We study the antiferromagnetic (AFM) and structural phase transitions in single-crystal $\mathrm{BaF}{\mathrm{e}}_2{\left(\mathrm{A}{\mathrm{s}}_{1-x}{\mathrm{P}}_x\right)}_2(x=0,0.3)$ at temperatures $T_N$ and $T_S$, respectively, by high-resolution ac microcalorimetry and SQUID magnetometry. The specific heat measurements of both as-grown and annealed $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ display a sharp peak at the AFM-structural transitions. A kink in the entropy of annealed $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ gives evidence for splitting of the two transitions by approximately 0.5 K. No additional features could be identified in the specific heat of both $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ and $\mathrm{BaF}{\mathrm{e}}_2{\left(\mathrm{A}{\mathrm{s}}_{0.7}{\mathrm{P}}_{0.3}\right)}_2$ in the temperature regions around $T^{*}>T_S$ where torque measurements [S. Kasahara et al., Nature 486, 382 (2012)] had revealed the “true” nematic phase transition, even though the Ginzburg-Landau model used to fit the magnetic torque data indicates that the expected thermal anomaly should be easily observable with our experimental resolution.

Lattice thermal conductivities of zincblende- and wurtzite-type compounds with 33 combinations of elements are calculated with the single-mode relaxation-time approximation and a full solution of the linearized phonon Boltzmann equation from first-principles anharmonic lattice dynamics calculations. In nine zincblende-type compounds, distributions of phonon linewidths (inverse phonon lifetimes) are discussed in detail. The phonon linewidths vary nonsmoothly with respect to wave vector, which is explained from the imaginary parts of the self-energies. It is presented that detailed combination of phonon-phonon interaction strength and three-phonon selection rule is critically important to determine phonon lifetime for these compounds. This indicates difficulty to predict phonon lifetime quantitatively without anharmonic force constants. However, it is shown that joint density of states weighted by phonon numbers, which is calculated only from harmonic force constants, can be potentially used for a screening of the lattice thermal conductivity of materials.

We study the response of a magnetic-field-driven superconducting qubit strongly coupled to a superconducting coplanar waveguide resonator. We observed a strong amplification/damping of a probing signal at different resonance points corresponding to a one- and two-photon emission/absorption. The sign of the detuning between the qubit frequency and the probe determines whether amplification or damping is observed. The larger blue detuned driving leads to two-photon lasing, while the larger red detuning cools the resonator. Our experimental results are in good agreement with the theoretical model of qubit lasing and cooling at the Rabi frequency.

Carbon nanotubes are quantum sources whose emission can be tuned at telecommunication wavelengths by appropriately choosing the diameter. Most applications require the smallest possible linewidth. Therefore, the study of the underlying dephasing mechanisms is of utmost interest. Here, we report on the low-temperature photoluminescence (PL) of high crystalline quality individual single-wall carbon nanotubes synthesized by laser ablation (L-SWNTs) and emitting at telecommunication wavelengths. A thorough statistical analysis of their emission spectra reveals a typical linewidth one order of magnitude narrower than that of most samples reported in the literature. The narrowing of the PL line of L-SWNTs is due to a weaker effective exciton-phonon coupling subsequent to a weaker localization of the exciton. These results suggest that exciton localization in SWNTs not only arises from interfacial effects, but that the intrinsic crystalline quality of the SWNT plays an important role.

