We study the loading of a nonequilibrium, dissipative system of composite bosons—exciton polaritons—into a one-dimensional periodic lattice potential. Utilizing momentum resolved photoluminescence spectroscopy, we observe a transition between an incoherent Bose gas and a polariton condensate, which undergoes further transitions between different energy states in the band-gap spectrum of the periodic potential with increasing pumping power. We demonstrate controlled loading into distinct energy bands by modifying the size and shape of the excitation beam. The observed effects are comprehensively described in the framework of a nonequilibrium model of polariton condensation. In particular, we implement a stochastic treatment of quantum and thermal fluctuations in the system and conclude that polariton-phonon scattering is a plausible energy relaxation mechanism enabling transitions from the highly nonequilibrium polariton condensate in the gap to the ground band condensation for large pump powers.

An ensemble of quantum dot excitons may be used for coherent information manipulation. Due to the ensemble inhomogeneity any optical information retrieval occurs in the form of a photon echo. We show that the inhomogeneity can lead to a significant deviation from the conventional echo timing sequence. Variation of the area of the initial rotation pulse, which generates excitons in a dot subensemble only, reveals this complex picture of photon echo formation. We observe a retarded echo for $\pi /2$ pulses, while for $3\pi /2$ the echo is advanced in time as evidenced through monitoring the Rabi oscillations in the time-resolved photon echo amplitude from (In,Ga)As/GaAs self-assembled quantum dot structures and confirmed by detailed calculations.

In typical topological insulator (TI) systems the TI is bordered by a non-TI insulator, and the surrounding conventional insulators, including vacuum, are not generally treated as part of the TI system. Here, we implement a material system where the roles are reversed, and the topological surface states form around the non-TI (instead of the TI) layers. This is realized by growing a layer of the tunable non-TI ${\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{I}{\mathrm{n}}_x\right)}_2\mathrm{S}{\mathrm{e}}_3$ in between two layers of the TI $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ using the atomically precise molecular beam epitaxy technique. On this tunable inverse topological platform, we systematically vary the thickness and the composition of the ${\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{I}{\mathrm{n}}_x\right)}_2\mathrm{S}{\mathrm{e}}_3$ layer and show that this tunes the coupling between the TI layers from strongly coupled metallic to weakly coupled, and finally to a fully decoupled insulating regime. This system can be used to probe the fundamental nature of coupling in TI materials and provides a tunable insulating layer for TI devices.

We report that anomalous low-energy excitation (ALE) peaks in the heat capacity emerging from single-crystal cage materials can be successfully rationalized in terms of a single unified exponential line for a variety of type-I clathrates by employing a parameter associated with the freedom of space and the modified radii of guest atoms estimated by band calculations. The origin of these low-energy excitations is interpreted in the framework of quasiharmonic van der Waals type guest-host interactions based on a unified picture with the help of first-principles calculations. It is shown that the influence of guest-host ionic and covalent bonding interactions on the phonon anharmonicity, which have so far been considered to play an important role, are not significant as long as high symmetry of the cage structure is preserved. The dominant van der Waals interactions explain the soft vibrational modes of the rattling, which suppress phonon transport and lead to the concept of “phonon-glass electron-crystal” (PGEC) for thermoelectric applications. A few exceptions existing in type-I clathrates, as indicated by deviations from the unified line, suggest that a quasiharmonic potential can become more asymmetric via lower symmetry of the cage structure, towards glasslike disordered states at even lower temperatures. Although the origin of the boson peaks appearing in disordered materials is still under debate due to incomplete information on the real structure, the understanding provided by the present paper for crystalline cage materials may provide information partly applicable to other disordered systems.

