In this work, we employ and critically evaluate a first-principles approach based on supercell calculations for predicting the magnetic critical order-disorder temperature $T_c$. As a model material we use the recently discovered nanolaminate $\mathrm{M}{\mathrm{n}}_2\mathrm{GaC}$. First, we derive the exchange interaction parameters $J_{ij}$ between pairs of Mn atoms on sites $i$ and $j$ of the bilinear Heisenberg Hamiltonian using the novel magnetic direct cluster averaging method (MDCA), and then compare the $J^{\prime }s$ from the MDCA calculations to the same parameters calculated using the Connolly-Williams method. We show that the two methods yield closely matching results, but observe that the MDCA method is computationally less effective when applied to highly ordered phases such as $\mathrm{M}{\mathrm{n}}_2\mathrm{GaC}$. Secondly, Monte Carlo simulations are used to derive the magnetic energy, specific heat, and $T_c$. For $\mathrm{M}{\mathrm{n}}_2\mathrm{GaC}$, we find $T_c=660\mathrm{K}$. The uncertainty in the calculated $T_c$ caused by possible uncertainties in the $J^{\prime }s$ is discussed and exemplified in our case by an analysis of the impact of the statistical uncertainties of the MDCA-derived $J^{\prime }s$, resulting in a $T_c$ distribution with a standard deviation of 133 K.

The importance of charge reservoir layers for supplying holes to the ${\mathrm{CuO}}_2$ planes of cuprate superconductors has long been recognized. Less attention has been paid to the screening of the charge transfer by the intervening ionic layers. We address this issue in the case of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$, where CuO chains supply the holes for the planes. We present a simple dielectric-screening model that gives a linear correlation between the relative displacements of ions along the $c$ axis, determined by neutron powder diffraction, and the hole density of the planes. Applying this model to the temperature-dependent shifts of ions along the $c$ axis, we infer a charge transfer of 5–10% of the hole density from the planes to the chains on warming from the superconducting transition to room temperature. Given the significant coupling of $c$-axis displacements to the average charge density, we point out the relevance of local displacements for screening charge modulations and note recent evidence for dynamic screening of in-plane quasiparticles. This line of argument leads us to a simple model for atomic displacements and charge modulation that is consistent with images from scanning-tunneling microscopy for underdoped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$.

The order of the superconducting phase transition is a classical problem. Single-component type-2 superconductors exhibit a continuous “inverted-$XY$” phase transition, as was first demonstrated for $U\left(1\right)$ lattice London superconductors by a celebrated duality mapping with subsequent backing by numerical simulations. Here we study this problem in multiband $U\left(1\right)$ London superconductors and find evidence that by contrast the model has a tricritical point. The superconducting phase transition becomes first order when the Josephson length is sufficiently large compared to the magnetic field penetration length. We present evidence that the fluctuation-induced dipolar interaction between vortex loops makes the phase transition discontinuous. We discuss that this mechanism is also relevant for the phase transitions in multicomponent gauge theories with higher broken symmetry.

We calculate the energy spectrum of quasiparticles trapped by a domain wall separating different time-reversal symmetry-breaking ground states in a hexagonal superconductor, such as ${\mathrm{UPt}}_3$. The bound-state energy is found to be strongly dependent on the gap symmetry, the domain-wall orientation, the quasiparticle's direction of semiclassical propagation, and the phase difference between the domains. We calculate the corresponding density of states and show how one can use its prominent features, in particular, the zero-energy singularity, to distinguish between different pairing symmetries.

The newly discovered Ising-type ferromagnet ${\mathrm{CeRu}}_2{\mathrm{Al}}_2\mathrm{B}$ exhibits an additional phase transition at $T_{\mathrm{N}}=14.2$ K before entering the ferromagnetic ground state at $T_{\mathrm{C}}=12.8$ K. We clarify the nature of this transition through high resolution neutron diffraction measurements. The data reveal the presence of a longitudinal incommensurate spin-density wave (SDW) in the temperature range of $T_{\mathrm{C}}<T<T_{\mathrm{N}}$. The propagation vector $\mathbf{q}\sim (0,0,0.148)$ is nearly temperature independent in this region and discontinuously locks into $\mathbf{q}=0$ at $T_{\mathrm{C}}$. Mean-field calculations of an effective Ising model indicate that the modulated SDW phase is stabilized by a strong competition between ferromagnetic and antiferromagnetic exchange interactions. This makes ${\mathrm{CeRu}}_2{\mathrm{Al}}_2\mathrm{B}$ a particularly attractive model system to study the global phase diagram of ferromagnetic heavy-fermion metals under the influence of magnetic frustration.