In a system with chiral topological order, there is a remarkable correspondence between the edge and entanglement spectra: the low-energy spectrum of the system in the presence of a physical edge coincides with the lowest part of the entanglement spectrum (ES) across a virtual cut of the system into two parts, up to rescaling and shifting. This correspondence is believed to be due to the existence of protected gapless edge modes. In this paper, we explore whether the edge-entanglement spectrum correspondence extends to nonchiral topological phases, where there are no protected gapless edge modes. Specifically, we consider the Wen-plaquette model, which is equivalent to the Kitaev toric code model and has ${\mathbb{Z}}_2$ topological order (quantum double of ${\mathbb{Z}}_2)$. The unperturbed Wen-plaquette model displays an exact correspondence: both the edge and entanglement spectra within each topological sector $a(a=1,\cdots ,4)$ are flat and equally degenerate. Here, we show, through a detailed microscopic calculation, that in the presence of generic local perturbations: (i) the effective degrees of freedom for both the physical edge and the entanglement cut consist of a (spin-$1/2)$ spin chain, with effective Hamiltonians $H_{\text{edge}}^a$ and $H_{\text{ent}}^a$, respectively, both of which have a ${\mathbb{Z}}_2$ symmetry enforced by the bulk topological order; (ii) there is in general no match between the low-energy spectra of $H_{\text{edge}}^a$ and $H_{\text{ent}}^a$, that is, there is no edge-ES correspondence. However, if supplement the ${\mathbb{Z}}_2$ topological order with a global symmetry (translational invariance along the edge/entanglement cut), i.e., by considering the Wen-plaquette model as a symmetry-enriched topological phase (SET), then there is a finite domain in Hamiltonian space in which both $H_{\text{edge}}^a$ and $H_{\text{ent}}^a$ realize the critical Ising model, whose low-energy effective theory is the $c=1/2$ Ising CFT. This is achieved because the presence of the global symmetry implies that the effective degrees of freedom of both the edge and entanglement cut are governed by Kramers-Wannier self-dual Hamiltonians, in addition to them being ${\mathbb{Z}}_2$ symmetric, which is imposed by the topological order. Thus, by considering the Wen-plaquette model as a SET, the topological order in the bulk together with the translation invariance of the perturbations along the edge/cut imply an edge-ES correspondence at least in some finite domain in Hamiltonian space.

Quasiparticle self-consistent (QS) $GW$ calculations are performed for bulk and monolayer ${\mathrm{V}}_2{\mathrm{O}}_5$. The orbital character of the bands and the bulk monolayer difference at the LDA level are discussed first. We find that the $\mathrm{QS}GW$ self-energy overestimates the gap by an unusually large amount. The main reason for this is identified to be the lattice polarization effect: The large LO-TO splittings in this polar material enhance the screening and reduce the screened Coulomb interaction affecting the gap. The effect is estimated to reduce the screened Coulomb interaction and hence the self-energy by a factor 0.38 (for bulk) and brings the calculated optical response functions in fairly good agreement with experiment. For monolayer ${\mathrm{V}}_2{\mathrm{O}}_5$ we find that the $\mathrm{QS}GW$ gap varies as $1/L$ with $L$ the size of the spacing between the monolayers in a supercell. This results from the long-range nature of the self-energy $\Sigma =iGW$ and the similar $1/L$ behavior of the dielectric screening.

A detailed analysis of the dynamics of excitons bound to two nitrogen atoms forming an isoelectronic center of $C_{2v}$ symmetry in GaAs is presented. The temperature dependence of photoluminescence (PL) intensities under both continuous and pulsed excitations reveals (1) overall decay rates significantly slower than that of spontaneous emission, (2) a decay rate anisotropy between states of orthogonal symmetry representations, and (3) a complementary behavior of relative intensities measured from states of identical symmetry representations. A comprehensive model of the exciton fine structure and the exciton dynamics allows the determination of the strength of the exchange and crystal-field interactions, the light- and heavy-hole splitting and composition of exciton states, the exciton capture time, the rates of spontaneous and nonradiative emission, and the rates of interlevel transfers induced by interactions with nuclear spins and by longitudinal acoustic phonons. It is found that the rate of electron spin flips is comparable to that measured in quantum dots but that the near degeneracy of light and heavy holes results in an efficient transfer channel between light- and heavy-hole states of identical symmetry representation.

We study the interplay between coherent light-matter interactions and nonresonant pulse propagation effects when ultrashort pulses propagate in room-temperature quantum dot (QD) semiconductor optical amplifiers (SOAs). The signatures observed on a pulse envelope after propagating in a transparent SOA, when coherent Rabi oscillations are absent, highlight the contribution of two-photon absorption (TPA), and its accompanying Kerr-like effect, as well as of linear dispersion, to the modification of the pulse complex electric field profile. These effects are incorporated into our previously developed finite-difference time-domain comprehensive model that describes the interaction between the pulses and the QD SOA. The present generalized model is used to investigate the combined effect of coherent and nonresonant phenomena in the gain and absorption regimes of the QD SOA. It confirms that in the QD SOA we examined, linear dispersion in the presence of the Kerr-like effect causes pulse compression, which counteracts the pulse peak suppression due to TPA, and also modifies the patterns which the coherent Rabi oscillations imprint on the pulse envelope under both gain and absorption conditions. The inclusion of these effects leads to a better fit with experiments and to a better understanding of the interplay among the various mechanisms so as to be able to better analyze more complex future experiments of coherent light-matter interaction induced by short pulses propagating along an SOA.

