The half-filled Landau level is widely believed to be described by the Halperin-Lee-Read theory of the composite Fermi liquid (CFL). In this paper, we develop a theory for the particle-hole conjugate of the CFL, the anti-CFL, which we argue to be a distinct phase of matter as compared with the CFL. The anti-CFL provides a possible explanation of a recent experiment [D. Kamburov et al., Phys. Rev. Lett. 113, 196801 (2014)] demonstrating that the density of composite fermions in GaAs quantum wells corresponds to the electron density when the filling fraction $\nu <\frac{1}{2}$ and to the hole density when $\nu >\frac{1}{2}$. We introduce a local field theory for the CFL and anti-CFL in the presence of a boundary, which we use to study CFL-insulator-CFL junctions, and the interface between the anti-CFL and CFL. We show that the CFL–anti-CFL interface allows partially fused boundary phases in which “composite electrons” can directly tunnel into “composite holes,” providing a nontrivial example of transmutation between topologically distinct quasiparticles. We discuss several observable consequences of the anti-CFL, including a predicted resistivity jump at a first-order transition between uniform CFL and anti-CFL phases. We also present a theory of a continuous quantum phase transition between the CFL and anti-CFL. We conclude that particle-hole symmetry requires a modified view of the half-filled Landau level, in the presence of strong electron-electron interactions and weak disorder, as a critical point between the CFL and the anti-CFL.

We use transport and neutron scattering to study electronic, structural, and magnetic properties of the electron-doped ${\mathrm{BaFe}}_{2-x}{\mathrm{Ni}}_x{\mathrm{As}}_2$ iron pnictides in uniaxial-strained and external-stress-free detwinned states. Using a specially designed in situ mechanical detwinning device, we demonstrate that the in-plane resistivity anisotropy observed in the uniaxial-strained tetragonal state of ${\mathrm{BaFe}}_{2-x}{\mathrm{Ni}}_x{\mathrm{As}}_2$ below a temperature $T^{*}$, previously identified as a signature of the electronic nematic phase, is also present in the stress-free tetragonal phase below $T^{**}(<T^{*})$. By carrying out neutron scattering measurements on ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and ${\mathrm{BaFe}}_{1.97}{\mathrm{Ni}}_{0.03}{\mathrm{As}}_2$, we argue that the resistivity anisotropy in the stress-free tetragonal state of iron pnictides arises from the magnetoelastic coupling associated with antiferromagnetic order. These results thus indicate that the local lattice distortion and nematic spin correlations are responsible for the resistivity anisotropy in the tetragonal state of stress-free iron pnictides, and suggest that resistivity anisotropy, spin excitation anisotropy, and orbital ordering found in the paramagnetic state of uniaxial-strained iron pnictides are due to the externally applied uniaxial strain via an enhanced nematic susceptibility.

We calculate the Rényi entropy of a positive integer order $M$ for a reduced density matrix of a single-level quantum dot connected to left and right leads. We exploit a $2\times 2$ modified Keldysh Green-function matrix obtained by the discrete Fourier transform of a $2M\times 2M$ multicontour Keldysh Green-function matrix. A moment-generating function of self-information is deduced from the analytic continuation of $M$ to the complex plane. We calculate the probability distribution of self-information and find that, within the Hartree approximation, the on-site Coulomb interaction affects rare events and modifies a bound of the probability distribution. A simple equality, from which an upper bound of the average, i.e., the entanglement entropy, would be inferred, is presented. For noninteracting electrons, the entanglement entropy is expressed with current cumulants of the full counting statistics of electron transport.

Paramagnetic luminescent point defects in diamond are increasingly important candidates for quantum information processing applications. Recently, the coherent manipulation of single silicon-vacancy defect spins has been demonstrated in chemical vapor deposited diamond samples where silicon may be introduced as a contamination in the growth process. Hydrogen impurity may simultaneously enter diamond too and form complexes with silicon-vacancy defects. However, relatively little is known about these complexes in diamond. Here we report plane-wave supercell density functional theory results on various complexes of silicon vacancy and hydrogen in diamond. We found a family of complexes of silicon, vacancies, and hydrogen atoms that are thermally stable in diamond with relatively low formation energies that might form yet unobserved or unidentified silicon-related defects. These complexes often show infrared optical transitions and are paramagnetic. We tentatively assign one of these complexes to a recently reported but yet unidentified infrared absorber center. We show that this center has a metastable triplet state and might exhibit a spin-selective decay to the ground state, thus it is an interesting candidate for quantum information processing applications. We also discuss here methodology aspects of calculating hyperfine parameters and intradefect level excitations in systems with notoriously complex electron states within hybrid density functional approach. We also demonstrate that a simplified approach using ab initio data can be very powerful to predict the relative intensities of the phonon replica associated with quasilocal vibration modes in the photoexcitation spectrum.

