidden tunnel barrier region occurs due to time-varying polarization and wave-function overlap in the gap which leads to a nonzero tunneling current in asymmetric device structures, even in an unbiased state. The photon energy is seen to act as an effective temperature-dependent bias in a uniform barrier asymmetric tunneling example problem. Higher-order terms in the density matrix expansion give rise to multiphoton enhanced tunneling currents that can be considered an extension of nonlinear optics where the nonlinear conductance plays a similar role as the nonlinear susceptibilities in the continuity equations.

The magnetoresistance of a material conveys various dynamic information about charge and spin carriers, inspiring both fundamental studies in physics and practical applications such as magnetic sensors, data storage, and spintronic devices. Magnetic impurities play a crucial role in the magnetoresistance as they induce exotic states of matter such as the quantum anomalous Hall effect in topological insulators and tunable ferromagnetic phases in dilute magnetic semiconductors. However, magnetically doped topological Dirac semimetals are hitherto lacking. Here, we report a systematic study of Cr-doped $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ thin films grown by molecular-beam epitaxy. With the Cr doping, $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ thin films exhibit unexpected negative transverse magnetoresistance and strong quantum oscillations, bearing a trivial Berry's phase and an enhanced effective mass. More importantly, with ionic gating the magnetoresistance of Cr-doped $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ thin films can be drastically tuned from negative to positive, demonstrating the strong correlation between electrons and the localized spins of the Cr impurities, which we interpret through the formation of magnetic polarons. Such a negative magnetoresistance under perpendicular magnetic field and its gate tunability have not been observed previously in the Dirac semimetal $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$. The Cr-induced topological phase transition and the formation of magnetic polarons in $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ provide insights into the magnetic interaction in Dirac semimetals as well as their potential applications in spintronics.

Lead telluride (PbTe) is a renowned thermoelectric material with high energy conversion efficiency in medium to high temperature range. However, the performance of PbTe at room temperature is poor due to its relatively high lattice thermal conductivity, which is difficult to be engineered due to its intrinsic very short phonon mean-free path. By performing systematic first-principles and molecular-dynamics simulations, we report that the room-temperature lattice thermal conductivity of PbTe can be reduced by almost one order of magnitude (86%) using the recent experimentally observed nanotwin structure. The mechanism responsible for the dramatic decrease of thermal conductivity strongly depends on the type and mass of atoms at the twin boundary. For PbTe nanotwinned structures with Te at the twin boundary, phonon transport is dominated by the phonon confinement effect and phonon-twin boundary scattering, and the thermal conductivity converges to the bulk value when half of the periodic length is larger than the dominant phonon mean-free path. The same phenomenon is found in another comparison system of KCl nanotwinned structures. However, when Pb is present at the twin boundary, a scattering mechanism occurs: anharmonicity induced by the twin boundary. Due to the mass difference between Pb and Te, the thermal resistance for Pb residing at the twin boundary is found to be one order of magnitude larger than the case with Te at the twin boundary, which results in much stronger phonon-twin boundary scattering. Consequently, the lowest thermal conductivity of such PbTe nanotwinned structure is only 0.4 W/mK, which is reduced by about sevenfold compared to the bulk value of 2.85 W/mK; finally, the converged thermal conductivity cannot restore the bulk value even when half of the periodic length is much larger than the dominant mean-free path. These results offer useful guidance for the development of PbTe-based thermoelectrics and also suggest that nanotwins are excellent building blocks for enhancing the performance of existing thermoelectrics.

We consider an effective model for graphene with interface-induced spin-orbit coupling and calculate the quantum Hall effect in the low-energy limit. We perform a systematic analysis of the contribution of the different terms of the effective Hamiltonian to the quantum Hall effect (QHE). By analyzing the spin splitting of the quantum Hall states as a function of magnetic field and gate voltage, we obtain different scaling laws that can be used to characterize the spin-orbit coupling in experiments. Furthermore, we employ a real-space quantum transport approach to calculate the quantum Hall conductivity and investigate the robustness of the QHE to disorder introduced by hydrogen impurities. For that purpose, we combine first-principles calculations and a genetic algorithm strategy to obtain a graphene-only Hamiltonian that models the impurity.