States of matter with a sharp Fermi surface but no well-defined Landau quasiparticles arise in a number of physical systems. Examples include (i) quantum critical points associated with the onset of order in metals; (ii) spinon Fermi-surface [U(1) spin-liquid] state of a Mott insulator; (iii) Halperin-Lee-Read composite fermion charge liquid state of a half-filled Landau level. In this work, we use renormalization group techniques to investigate possible instabilities of such non-Fermi liquids in two spatial dimensions to Cooper pairing. We consider the Ising-nematic quantum critical point as an example of an ordering phase transition in a metal, and demonstrate that the attractive interaction mediated by the order-parameter fluctuations always leads to a superconducting instability. Moreover, in the regime where our calculation is controlled, superconductivity preempts the destruction of electronic quasiparticles. On the other hand, the spinon Fermi surface and the Halperin-Lee-Read states are stable against Cooper pairing for a sufficiently weak attractive short-range interaction; however, once the strength of attraction exceeds a critical value, pairing sets in. We describe the ensuing quantum phase transition between (i) $\text{U}\left(1\right)$ and $Z_2$ spin-liquid states; (ii) Halperin-Lee-Read and Moore-Read states.

Recent experiments in ultracold atoms and photonic analogs have reported the implementation of artificial gauge fields in lattice systems, facilitating the realization of topological phases. Motivated by such advances, we investigate the Haldane honeycomb lattice tight-binding model, for bosons with local interactions at the average filling of one boson per site. We analyze the ground-state phase diagram and uncover three distinct phases: a uniform superfluid (SF), a chiral superfluid (CSF), and a plaquette Mott insulator with local current loops (PMI). Nearest-neighbor and next-nearest-neighbor currents distinguish CSF from SF, and the phase transition between them is first order. We apply bosonic dynamical mean-field theory and exact diagonalization to obtain the phase diagram, complementing numerics with calculations of excitation spectra in strong and weak coupling perturbation theory. The characteristic density fluctuations, current correlation functions, and excitation spectra are measurable in ultracold atom experiments.

We study how polaronic states form as a function of time due to strong electron-phonon coupling, starting from a hot electron distribution which is representative of a photoinduced metallic state immediately after laser excitation. For this purpose we provide the exact solution of the single-electron Holstein model within nonequilibrium dynamical mean-field theory. In particular, this allows us to reveal key features of the transient metallic state in the numerically most challenging regime, the adiabatic regime, in which phonon frequencies are smaller than the electronic bandwidth: Initial coherent phonon oscillations are strongly damped, leaving the system in a mixture of excited polaron states and metastable delocalized states. We compute the time-resolved photoemission spectrum, which makes it possible to disentangle two contributions. The existence of long-lived delocalized states suggests ways to externally control transient properties of photodoped metals.

We study the real-time dynamics of a highly excited charge carrier coupled to quantum phonons via a Holstein-type electron-phonon coupling. This is a prototypical example for the nonequilibrium dynamics in an interacting many-body system where excess energy is transferred from electronic to phononic degrees of freedom. We use diagonalization in a limited functional space (LFS) to study the nonequilibrium dynamics on a finite one-dimensional chain. This method agrees with exact diagonalization and the time-evolving block-decimation method, in both the relaxation regime and the long-time stationary state, and among these three methods it is the most efficient and versatile one for this problem. We perform a comprehensive analysis of the time evolution by calculating the electron, phonon and electron-phonon coupling energies, and the electronic momentum distribution function. The numerical results are compared to analytical solutions for short times, for a small hopping amplitude and for a weak electron-phonon coupling. In the latter case, the relaxation dynamics obtained from the Boltzmann equation agrees very well with the LFS data. We also study the time dependence of the eigenstates of the single-site reduced density matrix, which defines the so-called optimal phonon modes. We discuss their structure in nonequilibrium and the distribution of their weights. Our analysis shows that the structure of optimal phonon modes contains very useful information for the interpretation of the numerical data.