We present ${}^{75}\mathrm{As}$ nuclear magnetic resonance spin-lattice and spin-spin relaxation rate data in $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ and $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Cu}}_x{)}_2{\mathrm{As}}_2$ as a function of temperature, doping, and magnetic field. The relaxation curves exhibit a broad distribution of relaxation rates, consistent with inhomogeneous glassy behavior up to 100 K. The doping and temperature response of the width of the dynamical heterogeneity is similar to that of the nematic susceptibility measured by elastoresistance measurements. We argue that quenched random fields which couple to the nematic order give rise to a nematic glass that is reflected in the spin dynamics.

We revisit the well-known Mollwo-Ivey relation that describes the “universal” dependence of the absorption energies of F-type color centers on the lattice constant $a$ of alkali-halide crystals, $E_{\text{abs}}\propto a^{-n}.$ We perform both state-of-the-art ab initio quantum chemistry and post-DFT calculations of F-center absorption spectra. By “tuning” independently the lattice constant and the atomic species we show that the scaling with the lattice constant alone (keeping the elements fixed) would yield $n=2$ in agreement with the “particle-in-the-box” model. Keeping the lattice constant fixed and changing the atomic species enables us to quantify the ion-size effects which are shown to be responsible for the exponent $n\approx 1.8$.

We systematically investigate the ground-state and the spectral properties of antiferromagnets on a kagomé lattice with several common types of the planar anisotropy: $XXZ$, single-ion, and out-of-plane Dzyaloshinskii-Moriya. Our main focus is on the role of nonlinear, anharmonic terms, which are responsible for the quantum order-by-disorder effect and for the corresponding selection of the ground-state spin structure in many of these models. The $XXZ$ and the single-ion anisotropy models exhibit a quantum phase transition between the $\mathbf{q}=0$ and the $\sqrt{3}\times \sqrt{3}$ states as a function of the anisotropy parameter, offering a rare example of the quantum order-by-disorder fluctuations favoring a ground state which is different from the one selected by thermal fluctuations. The nonlinear terms are also shown to be crucial for a very strong near-resonant decay phenomenon leading to the quasiparticle breakdown in the kagomé-lattice antiferromagnets whose spectra are featuring flat or weakly dispersive modes. The effect is shown to persist even in the limit of large spin values and should be common to other frustrated magnets with flat branches of excitations. Model calculations of the spectrum of the $S=5/2$ Fe-jarosite with Dzyaloshinskii-Moriya anisotropy provide a convincing and detailed characterization of the proposed scenario.

We use a magnetic force microscope (MFM) to investigate single-vortex pinning and penetration depth in ${\mathrm{NdFeAsO}}_{1-x}{\mathrm{F}}_x$, one of the highest-$T_c$ iron-based superconductors. In fields up to 20 G, we observe a disordered vortex arrangement, implying that the pinning forces are stronger than the vortex-vortex interactions. We measure the typical force to depin a single vortex, $F_{\mathrm{depin}}\simeq 4.5$ pN, corresponding to a critical current up to $J_c\simeq 7\times {10}^5$ A/${\mathrm{cm}}^2$. Furthermore, our MFM measurements allow the first local and absolute determination of the superconducting in-plane penetration depth in ${\mathrm{NdFeAsO}}_{1-x}{\mathrm{F}}_x,{\lambda }_{ab}=320\pm 60$ nm, which is larger than previous bulk measurements.

Time-resolved terahertz time-domain spectroscopy (THz-TDS) is an ideal tool for probing photoinduced nonequilibrium metallic and superconducting states. Here, we focus on the interpretation of the two-dimensional response function $\mathrm{\Sigma }(\omega ;t)$ that it measures, examining whether it provides an accurate snapshot of the instantaneous optical conductivity $\sigma (\omega ;t)$. For the Drude model with a time-dependent carrier density, we show that $\mathrm{\Sigma }(\omega ;t)$ is not simply related to $\sigma (\omega ;t)$. The difference in the two response functions is most pronounced when the momentum relaxation rate of photocarriers is small, as would be the case in a system that becomes superconducting following pulsed photoexcitation. From the analysis of our model, we identify signatures of photoinduced superconductivity that could be seen by time-resolved THz-TDS.

