We demonstrate that the entropy per electron of a two-dimensional electron gas (2DEG) exhibits quantized peaks at resonances between the chemical potential and electron levels of size quantization. In the limit of no scattering, the peaks depend only on the subband quantization number and are independent of material parameters, shape of the confining potential, electron effective mass, and temperature. The quantization of entropy per electron is a signature of a Lifshitz phase transition in a 2DEG. In the presence of stationary disorder, the magnitude of peaks decreases. Its deviation from the quantized values is a direct measure of the disorder induced smearing of the electronic density of states.

States of matter with nontrivial topology have been classified by their bulk symmetry properties. However, by cutting the topological insulator into ribbons, the symmetry of the system is reduced. By constructing effective Hamiltonians containing the proper symmetry of the ribbon, we find that the nature of topological states is dependent on the reduced symmetry of the ribbon and the appropriate boundary conditions. We apply our model to the recently discovered two-dimensional topological crystalline insulators composed by IV-VI monolayers, where we verify that the edge terminations play a major role on the Dirac crossings. Particularly, we find that some bulk cuts lead to nonsymmorphic ribbons, even though the bulk material is symmorphic. The nonsymmorphism yields a new topological protection, where the Dirac cone is preserved for arbitrary ribbon width. The effective Hamiltonians are in good agreement with ab initio calculations.

Spectral diffusion arising from ${}^{29}\mathrm{Si}$ nuclear spin flip-flops, known to be a primary source of electron spin decoherence in silicon, is also predicted to limit the coherence times of neutral donor nuclear spins in silicon. Here, the impact of this mechanism on ${}^{31}\mathrm{P}$ nuclear spin coherence is measured as a function of ${}^{29}\mathrm{Si}$ concentration using $X$-band pulsed electron nuclear double resonance. The ${}^{31}\mathrm{P}$ nuclear spin echo decays show that decoherence is controlled by ${}^{29}\mathrm{Si}$ flip-flops resulting in both fast (exponential) and slow (nonexponential) spectral diffusion processes. The decay times span a range from 100 ms in crystals containing 50% ${}^{29}\mathrm{Si}$ to 3 s in crystals containing 1% ${}^{29}\mathrm{Si}$. These nuclear spin echo decay times for neutral donors are orders of magnitude longer than those reported for ionized donors in natural silicon. The electron spin of the neutral donors “protects” the donor nuclear spins by suppressing ${}^{29}\mathrm{Si}$ flip-flops within a “frozen core,” as a result of the detuning of the ${}^{29}\mathrm{Si}$ spins caused by their hyperfine coupling to the electron spin.

We present an efficient implementation of the parquet formalism that respects the asymptotic structure of the vertex functions at both single- and two-particle levels in momentum and frequency space. We identify the two-particle reducible vertex as the core function that is essential for the construction of the other vertex functions. This observation stimulates us to consider a two-level parameter reduction for this function to simplify the solution of the parquet equations. The resulting functions, which depend on fewer arguments, are coined “kernel functions.” With the use of the kernel functions, the open boundary of various vertex functions in Matsubara-frequency space can be faithfully satisfied. We justify our implementation by accurately reproducing the dynamical mean-field theory results from momentum-independent parquet calculations. The high-frequency asymptotics of the single-particle self-energy and the two-particle vertex are correctly reproduced, which turns out to be essential for the self-consistent determination of the parquet solutions. The current implementation is also feasible for the dynamical vertex approximation.

We investigate the conduction mechanisms of nitronyl-nitroxide (NIT) molecular radicals, as useful for the creation of nanoscopic molecular spintronic devices, finding that it does not correspond to standard Mott behavior, as previously postulated. We provide a complete investigation using transport measurements, low-energy, sub-THz spectroscopy and introducing differently substituted phenyl appendages. We show that a nontrivial surface-charge-limited regime is present in addition to the standard low-voltage Ohmic conductance. Scaling analysis allows one to determine all the main transport parameters for the compounds and highlights the presence of charge-trapping effects. Comparison among the different compounds shows the relevance of intermolecular stacking between the aromatic ring of the phenyl appendix and the NIT motif in the creation of useful electron transport channels. The importance of intermolecular pathways is further highlighted by electronic structure calculations, which clarify the nature of the electronic channels and their effect on the Mott character of the compounds.