Incorporating nanoparticles into superconducting materials has emerged as an efficient route to enhance their current-carrying capability. However, a thorough understanding of how these inclusions can be used in the most efficient way is still lacking. We address this problem of optimizing the vortex pinning landscape for randomly distributed metallic spherical inclusions using systematic large-scale numerical simulations of time-dependent Ginzburg-Landau equations. This approach allows us to predict the size and density of particles for which the highest critical current is realized. For a given particle size and magnetic field, the critical current reaches a maximum value at a particle density, which typically corresponds to 15%–23% of the total volume being replaced by the nonsuperconducting material. For a fixed diameter, this optimal particle density increases with the magnetic field. Moreover, we found that, as the magnetic field increased, the optimal particle diameter slowly decreases from 4.5 to 2.5 coherence lengths. This result shows that pinning landscapes have to be designed for specific applications taking into account relevant magnetic field scales.

We investigate magnetic properties of strongly interacting four component spin-3/2 ultracold fermionic atoms in the Mott insulator limit with one particle per site in an optical lattice with honeycomb symmetry. In this limit, atomic tunneling is virtual, and only the atomic spins can exchange. We find a competition between symmetry-breaking and liquidlike disordered phases. Particularly interesting are valence bond states with bond centered magnetizations, situated between the ferromagnetic and conventional valence bond phases. In the framework of a mean-field theory, we calculate the phase diagram and identify an experimentally relevant parameter region where a homogeneous SU(4) symmetric Affleck-Kennedy-Lieb-Tasaki–like valence bond state is present.

We study Landau-Zener-Stückelberg (LZS) interferometry in multilevel systems coupled to an Ohmic quantum bath. We consider the case of superconducting flux qubits driven by a dc+ac magnetic fields, but our results can apply to other similar systems. We find a dynamic transition manifested by a symmetry change in the structure of the LZS interference pattern, plotted as a function of ac amplitude and dc detuning. The dynamic transition is from an LZS pattern with nearly symmetric multiphoton resonances to antisymmetric multiphoton resonances at long times (above the relaxation time). We also show that the presence of a resonant mode in the quantum bath can impede the dynamic transition when the resonant frequency is of the order of the qubit gap. Our results are obtained by a numerical calculation of the finite time and the asymptotic stationary population of the qubit states, using the Floquet-Markov approach to solve a realistic model of the flux qubit considering up to ten energy levels.

We investigate the structure of vortex-bound states in spin-singlet chiral superconductors with $\left(d_{x^2-y^2}\pm i{d}_{xy}\right)$-wave and $\left(d_{xz}\pm i{d}_{yz}\right)$-wave pairing symmetries. It is found that vortices in the $\left(d_{xz}\pm i{d}_{yz}\right)$-wave state bind zero-energy states, which are dispersionless along the vortex line, forming a doubly degenerate Majorana flat band. Vortex-bound states of $\left(d_{x^2-y^2}\pm i{d}_{xy}\right)$-wave superconductors, on the other hand, exist only at finite energy. Using exact diagonalization and analytical solutions of tight-binding Bogoliubov-de Gennes Hamiltonians, we compute the energy spectrum of the vortex-bound states and the local density of states around the vortex and antivortex cores. We find that the tunneling conductance peak of the vortex is considerably broader than that of the antivortex. This difference can be used as a direct signature of the chiral order parameter symmetry.

The long standing problem of the $\alpha \text{−}\gamma$ phase transition in cerium metal is approached by treating all electrons at the same quantum mechanical level, using both hybrid functionals (PBE0 and HSE06) and exact exchange plus correlation in the random-phase approximation (EX+cRPA). The exact-exchange contribution in PBE0 and HSE06 is crucial to produce two distinct solutions that can be associated with the $\alpha$ and $\gamma$ phases. An analysis of the band structure and the electron density reveals a localization and delocalization behavior of the $f$ electrons in the $\gamma$ and $\alpha$ phases, respectively. However, a quantitative agreement with the extrapolated phase diagram to zero temperature is achieved only with EX+cRPA, based on the hybrid functional starting point. We predict that a pressure induced phase transition should exist at or close to $T=0\text{K}$. By adding entropic contributions we determine the pressure-temperature phase diagram, which is in reasonable agreement with experiment.

We perform a systematic study of the antiferromagnetic Heisenberg model on the Husimi lattice using numerical tensor-network methods based on projected entangled simplex states. The nature of the ground state varies strongly with the spin quantum number $S$. For $S=\frac{1}{2}$, it is an algebraic (gapless) quantum spin liquid. For $S=1$, it is a gapped, nonmagnetic state with spontaneous breaking of triangle symmetry (a trimerized simplex-solid state). For $S=2$, it is a simplex-solid state with a spin gap and no symmetry breaking; both integer-spin simplex-solid states are characterized by specific degeneracies in the entanglement spectrum. For $S=\frac{3}{2}$, and indeed for all spin values $S\ge \frac{5}{2}$, the ground states have ${120}^{\circ }$ antiferromagnetic order. In a finite magnetic field, we find that, irrespective of the value of $S$, there is always a plateau in the magnetization at $m=\frac{1}{3}$.