Whether a quantum bath can be approximated as classical Gaussian noise is a fundamental issue in central spin decoherence and also of practical importance in designing noise-resilient quantum control. Spin qubits based on bismuth donors in silicon have tunable interactions with nuclear spin baths and are first-order insensitive to magnetic noise at so-called clock transitions (CTs). This system is therefore ideal for studying the quantum/classical Gaussian nature of nuclear spin baths since the qubit-bath interaction strength determines the back-action on the baths and hence the adequacy of a Gaussian noise model. We develop a Gaussian noise model with noise correlations determined by quantum calculations and compare the classical noise approximation to the full quantum bath theory. We experimentally test our model through a dynamical decoupling sequence of up to 128 pulses, finding good agreement with simulations and measuring electron spin coherence times approaching 1 s—notably using natural silicon. Our theoretical and experimental study demonstrates that the noise from a nuclear spin bath is analogous to classical Gaussian noise if the back-action of the qubit on the bath is small compared to the internal bath dynamics, as is the case close to CTs. However, far from the CTs, the back-action of the central spin on the bath is such that the quantum model is required to accurately model spin decoherence.

The hyperfine field from dynamically polarized nuclei in $n$-GaAs is very spatially inhomogeneous, as the nuclear polarization process is most efficient near the randomly distributed donors. Electrons with polarized spins traversing the bulk semiconductor will experience this inhomogeneous hyperfine field as an effective fluctuating spin precession rate, and thus the spin polarization of an electron ensemble normal to the fluctuating hyperfine fields will relax. A theory of spin relaxation based on the theory of random walks is applied to such an ensemble precessing in an oblique magnetic field, and the precise form of the (unequal) longitudinal and transverse spin relaxation is analytically derived. To investigate this mechanism, electrical three-terminal Hanle measurements were performed on epitaxially grown ${\mathrm{Co}}_2\mathrm{MnSi}/n$-GaAs heterostructures fabricated into electrical spin injection devices. The proposed anisotropic spin relaxation mechanism is required to satisfactorily describe the Hanle line shapes when the applied field is oriented at large oblique angles.

Several observed transport and thermodynamic properties of the heavy-fermion compound ${\text{YbRh}}_2{\text{Si}}_2$ in the quantum critical regime are unusual and suggest that the fermionic quasiparticles are critical, characterized by a scale-dependent diverging effective mass. A theory based on the concept of critical quasiparticles scattering off antiferromagnetic spin fluctuations in a strong-coupling regime has been shown to successfully explain the unusual existing data and to predict a number of so far unobserved properties. In this paper, we point out a new feature of a magnetic field-tuned quantum critical point of a heavy-fermion metal: anomalies in the transport and thermodynamic properties caused by the freezing out of spin-flip scattering of critical quasiparticles and the scattering off collective spin excitations. We show a steplike behavior as a function of magnetic field of, e.g., the Hall coefficient and magnetoresistivity results, which accounts quantitatively for the observed behavior of these quantities. That behavior has been described as a crossover line $T^{*}\left(H\right)$ in the $T-H$ phase diagram of ${\text{YbRh}}_2{\text{Si}}_2$. Whereas some authors have interpreted this observation as signaling the breakdown of Kondo screening and an associated abrupt change of the Fermi surface, our results suggest that the $T^{*}$ line may be quantitatively understood within the picture of robust critical quasiparticles.

Ultrafast optical pump-probe spectroscopy is used to track carrier dynamics in the large-magnetoresistance material ${\mathrm{WTe}}_2$. Our experiments reveal a fast relaxation process occurring on a subpicosecond time scale that is caused by electron-phonon thermalization, allowing us to extract the electron-phonon coupling constant. An additional slower relaxation process, occurring on a time scale of $\sim 5$–15 ps, is attributed to phonon-assisted electron-hole recombination. As the temperature decreases from 300 K, the time scale governing this process increases due to the reduction of the phonon population. However, below $\sim 50$ K, an unusual decrease of the recombination time sets in, most likely due to a change in the electronic structure that has been linked to the large magnetoresistance observed in this material.

The interplay between charge, spin, and heat currents in magnetic nanostructures subjected to a temperature gradient has led to a variety of novel effects and promising applications studied in the fast-growing field of spin caloritronics. Here, we explore the magnetothermoelectrical properties of an individual magnetic domain wall in a permalloy nanowire. In thermal gradients of the order of few $\mathrm{K}/\mu \mathrm{m}$ along the long wire axis, we find a clear magneto-Seebeck signature due to the presence of a single domain wall. The observed domain wall magneto-Seebeck effect can be explained by the magnetization-dependent Seebeck coefficient of permalloy in combination with the local spin configuration of the domain wall.

We examine the quality of the local self-energy approximation, applied here to models of multiple quantum impurities coupled to an electronic bath. The local self-energy is obtained by solving a single-impurity Anderson model in an effective medium that is determined self-consistently, similar to the dynamical mean-field theory (DMFT) for correlated lattice systems. By comparing to exact results obtained by using the numerical renormalization group, we determine situations where “impurity-DMFT” is able to capture the physics of highly inhomogeneous systems and those cases where it fails. For two magnetic impurities separated in real space, the onset of the dilute limit is captured, but RKKY-dominated interimpurity singlet formation cannot be described. For parallel quantum dot devices, impurity-DMFT succeeds in capturing the underscreened Kondo physics by self-consistent generation of a critical pseudogapped effective medium. However, the quantum phase transition between high- and low-spin states upon tuning interdot coupling cannot be described.

