We study the temperature and pressure dependence of the structural and electronic properties of the iron pnictide superconductor BaFe${}_2$As${}_2$. We use density functional theory-based Born-Oppenheimer molecular dynamics simulations to investigate the system at temperatures from $T=5$ to 150 K and pressures from $P=0$ to 30 GPa. When increasing the pressure at low temperature, we find the two transitions from an orthorhombic to a tetragonal and to a collapsed tetragonal structure that are also observed in zero temperature structure relaxations and in experiment. These transitions are considerably smeared out at finite temperature, with the critical pressure for the first transition increasing with temperature. We also analyze the electronic structure of BaFe${}_2$As${}_2$ at finite temperature and the effect of the structural oscillations on the band structure and Fermi surface in comparison to known zero-temperature results. Our results should be helpful for resolving some open issues in experimental reports for BaFe${}_2$As${}_2$ under high pressure.

We study the entanglement entropies in one-dimensional open critical systems, whose effective description is given by a conformal field theory with boundaries. We show that, for pure-state systems formed by the ground state or by the excited states associated to primary fields, the entanglement entropies have a finite-size behavior that depends on the correlation of the underlying field theory. The analytical results are checked numerically, finding excellent agreement for the quantum chains ruled by the theories with central charge $c=1/2$ and 1.

We utilize single-electron tunneling spectroscopy to measure the discrete energy levels in a nanometer-scale cobalt particle at $T=60$ mK, and find effective single-electron spin $g$ factors $\approx 7.3$. These large $g$ factors do not result from the typical orbital contribution to $g$ factors, since the orbital angular momentum is quenched. Instead, they are due to nontrivial many-body excitations. A kink in the plot of conductance vs voltage and magnetic field is a signature of degenerate total spin on the particle. Spin-orbit interactions cause the new particle eigenstates to have “spin” that is an admixture of pure spin states. Fluctuations in the discrete energy level spacing allow for the total change in “spin” on the particle during a single-electron tunneling event to be $\Delta S^{\prime }=3/2$, leading to a $g$ factor of around 6.

We have measured the normalized gradient of the Casimir force between Au-coated surfaces of the sphere and the plate and equivalent Casimir pressure between two parallel Au plates at $T=77$K. These measurements have been performed by means of dynamic force microscope adapted for operating at low temperatures in the frequency shift technique. It was shown that the measurement results at $T=77$K are in a very good agreement with those at $T=300$K and with computations at $T=77$K using both theoretical approaches to the thermal Casimir force proposed in the literature. No thermal effect in the Casimir pressure was observed in the limit of experimental errors with the increase of temperature from $T=77$ to $300$K. Taking this into account, we have discussed the possible role of patch potentials in the comparison between measured and calculated Casimir pressures.

Germanium deposited on single-crystal Ag(111) substrates at about 1/3-monolayer coverage forms a stable $(\sqrt{3}\times \sqrt{3})R{30}^{\circ }$ superstructure as seen by reflection high-energy electron diffraction. In situ angle resolved photoelectron spectroscopy reveals a rich band structure consisting of conelike features that cross the Fermi surface at the center ($\overline{\Gamma }$ points) of the superstructure surface Brillouin zone (SBZ), exhibiting a high dispersion with the surface parallel wave vector. With the help of first-principles calculations, it can be inferred that most of the features are explained in terms of an Ag${}_2$Ge-ordered surface alloy corresponding to surface states localized in the first two layers. One of the bands has a saddle point near the $\overline{\mathrm{M}}$ point of the SBZ with a characteristic split-band behavior distinguishing Ge surface alloy from other similar systems.

We study analytically and numerically spin effects in ${\mathrm{MoS}}_2$ monolayer armchair quantum wires and quantum dots. The interplay between intrinsic and Rashba spin orbit interactions induced by an electric field leads to helical modes, giving rise to spin filtering in time-reversal-invariant systems. The Rashba spin orbit interaction can also be generated by spatially varying magnetic fields. In this case, the system can be in a helical regime with nearly perfect spin polarization. If such a quantum wire is brought into proximity to an $s$-wave superconductor, the system can be tuned into a topological phase, resulting in midgap Majorana fermions localized at the wire ends.

Using a large-$N$ approach, we study the effect of disorder in the Kondo-screened phase of heavy-fermion materials. We demonstrate that the strong feedback between the hybridization and the conduction electron charge density magnifies the effect of disorder, such that already small concentrations of defects strongly disorder the materials’ local electronic structure, while only weakly affecting their spatially averaged, thermodynamic properties. Finally, we show that the microscopic nature of defects can be identified through their characteristic signatures in the hybridization and quasiparticle interference spectrum.