The complex interdigitated phases have greatly frustrated attempts to document the basic features of the superconductivity in the alkali metal intercalated iron chalcogenides. Here, using elastic neutron scattering, energy-dispersive x-ray spectroscopy, and resistivity measurements, we elucidate the relations of these phases in ${\mathrm{Rb}}_x{\mathrm{Fe}}_y{\mathrm{Se}}_{2-z}{\mathrm{S}}_z$. We find (i) the iron content is crucial in stabilizing the stripe antiferromagnetic (AF) phase with rhombic iron vacancy order $(y\approx 1.5)$, the block AF phase with $\sqrt{5}\times \sqrt{5}$ iron vacancy order $(y\approx 1.6)$, and the iron vacancy-free phase $(y\approx 2)$; and (ii) the iron vacancy-free superconducting phase $(z=0)$ evolves into an iron vacancy-free metallic phase with sulfur substitution $(z>1.5)$ due to the progressive decrease of the electronic correlation strength. Both the stripe AF phase and the block AF phase are Mott insulators. The iron-rich compounds $(y>1.6)$ undergo a first order transition from an iron vacancy disordered phase at high temperatures into the $\sqrt{5}\times \sqrt{5}$ iron vacancy ordered phase and the iron vacancy-free phase below $T_s$. Our data demonstrate that there are miscibility gaps between these three phases. The existence of the miscibility gaps in the iron content is a key to understanding the relationship between these complicated phases.

A mechanism is proposed for the tantalizing evidence of polar Kerr effect in a class of high-temperature superconductors: the signs of the Kerr angle from two opposite faces of the same sample are identical and magnetic field training is nonexistent. The mechanism does not break global time-reversal symmetry, as in an antiferromagnet, and results in zero Faraday effect. It is best understood in a phenomenological model of bilayer cuprates, such as ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+\delta }$, in which intrabilayer tunneling nucleates a chiral $d$-density wave such that the individual layers have opposite chirality. Although specific to the chiral $d$-density wave, the mechanism may be more general to any quasi-two-dimensional orbital antiferromagnet in which time-reversal symmetry is broken in each plane, but not when averaged macroscopically.

An electron-hole liquid (EHL), a condensed liquidlike phase of free electrons and holes in a semiconductor, presents a unique system for exploring quantum many-body phenomena. While the behavior of EHLs is generally understood, less attention has been devoted to systematically varying the onset of their formation and resulting properties. We report on an experimental approach to tune the conditions of formation and characteristics using a combination of low excitation densities and high magnetic fields up to 90 T. Demonstration of this approach was carried out in indirect-band-gap $\mathrm{A}{\mathrm{l}}_{0.387}\mathrm{G}{\mathrm{a}}_{0.613}\mathrm{As}$. EHL droplets can be nucleated from one of two multiexciton complex states depending on the applied excitation density. Furthermore, the excitation density influences the carrier density of the EHL at high magnetic fields, where filling of successive Landau levels can be controlled. The ability to manipulate the formation pathway, temperature, and carrier density of the EHL phase under otherwise fixed experimental conditions makes our approach a powerful tool for studying condensed carrier phases in further detail.

We present a theoretical analysis of the intensity correlation functions for the spontaneous emission from a planar ensemble of self-assembled quantum dots. Using the quantum jump approach, we numerically simulate the evolution of the system and construct photon-photon delay time statistics that approximates the second-order correlation functions of the field. The form of this correlation function in the case of collective emission from a highly homogeneous ensemble qualitatively differs from that characterizing an ensemble of independent emitters (inhomogeneous ensemble of uncoupled dots). The signatures of collective emission are observed also in the case of an inhomogeneous but sufficiently strongly coupled ensemble. Different forms of the correlation functions are observed in the intensity autocorrelations and in cross correlations between various spectral ranges, revealing the quantum state projection associated with the detection event and the subsequent interaction-induced redistribution of occupations. The predicted effect of collective dynamics on the correlation functions appears under various excitation conditions. Thus, we show that the second-order correlation function of the emitted field provides a sensitive test of cooperative effects.