Here we report on the realization of tuning the intrinsic damping in the half-metallic Heusler compound ${\mathrm{Co}}_2\mathrm{MnSi}$ by substituting Co by Ir. The work includes theoretical calculations and experimental measurements on bulk and thin films samples. Control of damping is to remove unwanted magnetization motion and suppress signal echoes through uncontrolled precession of the magnetization for future implementation of this material into, e.g., current perpendicular plane–giant-magnetoresistance sensors. Density functional calculations revealed stable magnetization and increasing damping parameter with Iridium concentration, whereas the half metallicity could be retained. The calculations are consistent with experimental results from bulk and thin film samples of this report and elucidate the linear dependence of the Gilbert damping parameter on the substituent concentration. This report again demonstrates the inherent tunability of Heusler compounds, which constitutes a pivotal feature of this material class.

Exchange bias is observed in ferromagnetic/antiferromagnetic (FM/AF) layered stacks and in materials with neighboring ferromagnetic and antiferromagnetic granules. The latter is commonly observed in Ni-Mn-based martensitic Heusler alloys. In general, the exchange-bias effect is identified as horizontally shifted hysteresis loop when the system is field cooled from high temperatures. We report here loop shifts not only under field-cooled but also under zero-field-cooled conditions in magnetically granular martensitic ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{50-x}{\mathrm{Sn}}_x$ Heusler alloys in the compositional range $13.0\ge x\ge 8.9$. Under zero-field-cooled conditions, the initially applied field can carry the system over energy barriers and stabilize a spin-reconfigured state so that a negatively shifted hysteresis loop can also occur here as in the field-cooled state. Spin reconfiguration occurs when the relative size of the AF and FM regions as well as the relative strength of the of AF and FM interactions are in balance.

We perform a theoretical search for dynamical cross-correlated responses of three-dimensional topological superconductors and superfluids. It has been suggested that a gravitational topological term, which is analogous to the $\theta$ term in topological insulators, can be derived in three-dimensional time-reversal invariant topological superconductors and superfluids, and that the dynamical gravitational axion field can be realized by the fluctuation of the relative phase, i.e., by the Leggett mode between topological $p$-wave pairing and conventional $s$-wave pairing. In the presence of the dynamical gravitational axion field, we propose the emergence of the “chiral gravitomagnetic effect,” a thermal current generation by gravitomagnetic fields, i.e., by mechanical rotations. This effect can be regarded as a thermal counterpart of the chiral magnetic effect, which has been studied mainly in Weyl semimetals. We also show the occurrence of the anomalous thermal Hall effect in the bulk. We discuss a possible application of our study to the thermal responses of Weyl superconductors.

Using high-pressure magnetotransport techniques we have discovered superconductivity in $\mathrm{B}{\mathrm{i}}_2\mathrm{Te}$, a member of the infinitely adaptive ${\left(\mathrm{B}{\mathrm{i}}_2\right)}_m{\left(\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3\right)}_n$ series, whose end members, Bi and $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$, can be tuned to display topological surface states or superconductivity. $\mathrm{B}{\mathrm{i}}_2\mathrm{Te}$ has a maximum $T_{\mathrm{c}}=8.6\mathrm{K}$ at $P=14.5\mathrm{GPa}$ and goes through multiple high-pressure phase transitions, ultimately collapsing into a bcc structure that suggests a universal behavior across the series. High-pressure magnetoresistance and Hall measurements suggest a semimetal to metal transition near 5.4 GPa, which accompanies the hexagonal to intermediate phase transition seen via x-ray diffraction measurements. In addition, the linearity of $H_{\mathrm{c}2}\left(T\right)$ exceeds the Werthamer-Helfand-Hohenberg limit, even in the extreme spin-orbit scattering limit, yet is consistent with other strong spin-orbit materials. Considering these results in combination with similar reports on strong spin-orbit scattering materials seen in the literature, we suggest the need for a new theory that can address the unconventional nature of their superconducting states.