We fit finite-temperature path integral Monte Carlo calculations of the exchange-correlation energy of the $3\text{D}$ finite-temperature homogeneous electron gas in the warm-dense regime [$r_s\equiv {(3/4\pi n)}^{1/3}{a}_B^{-1}<40$ and $\Theta \equiv T/T_F>0.0625$]. In doing so, we construct a Padé approximant which collapses to Debye-Hückel theory in the high-temperature, low-density limit. Likewise, the zero-temperature limit matches the numerical results of ground-state quantum Monte Carlo, as well as analytical results in the high-density limit.

We study the band structure of the ${\text{Bi}}_2{\text{Se}}_3$ topological insulator (111) surface using angle-resolved photoemission spectroscopy. We examine the situation where two sets of quantized subbands exhibiting different Rashba spin splitting are created via bending of the conduction (CB) and the valence (VB) bands at the surface. While the CB subbands are strongly Rashba spin split, the VB subbands do not exhibit clear spin splitting. We find that CB and VB experience similar band bending magnitudes, which means, a spin-splitting discrepancy due to different surface potential gradients can be excluded. On the other hand, by comparing the experimental band structure to first-principles LMTO band structure calculations, we find that the strongly spin-orbit coupled Bi 6$p$ orbitals dominate the orbital character of CB, whereas their admixture to VB is rather small. The spin-splitting discrepancy is, therefore, traced back to the difference in spin-orbit coupling between CB and VB in the respective subbands’ regions.

Bias voltage dependent scattering of the topological surface state is studied by scanning tunneling microscopy/spectroscopy for a clean surface of the topological insulator Bi${}_2$Te${}_2$Se. A strong warping of constant energy contours in the unoccupied part of the spectrum is found to lead to a spin-selective scattering. The topological surface state persists to higher energies in the unoccupied range far beyond the Dirac point, where it coexists with the bulk conduction band. This finding sheds light on the spin and charge dynamics over a wide energy range and opens a way to designing optospintronic devices.

The surface of a 3+1d-topological insulator hosts an odd number of gapless Dirac fermions when charge conjugation and time-reversal symmetries are preserved. Viewed as a purely 2+1d system, this surface theory would necessarily explicitly break parity and time-reversal when coupled to a fluctuating gauge field. Here, we explain why such a state can exist on the boundary of a 3+1d system without breaking these symmetries, even if the number of boundary components is odd. This is accomplished from two complementary perspectives: topological quantization conditions and regularization. We first discuss the conditions under which (continuous) large gauge transformations may exist when the theory lives on a boundary of a higher-dimensional space-time. Next, we show how the higher-dimensional bulk theory is essential in providing a parity-invariant regularization of the theory living on the lower-dimensional boundary or defect.

The strong time-reversal symmetric topological insulator in three space dimensions features gapless surface states in the form of massless Dirac fermions. We study these surface states with the method of bosonization, and find that the resulting bosonic theory has a topological contribution due to the parity anomaly of the surface Dirac fermions. We argue that the presence of a quantum anomaly is, in fact, the main reason for the existence of a surface state, by the principle that anomalies of a surface and bulk must cancel. Inspecting other classes of topological insulators, we argue that this principle holds in general. Moving beyond purely topological considerations, we incorporate the dynamics of the surface electron states into the bosonic theory. Additionally, we discuss the thermodynamics of the bosonic theory and propose a representation of the surface Dirac fermions in terms of the bosonic fields.

We report on spatially resolved pump-probe MOKE spectroscopy measurements of electron spins in bulk $n$-type GaAs. For low lattice temperatures these measurements are significantly compromised by local changes in the excitonic magneto-optical response which are not related to the electron spin polarization. These local changes in the excitonic Kerr spectrum are due to carrier heating caused by above-band-gap optical excitation. When hot electrons are present, a measurement of the local Kerr rotation at a fixed arbitrary probe wavelength does not necessarily correctly reveal the local electron spin polarization. By analyzing the spatial dependence of the full excitonic Kerr rotation spectrum we determine the local excitonic spin splitting energy from which we obtain the true lateral electron spin polarization profile.

We theoretically develop a partial-wave method to study multiple scattering of massive Dirac fermions on the surface of topological insulators (TI) with strong warping effect. The cross sections (CS), local density of states (LDOS), and charge current density (CCD) for single-, two-, and three-centered magnetic scattering cases are discussed. We demonstrate that the $s$-wave approximation is not advisable and convergent for magnetic impurity scattering. Similar to the case of nonmagnetic impurity scattering, a sharp resonance peak at the band edge of the gapped TI is found in the total CS for the magnetic impurity scattering. Furthermore, we show that with increasing the Fermi energy, the power-law decay of LDOS is modified due to the warping effect of the Fermi surface. The CCD patterns are also modified remarkably by the strong warping.