We report a high-pressure study of simultaneous low-temperature electrical resistivity and Hall effect measurements on high quality single-crystalline ${\mathrm{KFe}}_2{\mathrm{As}}_2$ using designer diamond anvil cell techniques with applied pressures up to 33 GPa. In the low-pressure regime, we show that the superconducting transition temperature $T_c$ finds a maximum onset value of 7 K near 2 GPa, in contrast to previous reports that find a minimum $T_c$ and reversal of pressure dependence at this pressure. Upon applying higher pressures, this $T_c$ is diminished until a sudden drastic enhancement occurs coincident with a first-order structural phase transition into a collapsed tetragonal phase. The appearance of a distinct superconducting phase above 13 GPa is also accompanied by a sudden reversal of dominant charge carrier sign, from hole- to electron-like, which agrees with our band structure calculations predicting the emergence of an electron pocket and diminishment of hole pockets upon Fermi surface reconstruction. Our results suggest the high-temperature superconducting phase in ${\mathrm{KFe}}_2{\mathrm{As}}_2$ is substantially enhanced by the presence of nested electron and hole pockets, providing the key ingredient of high-$T_c$ superconductivity in iron pnictide superconductors.

Interfacial superconductivity is observed in a variety of heterostructures composed of different materials including superconducting and nonsuperconducting (at appropriate doping and temperatures) cuprates and iron-based pnictides. The origin of this superconductivity remains in many cases unclear. Here, we propose a general mechanism of interfacial superconductivity for systems with competing order parameters. We assume that parameters characterizing the material allow formation of another order like charge- or spin-density wave competing and prevailing superconductivity in the bulk (hidden superconductivity). Diffusive electron scattering on the interface results in a suppression of this order and releasing the superconductivity. Our theory is based on the use of Ginzburg-Landau equations applicable to a broad class of systems. We demonstrate that the local superconductivity appears in the vicinity of the interface and the spatial dependence of the superconducting order parameter $\Delta \left(x\right)$ is described by the Gross-Pitaevskii equation. Solving this equation we obtain quantized values of temperature and doping levels at which $\Delta \left(x\right)$ appears. Remarkably, the local superconductivity shows up even in the case when the rival order is only slightly suppressed and may arise also on the surface of the sample (surface superconductivity).

We present an analytic theory of quantum criticality in quasi-one-dimensional topological Anderson insulators. We describe these systems in terms of two parameters $(g,\chi )$ representing localization and topological properties, respectively. Certain critical values of $\chi$ (half-integer for $\mathbb{Z}$ classes, or zero for ${\mathbb{Z}}_2$ classes) define phase boundaries between distinct topological sectors. Upon increasing system size, the two parameters exhibit flow similar to the celebrated two-parameter flow of the integer quantum Hall insulator. However, unlike the quantum Hall system, an exact analytical description of the entire phase diagram can be given in terms of the transfer-matrix solution of corresponding supersymmetric nonlinear sigma models. In ${\mathbb{Z}}_2$ classes we uncover a hidden supersymmetry, present at the quantum critical point.

Magnetic moments intercalated into layered transition metal dichalcogenides are an excellent system for investigating the rich physics associated with magnetic ordering in a strongly anisotropic, strong spin-orbit coupling environment. We examine electronic transport and magnetization in ${\mathrm{Fe}}_{0.28}{\mathrm{TaS}}_2$, a highly anisotropic ferromagnet with a Curie temperature $T_{\mathrm{C}}\sim 68.8$ K. We find anomalous Hall data confirming a dominance of spin-orbit coupling in the magnetotransport properties of this material, and a remarkably large field-perpendicular-to-plane magnetoresistance (MR) exceeding 60% at 2 K, much larger than the typical MR for bulk metals, and comparable to state-of-the-art giant MR in thin film heterostructures, and smaller only than colossal MR in Mn perovskites or high mobility semiconductors. Even within the ${\mathrm{Fe}}_x{\mathrm{TaS}}_2$ series, for the current $x=0.28$ single crystals the MR is nearly $100\times$ higher than that found previously in the commensurate compound ${\mathrm{Fe}}_{0.25}{\mathrm{TaS}}_2$. After considering alternatives, we argue that the large MR arises from spin-disorder scattering in the strong spin-orbit coupling environment, and suggest that this can be a design principle for materials with large MR.