Resonantly enhanced Raman scattering in dielectric nanostructures has been recently proven to be an efficient tool for nanothermometry and for the experimental determination of their mode composition. In this paper we develop a rigorous analytical theory based on the Green's function approach to calculate the Raman emission from crystalline high-index dielectric nanoparticles. As an example, we consider silicon nanoparticles which have a strong Raman response due to active optical phonon modes. We relate enhancement of Raman signal emission to the Purcell effect due to the excitation of Mie modes inside the nanoparticles. We also employ our numerical approach to calculate inelastic Raman emission in more sophisticated geometries, which do not allow a straightforward analytical form of the Green's function. The Raman response from a silicon nanodisk has been analyzed with the proposed method, and the contribution of various Mie modes has been revealed.

We study the effects of strong impurities on heat transfer in a coupled electron-phonon system in disordered graphene. A detailed analysis of the electron-phonon heat exchange assisted by such an impurity through the “resonant supercollision” mechanism is presented. We further explore the local modification of heat transfer in a weakly disordered graphene due to a resonant scatterer and determine spatial profiles of the phonon and electron temperature around the scatterer under electrical driving. Our results are consistent with recent experimental findings on imaging resonant dissipation from individual atomic defects.

We study the electronic and scattering properties of graphene with moderate Rashba spin-orbit coupling (SOC). The Rashba SOC in graphene tends to distort the band structure and gives rise to a trigonally warped Fermi surface. For electrons at a pronouncedly warped Fermi surface, the spin direction exhibits a staircase profile as a function of the momentum, making an unusual spin texture. We also study the spin-resolved scattering on a Rashba barrier and find that the trigonal warping is essential for producing spin polarization of the transmitted current. Particularly, both the direction and strength of the spin polarization can be controlled by kinds of electric methods. Our work unveils that not only SOC but also the geometry of the Fermi surface is important for generating spin polarization.

Defect formation in LiF, which is used as an observation window in ramp and shock experiments, has significant effects on its transmission properties. Given the extreme conditions of the experiments it is hard to measure the change in transmission directly. Using molecular dynamics, we estimate the change in conductivity as a function of the concentration of likely point and extended defects using a Green-Kubo technique with careful treatment of size effects. With this data, we form a model of the mean behavior and its estimated error; then, we use this model to predict the conductivity of a large sample of defective LiF resulting from a direct simulation of ramp compression as a demonstration of the accuracy of its predictions. Given estimates of defect densities in a LiF window used in an experiment, the model can be used to correct the observations of thermal energy through the window. In addition, the methodology we develop is extensible to modeling, with quantified uncertainty, the effects of a variety of defects on the thermal conductivity of solid materials.

We report muon spin relaxation ($\mu \mathrm{SR}$) measurements of optimally doped and overdoped ${\mathrm{Bi}}_{2+x}{\mathrm{Sr}}_{2-x}{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ single crystals that reveal the presence of a weak temperature-dependent quasistatic internal magnetic field of electronic origin in the superconducting and pseudogap (PG) phases. In both samples the internal magnetic field persists up to 160 K, but muon diffusion prevents following the evolution of the field to higher temperatures. We consider the evidence from our measurements in support of PG order parameter candidates, namely, electronic loop currents and magnetoelectric quadrupoles.

Pristine $1\mathit{T}-{\mathrm{TiTe}}_2$ single crystal has been studied for resistance and magnetoresistance behavior under quasihydrostatic and nonhydrostatic compressions. While the semimetallic state is retained in nearly hydrostatic pressures, small nonhydrostatic compression leads to an abrupt change in low-temperature resistance, a signature of possible charge density wave (CDW) ordering, that eventually collapses above 6.2 GPa. Superconductivity emerges at $\sim 5$ GPa, rapidly increasing to a critical temperature ($T_c$) of 5.3 K at 12 GPa, irrespective of pressure condition. Pressure studies thus evidence that $1\mathit{T}-{\mathrm{TiTe}}_2$ exhibits superconductivity irrespective of the formation of the CDW-like state, implying the existence of phase-separated domains. Most surprisingly, the superconducting state persists upon decompression, establishing a novel phase diagram with suppressed $P$ scale. The pressure quenchable superconductivity, of multiband nature and relatively high upper critical field, makes $1\mathit{T}-{\mathrm{TiTe}}_2$ unique among other layered dichalcogenides.