The microscopic origin of the two-dimensional electron system (2DES) at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ heterointerface remains a topic of debate. Among others, mechanisms involving surface defects acting as electron donors for the 2DES have been proposed recently. Since samples are usually investigated after exposure to air, adsorbates are a plausible candidate for such surface defects. We therefore used in situ photoelectron spectroscopy and in situ resistivity measurements to study the impact of adsorbates on the 2DES. The adsorbate coverage caused by storage in air is indeed found to induce additional charge carriers at the interface, compared to the adsorbate-free surface. In controlled adsorption experiments with water and hydrogen we identify adsorbed atomic hydrogen to induce additional charge carriers at the interface, differing from molecular water adsorption reported for samples exposed to macroscopically large volumes of water in air.

We study ballistic transport of Dirac fermions in silicene through arrays of barriers, of width $d$, in the presence of an exchange field $M$ and a tunable potential of height $U$ or depth $-U$. The spin- and valley-resolved conductances as functions of $U$ or $M$, exhibit resonances away from the Dirac point (DP) and close to it a pronounced dip that becomes a gap when a critical electric field $E_z$ is applied. This gap widens by increasing the number of barriers and can be used to realize electric field-controlled switching of the current. The spin $p_s$ and valley $p_v$ polarizations of the current near the DP increase with $E_z$ or $M$ and can reach 100% for certain of their values. These field ranges widen significantly by increasing the number of barriers. Also, $p_s$ and $p_v$ oscillate nearly periodically with the separation between barriers or wells and can be inverted by reversing $M$.

Ab initio electronic structure calculations show that the recently identified quasi-two-dimensional electride ${\mathrm{Y}}_2\mathrm{C}$ is a weak itinerant ferromagnet or is at least close to a ferromagnetic instability. The ferromagnetism is induced by the electride electrons, which are loosely bound around interstitial sites and overlap with each other to form two-dimensional interlayer conduction bands. The semimetallicity and two-dimensionality of the band structure are the keys to understanding this ferromagnetic instability.

Typically point defects are modeled by adding, removing, or exchanging at most few atoms around a given lattice site. We demonstrate the possibility of formation of extended antisite defects that involve complex, nonlocal atomic rearrangements, which cannot be captured within a simple point defect model. We illustrate the formation of extended antisite defects in ${\mathrm{Cu}}_2{\mathrm{SnS}}_3$ and ${\mathrm{Cu}}_2{\mathrm{SnZnS}}_4$ solar absorbers where they lower the formation energy by up to about 1 eV per defect. These extended antisite configurations can dramatically change the stoichiometries and doping properties of multinary semiconductors that have a propensity towards disorder.

We show experimental and theoretical evidence that BiTeI hosts a disordered metallic state named diffusive helical Fermi liquid, characterized by a pair of concentric spin-chiral Fermi surfaces with negligible intervalley scattering. Key experimental observations are extreme disparity of the mobility between inner and outer helical Fermi surfaces near the Weyl point and existence of the so-called universal scaling behavior for the Hall resistivity. Although the extreme enhancement of the inner-Fermi-surface mobility near the Weyl point is quantitatively explained within the self-consistent Born approximation, the existence of universal scaling in the Hall resistivity shows its breakdown, implying necessity of mass renormalization in the inner Fermi-surface beyond the independent electron picture.

We report on the mean-field study of the chiral magnetic effect (CME) in static magnetic fields within a simple model of parity-breaking Weyl semimetal given by the lattice Wilson-Dirac Hamiltonian with constant chiral chemical potential. We consider both the mean-field renormalization of the model parameters and nontrivial corrections to the CME originating from resummed ladder diagrams with arbitrary number of loops. We find that onsite repulsive interactions affect the chiral magnetic conductivity almost exclusively through the enhancement of the renormalized chiral chemical potential. Our results suggest that nontrivial corrections to the chiral magnetic conductivity due to interfermion interactions are not relevant in practice since they only become important when the CME response is strongly suppressed by the large gap in the energy spectrum.

Massless Dirac particles on the helicoid are theoretically investigated. With its possible application being helical graphene, we explore how the peculiarities of Dirac particles appear on the curved, screw-symmetric surface. Zweibein is used to derive the massless Dirac equation on the helicoid and on general curved surfaces. We show that bound states of massless Dirac electrons on the helicoid are absent, and thus the system is fully characterized by the scattering probabilities and the phase shifts. We obtain these quantities from numerically calculated wave functions. We find the local density of states and the phase shifts behave characteristically around the axis of the helicoid. Bound states of massive Dirac electrons on the surface are also shown to be absent as an extension of the above result on massless Dirac electrons. A comparison with the nonrelativistic case is also made.