The influence of Zn and Ni impurities on the normal-state pseudogap of underdoped $\text{high-}{T}_c$ cuprate superconductors is studied using exact diagonalization of effective $t-J\text{-like}$ Hamiltonians describing low energy electronic excitations of the ${\mathrm{CuO}}_2$ plane with some of the copper ions replaced with Zn/Ni. The Ni case Hamiltonian has been obtained by a sequence of approximations from a more complete model involving Cu $3d$, Ni $3d$, and O $2p$ orbitals. Our main findings are: (i) The width ${\mathrm{\Omega }}_{\mathrm{PG}}$ of the pseudogap occurring in the many body density of states, and manifesting itself also in the $c\text{-axis}$ infrared conductivity, decreases with increasing Zn concentration as a consequence of a suppression of short range spin correlations. (ii) In the case of one hole and one Ni impurity, the hole is—for realistic values of the model parameters—weakly bound to the Ni site. This causes a slight increase of ${\mathrm{\Omega }}_{\mathrm{PG}}$ with respect to the pure case. (iii) Based on this result and further results for 1–2 holes and 1–2 Ni impurities, we suggest that in the real Ni substituted ${\mathrm{CuO}}_2$ plane ${\mathrm{\Omega }}_{\mathrm{PG}}$ is larger than in the pure case due to the binding of the doped holes to the Ni sites and effective underdoping. Our findings clarify the trends observed in the $c\text{-axis}$ infrared conductivity data of Zn and Ni substituted (Sm,Nd)${\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ crystals.

The lattice thermal conductivity (${\kappa }_{\mathrm{L}}$) of ${\mathrm{PbTiO}}_3$ is estimated using a combination of ab initio calculations and the semiclassical Boltzmann transport equation. The computed ${\kappa }_{\mathrm{L}}$ is remarkably low, nearly comparable with the ${\kappa }_{\mathrm{L}}$ of good thermoelectric materials such as PbTe. In addition, a semiclassical analysis of the electronic transport quantities is presented, which suggests excellent thermoelectric properties, with a figure of merit $zT$ well over 1 for a wide range of temperatures. For thermoelectric applications, the ${\kappa }_{\mathrm{L}}$ could be further reduced by utilizing different morphologies and compositions.

The redistribution of the electron density in a material during laser irradiation can have a significant impact on its structural dynamics. This electronic excitation can be incorporated into two temperature molecular dynamics (2T-MD) simulations through the use of electronic temperature dependent potentials. Here, we study the structural dynamics of laser irradiated tungsten nanofilms using 2T-MD simulations with an electronic temperature dependent potential and compare the results to equivalent simulations that employ a ground-state interatomic potential. Electronic excitation leads to an expansion of the crystal and a decrease in the melting point of tungsten. During laser irradiation these factors ensure that the threshold fluences to the different melting regimes are reduced. Furthermore, both heterogenous and homogeneous melting are predicted to occur more rapidly due to excitation and oscillations in the film thickness will be accentuated.

We consider the response of etalons created by a combination of a conventional mirror and a metasurface composed of a periodic lattice of metal scatterers with a resonant response. This geometry has been used previously for hybridization of localized plasmons with Fabry-Perot resonances and for perfect absorption, in so-called Salisbury screens. The particular aspect we address is if one can assume an environment-independent reflectivity for the metasurface when calculating the reflectivity of the composite system, as in a standard Fabry-Perot analysis, or whether the fact that the metasurface interacts with its own mirror image renormalizes its response. Using lattice sum theory, we take into account all possible retarded dipole-dipole interactions of scatterers in the metasurface amongst each other and through the mirror. We show that while a layer-by-layer Fabry-Perot formalism captures the main qualitative features of metasurface etalons, in fact the mirror modifies both the polarizability and reflectivity of the metasurface in a fashion that is akin to Drexhage's modification of the radiative properties of a single dipole.