We study the real-time dynamics of a translationally invariant quantum spin chain, based on the East kinetically constrained glass model, in search for evidence of many-body localization in the absence of disorder. Numerical simulations indicate a change, controlled by a coupling parameter, from a regime of fast relaxation-corresponding to thermalization-to a regime of very slow relaxation. This slowly relaxing regime is characterized by dynamical features usually associated with nonergodicity and many-body localization (MBL): memory of initial conditions, logarithmic growth of entanglement entropy, and nonexponential decay of time correlators. We show that slow relaxation is a consequence of sensitivity to spatial fluctuations in the initial state. While numerical results and physical considerations indicate that relaxation time scales grow markedly with size, our finite size results are consistent both with an MBL transition, expected to only occur in disordered systems, and with a pronounced quasi-MBL crossover.

In this paper, we show how to discretize the Abelian Chern-Simons gauge theory on generic planar lattices/graphs (with or without translational symmetries) embedded in arbitrary two-dimensional closed orientable manifolds. We find that, as long as a one-to-one correspondence between vertices and faces can be defined on the graph such that each face is paired up with a neighboring vertex (and vice versa), a discretized Abelian Chern-Simons theory can be constructed consistently. We further verify that all the essential properties of the Chern-Simons gauge theory are preserved in the discretized setup. In addition, we find that the existence of such a one-to-one correspondence is not only a sufficient condition for discretizing a Chern-Simons gauge theory but, for the discretized theory to be nonsingular and to preserve some key properties of the topological field theory, this correspondence is also a necessary one. A specific example will then be provided, in which we discretize the Abelian Chern-Simons gauge theory on a tetrahedron.

We present a method for determining correlations in a gas of indirect excitons in a semiconductor quantum well structure. The method involves subjecting the excitons to a periodic electrostatic potential that causes modulations of the exciton density and photoluminescence (PL). Experimentally measured amplitudes of energy and intensity modulations of exciton PL serve as an input to a theoretical estimate of the exciton correlation parameter and temperature. We also present a proof-of-principle demonstration of the method for determining the correlation parameter and discuss how its accuracy can be improved.

The Majorana code is an example of a stabilizer code where the quantum information is stored in a system supporting well-separated Majorana bound states (MBSs). We focus on one-dimensional realizations of the Majorana code, as well as networks of such structures, and investigate their lifetime when coupled to a parity-preserving thermal environment. We apply the Davies prescription, a standard method that describes the basic aspects of a thermal environment, and derive a master equation in the Born-Markov limit. We first focus on a single wire with immobile MBSs and perform error correction to annihilate thermal excitations. In the high-temperature limit, we show both analytically and numerically that the lifetime of the Majorana qubit grows logarithmically with the size of the wire. We then study a trijunction with four MBSs when braiding is executed. We study the occurrence of dangerous error processes that prevent the lifetime of the Majorana code from growing with the size of the trijunction. The origin of the dangerous processes is the braiding itself, which separates pairs of excitations and renders the noise nonlocal; these processes arise from the basic constraints of moving MBSs in one-dimensional (1D) structures. We confirm our predictions with Monte Carlo simulations in the low-temperature regime, i.e., the regime of practical relevance. Our results put a restriction on the degree of self-correction of this particular 1D topological quantum computing architecture.

Photon-based spectroscopies have played a central role in exploring the electronic properties of crystalline solids and thin films. Though they remain a powerful tool for probing the electronic properties of nanostructures, they are limited by lack of spatial resolution. On the other hand, electron-based spectroscopies, e.g., electron energy loss spectroscopy (EELS), are now capable of subangstrom spatial resolution. Core-loss EELS, a spatially resolved analog of x-ray absorption, has been used extensively in the study of inhomogeneous complex systems. In this paper, we demonstrate that low-loss EELS in an aberration-corrected scanning transmission electron microscope, which probes low-energy excitations, combined with a theoretical framework for simulating and analyzing the spectra, is a powerful tool to probe low-energy electron excitations with atomic-scale resolution. The theoretical component of the method combines density functional theory–based calculations of the excitations with dynamical scattering theory for the electron beam. We apply the method to monolayer graphene in order to demonstrate that atomic-scale contrast is inherent in low-loss EELS even in a perfectly periodic structure. The method is a complement to optical spectroscopy as it probes transitions entailing momentum transfer. The theoretical analysis identifies the spatial and orbital origins of excitations, holding the promise of ultimately becoming a powerful probe of the structure and electronic properties of individual point and extended defects in both crystals and inhomogeneous complex nanostructures. The method can be extended to probe magnetic and vibrational properties with atomic resolution.