The application of graphene to electronic and optoelectronic devices is limited by the absence of reliable semiconducting variants of this material. A promising candidate in this respect is graphene oxide, with a band gap on the order of $\sim 5\mathrm{eV}$, however, this has a finite density of states at the Fermi level. Here, we examine the electronic structure of three variants of half -fluorinated carbon on Sic(0001), i.e., the $\left(6\sqrt{3}\times 6\sqrt{3}\right)$ $R{30}^{\circ }$ C/SiC “buffer layer,” graphene on this $\left(6\sqrt{3}\times 6\sqrt{3}\right)$ $R{30}^{\circ }$ C/SiC buffer layer, and graphene decoupled from the SiC substrate by hydrogen intercalation. Using angle-resolved photoemission, core level photoemission, and x-ray absorption, we show that the electronic, chemical, and physical structure of all three variants is remarkably similar, exhibiting a large band gap and a vanishing density of states at the Fermi level. These results are explained in terms of first-principles calculations. This material thus appears very suitable for applications, even more so since it is prepared on a processing-friendly substrate. We also investigate two separate UV photon-induced modifications of the electronic structure that transform the insulating samples (6.2-eV band gap) into semiconducting ($\sim 2.5$-eV band gap) and metallic regions, respectively.

The anisotropy of the electronic transition is a well-known characteristic of low-dimensional transition-metal dichalcogenides, but their layer-thickness dependence has not been properly investigated experimentally until now. Yet, it not only determines the optical properties of these low-dimensional materials, but also holds the key in revealing the underlying character of the electronic states involved. Here we used both angle-resolved electron energy-loss spectroscopy and spectral analysis of angle-integrated spectra to study the evolution of the anisotropic electronic transition involving the low-energy valence electrons in the freestanding $\mathrm{Mo}{\mathrm{S}}_2$ layers with different thicknesses. We are able to demonstrate that the well-known direct gap at 1.8 eV is only excited by the in-plane polarized field while the out-of-plane polarized optical gap is 2.4 ± 0.2 eV in monolayer $\mathrm{Mo}{\mathrm{S}}_2$. This contrasts with the much smaller anisotropic response found for the indirect gap in the few-layer $\mathrm{Mo}{\mathrm{S}}_2$ systems. In addition, we determined that the joint density of states associated with the indirect gap transition in the multilayer systems and the corresponding indirect transition in the monolayer case has a characteristic three-dimensional-like character. We attribute this to the soft-edge behavior of the confining potential and it is an important factor when considering the dynamical screening of the electric field at the relevant excitation energies. Our result provides a logical explanation for the large sensitivity of the indirect transition to thickness variation compared with that for the direct transition, in terms of quantum confinement effect.

We study current-current correlation in an electronic analog of a beam splitter realized with edge channels of a fractional quantum Hall liquid at Laughlin filling fractions. In analogy with the known result for chiral electrons [M. Büttiker, Phys. Rev. B 46, 12485 (1992)], if the currents are measured at points located after the beam splitter, we find that the zero frequency equilibrium correlation vanishes due to the chiral propagation along the edge channels. Furthermore, we show that the current-current correlation, normalized to the tunneling current, exhibits clear signatures of the Laughlin quasiparticles' fractional statistics.

We present Fermi's “golden rule” calculations of the optical carrier injection and the coherent control of current injection in graphene nanoribbons with zigzag geometry, using an envelope function approach. This system possesses strongly localized states (flat bands) with a large joint density of states at low photon energies; for ribbons with widths above a few tens of nanometers, this system also posses large number of (nonflat) states with maxima and minima close to the Fermi level. Consequently, even with small dopings the occupation of these localized states can be significantly altered. In this work, we calculate the relevant quantities for coherent control at different chemical potentials, showing the sensitivity of this system to the occupation of the edge states. We consider coherent control scenarios arising from the interference of one-photon absorption at $2\hslash \omega$ with two-photon absorption at $\hslash \omega$, and those arising from the interference of one-photon absorption at $\hslash \omega$ with stimulated electronic Raman scattering (virtual absorption at $2\hslash \omega$ followed by emission at $\hslash \omega )$. Although at large photon energies these processes follow an energy-dependence similar to that of 2D graphene, the zigzag nanoribbons exhibit a richer structure at low photon energies, arising from divergences of the joint density of states and from resonant absorption processes, which can be strongly modified by doping. As a figure of merit for the injected carrier currents, we calculate the resulting swarm velocities. Finally, we provide estimates for the limits of validity of our model.

Here we investigate the growth of the pentacene thin film on the quartz (0001) surface. The evolution of the optical properties of the pentacene thin film has been monitored in situ by optical spectroscopy and the thin-film morphology was investigated ex situ by atomic force spectroscopy. We find that (001)-oriented thin-film phase pentacene crystallites grow on a one-monolayer-thick interfacial layer. The optical and structural properties of the first monolayer determined in situ and ex situ, respectively, differ substantially from those of the multilayers. Most importantly, the distinct optical properties of the first monolayer are preserved when the interface becomes buried during the subsequent pentacene thin-film formation.