Using density functional calculations we have investigated the local spin moment formation and lattice deformation in graphene when an isolated vacancy is created. We predict two competing equilibrium structures: a ground-state planar configuration with a saturated local moment of 1.5 ${\mu }_B$ and a metastable nonplanar configuration with a vanishing magnetic moment, at a modest energy expense of 50 meV. Though nonplanarity relieves the lattice of vacancy-induced strain, the planar state is energetically favored due to maximally localized defect states $(v\sigma$, $v\pi )$. In the planar configuration, charge transfer from itinerant (Dirac) states weakens the spin polarization of $v\pi$ yielding a fractional moment, which is aligned parallel to the unpaired $v\sigma$ electron through Hund's coupling. As a by-product, the Dirac states $\left(d\pi \right)$ of the two sublattices undergo a minor spin polarization and couple antiferromagnetically. In the nonplanar configuration, the absence of orthogonal symmetry allows interaction between $v\sigma$ and local $d\pi$ states, to form a hybridized $v{\sigma }^{\prime }$ state. The nonorthogonality also destabilizes the Hund's coupling, and an antiparallel alignment between $v\sigma$ and $v\pi$ lowers the energy. The gradual spin reversal of $v\pi$ with increasing nonplanarity opens up the possibility of an intermediate structure with a balanced $v\pi$ spin population. If such a structure is realized under external perturbations, diluted vacancy concentration may lead to $v\sigma$-based spin-$\frac{1}{2}$ paramagnetism. Carrier doping, electron or hole, does not alter the structural stability. However, the doping proportionately changes the occupancy of $v\pi$ state and hence the net magnetic moment.

We study a thermoelectric ratchet consisting of an array of disordered nanowires arranged in parallel on top of an insulating substrate and contacted asymmetrically to two electrodes. Transport is investigated in the Mott hopping regime, when localized electrons can propagate through the nanowires via thermally assisted hops. When the electronic temperature in the nanowires is different from the phononic one in the substrate, we show that a finite electrical current is generated even in the absence of driving forces between the electrodes. We discuss the device performance both as an energy harvester, when an excess heat from the substrate is converted into useful power, and as a refrigerator, when an external power is supplied to cool down the substrate.

We revisit, in the framework of the Landauer theory, the thermal emission in far-field regime, of arbitrary indefinite planar media and finite size systems. We prove that the flux radiated by the former is bounded by the blackbody emission while, for the second, there is in principle, no upper limit demonstrating such possibility for a super-Planckian thermal emission with finite size systems.

The random field $S=\frac{1}{2}$ Heisenberg chain exhibits a dynamical many body localization transition at a critical disorder strength, which depends on the energy density. At weak disorder, the eigenstate thermalization hypothesis (ETH) is fulfilled on average, making local observables smooth functions of energy, whose eigenstate-to-eigenstate fluctuations decrease exponentially with system size. We demonstrate the validity of ETH in the thermal phase as well as its breakdown in the localized phase and show that rare states exist which do not strictly follow ETH, becoming more frequent closer to the transition. Similarly, the probability distribution of the entanglement entropy at intermediate disorder develops long tails all the way down to zero entanglement. We propose that these low entanglement tails stem from localized regions at the subsystem boundaries which were recently discussed as a possible mechanism for subdiffusive transport in the ergodic phase.

We analyze the general circumstances that can lead to the negative longitudinal/transverse optical mode splitting (the Lyddane-Sachs-Teller splitting). As a specific example, we consider the optical phonon frequencies of perovskite $\mathrm{LaAl}{\mathrm{O}}_3$ computed within density functional theory and compare them to prior experimental reports. The experimental results and theoretical calculation convincingly show that one of the optical mode splittings is indeed negative. The specific optical mode in experiment experiencing the negative splitting is consistent with our analysis. As a typical negative splitting is rather small and close to the experimental resolution, our analysis provides a firm theoretical basis for its existence.