Antiferromagnets (AFMs) exhibit intrinsic magnetization when the order parameter spatially varies. This intrinsic spin is present even at equilibrium and can be interpreted as a twisting of the homogeneous AFM into a state with a finite spin. Because magnetic moments couple directly to external magnetic fields, the intrinsic magnetization can alter the dynamics of antiferromagnetic textures under such influence. Starting from the discrete Heisenberg model, we derive the continuum limit of the free energy of AFMs in the exchange approximation and explicitly rederive that the spatial variation of the antiferromagnetic order parameter is associated with an intrinsic magnetization density. We calculate the magnetization profile of a domain wall and discuss how the intrinsic magnetization reacts to external forces. We show conclusively, both analytically and numerically, that a spatially inhomogeneous magnetic field can move and control the position of domain walls in AFMs. By comparing our model to a commonly used alternative parametrization procedure for the continuum fields, we show that the physical interpretations of these fields depend critically on the choice of parametrization procedure for the discrete-to-continuous transition. This can explain why a significant amount of recent studies of the dynamics of AFMs, including effective models that describe the motion of antiferromagnetic domain walls, have neglected the intrinsic spin of the textured order parameter.

Using first-principles calculations, we find a dynamically stable structure of Li-intercalated bilayer ${\mathrm{MoS}}_2$, of which two monolayers are mirror images of each other. It is shown that the stable Li-intercalated bilayer ${\mathrm{MoS}}_2$ can become a BCS superconductor with superconducting transition temperature $T_c$ evaluated over 10 K. The partial charge transfer from Li atoms to inner S planes of two ${\mathrm{MoS}}_2$ monolayers leads to a semiconductor-metal transition and an ionic interlayer interaction so as to increase the screening effect within the layers, soften phonon modes, and enhance electron-phonon couplings. Our finding suggests further experimental efforts directed toward an understanding of quasi-two-dimensional superconductivity.

A systematic investigation of electronic, lattice dynamical, and superconducting properties of the early actinides Ac, Th, and Pa within the framework of density functional perturbation theory is presented. Spin-orbit coupling is included self-consistently in the calculation of all relevant quantities. The largest coupling constant is obtained for Ac, which with $\lambda \approx 0.97$ is close to the strong coupling regime, and where a sizable superconducting transition temperature of 4–5 K is predicted. For Th and Pa, a reduced $\lambda \approx 0.5$ is found in accordance with the observed smaller $T_c$. By comparing with scalar-relativistic calculations we found that spin-orbit coupling results in a hardening of the phonon spectrum for all cases. Its influence on $\lambda$ is almost negligible for Ac and Pa. In contrast, for Th spin-orbit interaction invokes band splittings near the Fermi level, which reduces $\lambda$ by 21%.

The search for novel materials with Dirac cone band dispersion is one of the most challenging and important works for both fundamental physics and technological applications. Here, we studied the electronic structure of FeSe thin films grown on $\mathrm{SrTi}{\mathrm{O}}_3$ substrates by angle-resolved photoemission spectroscopy (ARPES). We revealed the existence of Dirac cone band dispersions in FeSe thin films thicker than 1 unit cell below the nematic transition temperature, whose apexes are located −10 meV below Fermi energy. The evolution of electronic structures for FeSe thin films as a function of temperature, thickness, and cobalt doping were systematically studied. The Dirac cones coexist with the nematicity in FeSe and disappear when nematicity is suppressed. Our results provide useful guidelines for understanding the novel electronic structure, nematicity, and superconductivity in the FeSe system.

Intensive effort has recently been made in search of topological insulators (TIs) that have great potential in spintronics applications. In this paper, a novel concept of overlayer induced interfacial TI phase in conventional semiconductor surface is proposed. The first-principles calculations demonstrate that a $p$-band-element $X$ $(X=\text{In}$, Bi, and Pb) decorated $d$-band surface, such as Au/Si(111) surface $[X$/Au/Si(111)] of an existing experimental system, offers a promising prototype for TIs. Specifically, Bi/Au/Si(111) and Pb/Au/Si(111) are identified to be large-gap TIs. A $p\text{−}d$ band inversion mechanism induced by growth of $X$ in the Au/Si(111) surface is revealed to function at different coverage of $X$ with different lattice symmetries, suggesting a general approach of interface orbital engineering of large-gap TIs via tuning the interfacial atomic orbital position of $X$ relative to Au.