The structure and superfluid response of nanoscale size ${}^4\mathrm{He}$ clusters enclosed in spherical cavities are studied by computer simulations. The curved surface causes the formation of well-defined concentric shells, thus imparting to the system a very different structure from that of freestanding clusters. On a strongly attractive substrate, superfluidity is only observed at a low density, in the single layer coating the inner surface of the cavity. If the substrate is very weak (e.g., Li), on the other hand, a superfluid two-shell structure can form, whose physical properties interpolate between two and three dimensions. It is shown how experimental signatures of this physical behavior can be detected through measurements of the momentum distribution.

We measured the absolute lengths of three single-crystal silicon samples by means of an imaging Twyman-Green interferometer in the temperature range from 7 K to 293 K with uncertainties of about 1 nm. From these measurements we extracted the coefficient of thermal expansion with uncertainties on the order of $1\times {10}^{-9}/\mathrm{K}$. To access the functional dependence of the length on the temperature, usually polynomials are fitted to the data. Instead we used a physically motivated model equation with seven fit parameters for the whole temperature range. The coefficient of thermal expansion is obtained from the derivative of the best fit. The measurements conducted in 2012 and 2014 demonstrate a high reproducibility, and the agreement of two independently produced samples supports single-crystal silicon as a reference material for thermal expansion. Although the results for all three samples agree with each other and with measurements performed at other institutes, they significantly differ from the currently recommended values for the thermal expansion of crystalline silicon.

Usually photons are not conserved in their interaction with matter. Consequently, for the thermodynamics of photons, while we have a concept of temperature for energy conservation, there is no equivalent chemical potential for particle number conservation. However, the notion of a chemical potential is crucial in understanding a wide variety of single- and many-body effects, from transport in conductors and semiconductors to phase transitions in electronic and atomic systems. Here we show how a direct modification of the system-bath coupling via parametric oscillation creates an effective chemical potential for photons even in the thermodynamic limit. In particular, we show that the photonic system equilibrates to the temperature of the bath, with a tunable chemical potential that is set by the frequency of the parametric coupler. Specific implementations, using circuit-QED or optomechanics, are feasible using current technologies, and we show a detailed example demonstrating the emergence of Mott insulator–superfluid transition in a lattice of nonlinear oscillators. Our approach paves the way for quantum simulation, quantum sources, and even electronlike circuits with light.

Anisotropic magnetoresistance measurements show that the magnetization state in epitaxial ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/{\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}$ bilayers exhibits aging over a wide range of temperatures and magnetic fields. The observed power-law time dependence indicates that aging is characterized by a wide range of activation time scales. Aging characteristics are also inconsistent with a superposition of independent activation barriers expected for Arrhenius-type relaxation. The observed effects indicate a fundamental connection with the critical phenomena in complex condensed matter systems.

We present Co substitution effects in ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y-z}{\mathrm{Co}}_z{\mathrm{Se}}_2$ ($0.06\le z\le 1.73$) single-crystal alloys. By 3.5% of Co doping superconductivity is suppressed, whereas phase separation of semiconducting ${\mathrm{K}}_2{\mathrm{Fe}}_4{\mathrm{Se}}_5$ and superconducting/metallic ${\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{Se}}_2$ is still present. We show that the arrangement and distribution of the superconducting phase (stripe phase) are connected with the arrangement of K, Fe, and Co atoms. Semiconducting spin glass is found in proximity to the superconducting state, persisting for large Co concentrations. At high Co concentrations a ferromagnetic metallic state emerges above the spin glass. This is coincident with changes of the unit cell and arrangement and connectivity of the stripe conducting phase.

Superconductivity in the quasi-one-dimensional material ${\mathrm{Li}}_{0.9}{\mathrm{Mo}}_6{\mathrm{O}}_{17}$ is analyzed based on a multiorbital extended Hubbard model. We found strong charge fluctuations at two different momenta ${\mathbf{Q}}_{\mathbf{1}}$ and ${\mathbf{Q}}_{\mathbf{2}}$ giving rise to two different charge-ordered phases. Evaluating the superconducting vertex, we found superconductivity near strong charge fluctuations at ${\mathbf{Q}}_{\mathbf{1}}$. The order parameter has $p$-wave symmetry with nodes on the Fermi surface. The metallic state displays a characteristic charge collective mode ${\mathbf{Q}}_{\mathbf{1}}$ due to nesting and, for on-site Hubbard repulsion sufficiently large, a charge critical mode ${\mathbf{Q}}_{\mathbf{2}}$ driven by Coulomb repulsion, which softens at the proximity to the transition. The results are quite robust for different coupling parametrizations. A phase diagram discussing the relevance of the model to the physics of the material is proposed.