A concept for acoustic metamaterials consisting of a cellular medium with fluid-filled cells is fabricated and studied experimentally. In such a system, the fluid and solid structure explicitly interact, and elastic wave propagation is coupled to both phases. Focusing here on shear wave behavior, we confirm previous numerical studies in three steps. We first measure the material deformations pertaining to three qualitatively different shear wave modes in the frequency range below 3.5 kHz. We then measure the group velocity and demonstrate that, within a certain frequency interval, the group and phase velocity have opposite signs. This shows that the system acts as a negative-index metamaterial. Finally, we confirm the presence of band gaps due to the locally resonant behavior of the cell walls. The demonstrated concept of a closed, fluid-filled cellular material as an acoustic metamaterial opens a wide space for applications.

The sodium iron phosphite $\mathrm{NaF}{\mathrm{e}}_3{\left(\mathrm{HP}{\mathrm{O}}_3\right)}_2{\left({\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_3\right)}_6$ is studied by ac-magnetic susceptibility, pulsed-field magnetization, specific heat, and high-frequency electron spin resonance (HF-ESR) measurements combined with Mössbauer spectroscopy and density-functional calculations. We show that this compound develops ferrimagnetic order below $T_C=9.5\mathrm{K}$ and reveals a magnetization plateau at 1/3 saturation. The plateau extends to $B_{\mathrm{c}}\sim 8\mathrm{T}$, whereas above $B_{\mathrm{c}}$ the magnetization increases linearly until reaching saturation at $B_s\sim 27\mathrm{T}$. The Mössbauer spectroscopy reveals two magnetically nonequivalent iron sites with the 2:1 ratio. The HF-ESR spectra are consistent with a two-sublattice ferrimagnet and additionally pinpoint weak magnetic anisotropy as well as short-range spin order that persists well above $T_C$. The ferrimagnetic order in the title compound is stabilized by a network of purely antiferromagnetic exchange interactions. The spin lattice comprises layers coinciding with the crystallographic (10-1) planes, with stronger couplings $J_i\sim 1.5\mathrm{K}$ within the layers and weaker couplings $J_i=0.3-0.4\mathrm{K}$ between the layers. Both intralayer and interlayer couplings are frustrated. The ensuing ferrimagnetic order arises from a subtle interplay of the frustrated but nonequivalent exchange couplings.

Using photoemission electron microscopy with x-ray magnetic circular dichroism as a contrast mechanism, new insights into the all-optical magnetization switching (AOS) phenomenon in GdFe-based rare-earth transition-metal ferrimagnetic alloys are provided. From a sequence of static images taken after single linearly polarized laser pulse excitation, the repeatability of AOS can be quantified with a correlation coefficient. It is found that low coercivity enables thermally activated domain-wall motion, limiting in turn the repeatability of the switching. Time-resolved measurements of the magnetization dynamics reveal that while AOS occurs below and above the magnetization compensation temperature $T_{\mathrm{M}}$, it is not observed in GdFe samples where $T_{\mathrm{M}}$ is absent. Finally, AOS is experimentally demonstrated against an applied magnetic field of up to 180 mT.

We investigate thickness-limited size effects on the thermal conductivity of amorphous silicon thin films ranging from 3 to 1636 nm grown via sputter deposition. While exhibiting a constant value up to $\sim 100$ nm, the thermal conductivity increases with film thickness thereafter. The thickness dependence we demonstrate is ascribed to boundary scattering of long wavelength vibrations and an interplay between the energy transfer associated with propagating modes (propagons) and nonpropagating modes (diffusons). A crossover from propagon to diffuson modes is deduced to occur at a frequency of $\sim 1.8$ THz via simple analytical arguments. These results provide empirical evidence of size effects on the thermal conductivity of amorphous silicon and systematic experimental insight into the nature of vibrational thermal transport in amorphous solids.

Recent experimental work on ${\mathrm{Mn}}_2{\mathrm{Ru}}_x\mathrm{Ga}$ demonstrates its potential as a compensated ferrimagnetic half metal (CFHM). Here we present a set of high-throughput ab initio density functional theory calculations and a detailed experimental characterization that enable us to correctly describe the nominal ${\mathrm{Mn}}_2{\mathrm{Ru}}_x\mathrm{Ga}$ thin films, in particular, with regard to site disorder and defects. We then construct models that accurately capture all the key features of the Mn-Ru-Ga system, including magnetic compensation and the spin gap at the Fermi level. We find that electronic doping is necessary, which is achieved with a Mn/Ga ratio smaller than two. Our study shows how composition and substrate-induced biaxial strain can be combined to design a ferrimagnetic half metal with a compensation point close to room temperature.