We theoretically consider the effect of plasmon collective modes on the frequency-dependent conductivity of graphene in the presence of the random static potential of charged impurities. We develop an equation of motion approach suitable for the relativistic Dirac electrons in graphene that allows analytical high-frequency asymptotic solution ($\omega \tau \gg 1$ where $\tau$ is the scattering time) in the presence of both disorder and interaction. We show that the presence of the gapless plasmon pole (in graphene the plasmon frequency vanishes at long wavelengths as the square root of wave number) in the inverse dynamical dielectric function of graphene gives rise to a strong variation with frequency of the screening effect of the relativistic electron gas in graphene on the potential of charged impurities. The resulting frequency-dependent impurity scattering rate gives rise to a broad peak in the frequency-dependent graphene optical conductivity with the amplitude and the position of the peak being sensitive to the detailed characteristics of disorder and interaction in the system. This sample-dependent (i.e., disorder, electron density, and interaction strength) redistribution of the spectral weight in the frequency-dependent graphene conductivity may have already been experimentally observed in optical measurements.

An ab initio based theoretical approach to describe nonequilibrium many-body effects in molecular transport is developed. We introduce a basis of localized molecular orbitals and formulate the many-body model in this basis. In particular, the Hubbard-Anderson Hamiltonian is derived for single-molecule junctions with intermediate coupling to the leads. As an example we consider a benzenedithiol junction with gold electrodes. An effective few-level model is obtained from which spectral and transport properties are computed and are analyzed. Electron-electron interaction crucially affects transport and induces multiscale Coulomb blockade at low biases. At large bias, transport through asymmetrically coupled molecular edge states results in the occurrence of “anomalous” conductance features, i.e., of peaks with unexpectedly large/small height or even not located at the expected resonance energies.

In this paper, instabilities of the isotropic metallic phase in iron pnictides are investigated. The relevant quartic fermionic interaction terms in the model are identified using phase space arguments. Using the functional integral formalism, a Hubbard-Stratonovich transformation is used to decouple these quartic terms. This procedure introduces several bosonic fields which describe the low-energy collective modes of the system. By studying the behavior of these collective modes, a possible instability is found in the forward scattering channel of the isotropic phase driven by magnetic fluctuations. Using mean field analysis, we obtain a static and homogeneous ground state. This ground state is metallic, but the electron Fermi pockets are distorted unequally at different pockets in momentum space. This results in a desirable nematic ordering which breaks the lattice $C_4$ symmetry but preserves translational symmetry and may explain several experimental observations.

Absolute resistivity measurements as a function of temperature from optimally doped YBa${}_2$Cu${}_3$O${}_{7-\delta }$, La${}_{2-x}$Sr${}_x$CuO${}_4$, Bi${}_2$Sr${}_2$Ca${}_1$Cu${}_2$O${}_{8-x}$, and (Ca${}_{0.1}$La${}_{0.9}$)(Ba${}_{1.65}$La${}_{0.35}$)Cu${}_3$O${}_y$ thin films are reported. Special attention is given to the measurement geometrical factors and the resistivity slope between $T_c$ and $T^{*}$. The results are compared with a strong-coupling theory for the resistivity derivative near $T_c$, which is based on hard core bosons, and with several weak-coupling theories, which are BCS based. Surprisingly, our results agree with both paradigms. The implications of these findings and the missing calculations needed to distinguish between the two paradigms are discussed.

We have observed the Autler-Townes doublet in a superconducting Al/AlO${}_X$/Al transmon qubit that acts as an artificial atom embedded in a three-dimensional Cu microwave cavity at a temperature of 22 mK. Using pulsed microwave spectroscopy, the three lowest transmon levels are isolated, eliminating unwanted effects of higher qubit modes and cavity modes. The long coherence time ($\sim$$40\mu \text{s}$) of the transmon enables us to observe a clear Autler-Townes splitting at drive amplitudes much smaller than the transmon level anharmonicity (177 MHz). Three-level density matrix simulations with no free parameters provide excellent fits to the data. At maximum separation, the fidelity of a dark state achieved in this experiment is estimated to be $99.6\%–99.9\%$.

Using pair potentials fitted to an $ab$ initio database, combined with replica-exchange simulated annealing, we show that the complex, quasicrystal-related Al${}_{11}$Ir${}_4$ compound contains a nonstandard version of the “pseudo-Mackay” icosahedral cluster, with nonicosahedral inner Al${}_{10}$Ir and/or Al${}_9$Ir clusters that exist in various orientations and account for partial occupancies in the reported structure. We found two different compositions with different orientationally ordered grounds, each doubling the (cubic) unit cell and each reached by a first-order thermal transition. One of these is metallic and the other is insulating.