We have studied the temperature induced $0-\pi$ thermodynamic transition in Nb/PdNi/Nb superconductor/ferromagnetic/superconductor (SFS) heterostructures by microwave measurements of the superfluid density. We have observed a shift in the transition temperature with the ageing of the heterostructures, suggesting that structural and/or chemical changes took place. Motivated by the electrodynamics findings, we have extensively studied the local structural properties of the samples by means of x-ray absorption spectroscopy (XAS) technique, and the compositional profile by time-of-flight secondary ion mass spectrometry (ToF–SIMS). We found that the samples have indeed changed their properties, in particular for what concerns the interfaces and the composition of the ferromagnetic alloy layer. The structural and compositional data are consistent with the shift of the $0-\pi$ transition toward the behavior of heterostructures with different F layers. An important emerging indication to the physics of SFS is the weak relevance of the ideality of the interfaces: even in aged samples, with less-than-ideal interfaces, the temperature-induced $0-\pi$ transition is still detectable albeit at a different critical F thickness.

We consider the impact of the competition among superconductivity, spin density wave, and nematic order in iron pnictides and show that the ordering competition substantially reshapes the global phase diagram. We perform a detailed renormalization group analysis of an effective field theory of iron pnictides and derive the flow equations of all the physical parameters. Using these results, we find that superconductivity can be strongly suppressed by the ordering competition, and we also extract the $T$ dependence of the superfluid density. Moreover, the phase transitions may become first order. Interestingly, our renormalization group (RG) analysis reveals that the nematic order exists only in an intermediate temperature region, $T_m<T<T_n$, but is destroyed at $T>T_n$ by thermal fluctuation and at $T<T_m$ by the ordering competition. This anomalous existence of nematic order leads to a backbending of the nematic transition line on the phase diagram, consistent with the observed reentrance of tetragonal structure at low temperatures. A modified phase diagram is obtained based on the RG results.

Line defects in two-dimensional (2D) materials greatly modulate various properties of their pristine form. Using ab initio molecular dynamics (AIMD) simulations, we investigate the structural reconstructions of different kinds of grain boundaries in the silicene sheets. It is evident that depending upon the presence of silicon adatoms and edge shape of grain boundaries (i.e., armchair or zigzag), stable extended line defects (ELDs) can be introduced in a controlled way. Further studies show the stability of these line-defects in silicene, grown on Ag(111) surface at room-temperature. Importantly, unlike most of the 2D sheet materials such as graphene and hexagonal boron nitride, 5-5-8 line defects modify the nonmagnetic semimetallic pristine silicene sheet to spin-polarized metal. As ferromagnetically ordered magnetic moments remain strongly localized at the line defect, a one-dimensional spin channel gets created in silicene. Interestingly, these spin channels are quite stable because, unlike the edge of nanoribbons, structural reconstruction or contamination cannot destroy the ordering of magnetic moments here. Zigzag silicene nanoribbons with a 5-5-8 line defect also exhibit various interesting electronic and magnetic properties depending upon their width as well as the nature of the magnetic coupling between edge and defect spin states. Upon incorporation of other ELDs, such as 4-4-4 and 4-8 defects, 2D sheets and nanoribbons of silicene show a nonmagnetic metallic or semiconducting ground state. Highlighting the controlled formation of ELDs and consequent emergence of technologically important properties in silicene, we propose new routes to realize silicene-based nanoelectronic and spintronic devices.

The coupled-wires approach has been shown to be useful in describing two-dimensional strongly interacting topological phases. In this manuscript, we extend this approach to three-dimensions, and construct a model for a fractional strong topological insulator. This topologically ordered phase has an exotic gapless state on the surface, called a fractional Dirac liquid, which cannot be described by the Dirac theory of free fermions. Like in noninteracting strong topological insulators, the surface is protected by the presence of time-reversal symmetry and charge conservation. We show that upon breaking these symmetries, the gapped fractional Dirac liquid presents unique features. In particular, the gapped phase that results from breaking time-reversal symmetry has a halved fractional Hall conductance of the form ${\sigma }_{xy}=\frac{1}{2}\frac{e^2}{mh}$ if the filling is $\nu =1/m$. On the other hand, if the surface is gapped by proximity coupling to an $s$-wave superconductor, we end up with an exotic topological superconductor. To reveal the topological nature of this superconducting phase, we partition the surface into two regions: one with broken time-reversal symmetry and another coupled to a superconductor. We find a fractional Majorana mode, which cannot be described by a free Majorana theory, on the boundary between the two regions. The density of states associated with tunneling into this one-dimensional channel is proportional to ${\omega }^{m-1}$, in analogy to the edge of the corresponding Laughlin state.

We calculate the Kadowaki-Woods ratio (KWR) in Fermi liquids with arbitrary band structures. We find that, contrary to the single-band case, the ratio is not generally independent of the effects of electronic correlations (universal). This is very surprising given the experimental findings of a near universal KWR in many multiband strongly correlated metals. We identify a limit where the universality of the ratio is recovered. We discuss the KWR in Dirac semimetals and find that the KWR is independent of correlations, but strongly dependent on the doping of the system: for massless fermions, the KWR is proportional to the inverse square of the carrier density.