Semiconductors under femtosecond x-ray irradiation are transiently excited to nonequilibrium states. This can lead to observable material modifications. During the excitation and relaxation dynamics, optical properties of the solid are changing, affected by both transient electron excitation as well as the evolution of the atomic structure. In this paper we apply a unified hybrid model to trace these two effects. Transient evolution of the optical properties is calculated within the transferable tight-binding approach. The presented methodology of calculation of the complex dielectric function proves to be capable of describing changes in the optical parameters during the phase transitions, when the solids are driven out of equilibrium by intense laser pulses, in a reasonable agreement with experiments.

In the temperature-concentration phase diagram of most iron-based superconductors, antiferromagnetic order is gradually suppressed to zero at a critical point, and a dome of superconductivity forms around that point. The nature of the magnetic phase and its fluctuations is of fundamental importance for elucidating the pairing mechanism. In ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ and ${\text{Ba}}_{1-x}{\text{Na}}_x{\text{Fe}}_2{\text{As}}_2$, it has recently become clear that the usual stripelike magnetic phase, of orthorhombic symmetry, gives way to a second magnetic phase, of tetragonal symmetry, near the critical point, in the range from $x=0.24$ to $x=0.28$ for ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$. In a prior study, an unidentified phase was discovered for $x<0.24$ but under applied pressure, whose onset was detected as a sharp anomaly in the resistivity. Here we report measurements of the electrical resistivity of ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ under applied hydrostatic pressures up to 2.75 GPa, for $x=0.22$, 0.24, and 0.28. The critical pressure above which the unidentified phase appears is seen to decrease with increasing $x$ and vanish at $x=0.24$, thereby linking the pressure-induced phase to the tetragonal magnetic phase observed at ambient pressure. In the temperature-concentration phase diagram of ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$, we find that pressure greatly expands the tetragonal magnetic phase, while the stripelike phase shrinks. This reveals that pressure may be a powerful tuning parameter with which to explore the interplay between magnetism and superconductivity in this material.

We present a large-$S$ study of a quantum spin ice Hamiltonian, introduced by Huang et al. [Phys. Rev. Lett. 112, 167203 (2014)], on the kagome lattice. This model involves a competition between the frustrating Ising term of classical kagome ice, a Zeeman magnetic field $h$, and a nearest-neighbor transverse spin-flip term $S_i^x{S}_j^x-S_i^y{S}_j^y$. Recent quantum Monte Carlo (QMC) simulations by Carrasquilla et al. [Nat. Commun. 6, 7421 (2015)], uncovered lobes of a disordered phase for large Ising interaction and $h\ne 0$—a putative quantum spin liquid phase. Here, we examine the nature of this model using large-$S$ expansion. We show that the ground state properties generally have the same trends with those observed in QMC simulations. In particular, the large-$S$ ground state phase diagram captures the existence of the disordered lobes.

We investigate the electronic dispersion of the high-$T_c$ cuprate superconductors using the fully self-consistent version of the phenomenological model, where charge planar quasiparticles are coupled to spin fluctuations. The inputs we use, the underlying (bare) band structure and the spin susceptibility $\chi$, are extracted from fits of angle-resolved photoemission and inelastic neutron scattering data of underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.6}$ by T. Dahm and coworkers [Nat. Phys. 5, 217 (2009)]. Our main results are as follows: (i) We have confirmed the finding by Dahm and coworkers that the main nodal kink is, for the present values of the input parameters, determined by the upper branch of the hourglass of $\chi$. We demonstrate that the properties of the kink depend qualitatively on the strength of the charge-spin coupling. (ii) The effect of the resonance mode of $\chi$ on the electronic dispersion strongly depends on its kurtosis in the quasimomentum space. A low (high) kurtosis implies a negligible (considerable) effect of the mode on the dispersion in the near-nodal region. (iii) The energy of the kink decreases as a function of the angle $\theta$ between the Fermi surface cut and the nodal direction, in qualitative agreement with recent experimental observations. We clarify the trend and make a specific prediction concerning the angular dependence of the kink energy in underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.6}$.