A series of infrared absorption experiments and complementary theory have been performed to determine the properties of OH and OD centers in $\mathrm{I}{\mathrm{n}}_2{\mathrm{O}}_3$ single crystals. Annealing $\mathrm{I}{\mathrm{n}}_2{\mathrm{O}}_3$ samples in ${\mathrm{H}}_2$ or ${\mathrm{D}}_2$ at temperatures near 450 °C produces an $n-\text{type}$ layer $\approx 0.06\mathrm{mm}$ thick with an $n-\text{type}$ doping of $1.6\times {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$. The resulting free-carrier absorption is correlated with an OH center with a vibrational frequency of $3306\mathrm{c}{\mathrm{m}}^{-1}$ that we associate with interstitial ${\mathrm{H}}^{+}$. Additional O-H (O-D) vibrational lines are assigned to metastable configurations of the interstitial ${\mathrm{H}}^{+}\left({\mathrm{D}}^{+}\right)$ center and complexes of H (D) with In vacancies. Unlike other oxides studied recently where H trapped at an oxygen vacancy is the dominant shallow donor (ZnO and $\mathrm{Sn}{\mathrm{O}}_2$, for example), interstitial ${\mathrm{H}}^{+}$ is found to be the dominant H-related shallow donor in $\mathrm{I}{\mathrm{n}}_2{\mathrm{O}}_3$.

The inclusion of the global effects of semiconductor doping poses a unique challenge for first-principles simulations, because the typically low concentration of dopants renders an explicit treatment intractable. Furthermore, the width of the space-charge region (SCR) at charged surfaces often exceeds realistic supercell dimensions. Here, we present a multiscale technique that fully addresses these difficulties. It is based on the introduction of a charged sheet, mimicking the SCR-related field, along with free charge which mimics the bulk charge reservoir, such that the system is neutral overall. These augment a slab comprising “pseudoatoms” possessing a fractional nuclear charge matching the bulk doping concentration. Self-consistency is reached by imposing charge conservation and Fermi level equilibration between the bulk, treated semiclassically, and the electronic states of the slab, which are treated quantum-mechanically. The method, called CREST—the charge-reservoir electrostatic sheet technique—can be used with standard electronic structure codes. We validate CREST using a simple tight-binding model, which allows for comparison of its results with calculations encompassing the full SCR explicitly. Specifically, we show that CREST successfully predicts scenarios spanning the range from no to full Fermi level pinning. We then employ it with density functional theory, obtaining insight into the doping dependence of the electronic structures of the metallic “clean-cleaved” Si(111) surface and its semiconducting $(2\times 1)$ reconstructions.

Lightly fullerene-doped polymers are suitable composite systems to study spin-dependent bimolecular interactions among charge excitations due to their long lifetimes in these systems. These interactions can affect the photocurrent as well as the open-circuit voltage in an organic solar cell. Combining photoluminescence detected magnetic resonance (PLDMR) and electrically detected magnetic resonance (EDMR) spectroscopies we study films and devices of $\text{poly}(p$-phenylene vinylene) polymers poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) and superyellow PPV (SY-PPV) lightly doped with various fullerene derivatives [6,6]-phenyl ${\mathrm{C}}_{61}$ butyric acid methyl ester (PCBM), bis[60]PCBM (bis-PCBM), indene-${\mathrm{C}}_{60}$ bisadduct (ICBA), and [6,6]-phenyl ${\mathrm{C}}_{71}$ butyric acid methyl ester (PC70BM). (i) We demonstrate strong fullerene triplet exciton (TE) production in SY-PPV:fullerene blends, whereas this is absent in MDMO-PPV:PCBM and only very weak in MDMO-PPV:ICBA. The low TE production in blends with MDMO-PPV is attributed to a weaker singlet-singlet energy-transfer coupling and an unfavorable triplet level alignment between the blend components. (ii) The fullerene TE spectra are analyzed on the basis of a single type of triplet excitation in PCBM, bis-PCBM, and ICBA, and two triplet species in PC70BM which are attributed to the α- and β-type isomers of the latter molecule. (iii) The sign change with increasing temperature of the $g\sim 2$ sharp central line in photo-EDMR, which is observed both in pristine SY-PPV and in blends with fullerene, is correlated to a transition from dominant TE-polaron annihilation to nongeminate polaron recombination processes.

We introduce a “second-quantized” representation of the ring of symmetric functions to further develop a purely second-quantized, or “lattice,” approach to the study of zero modes of frustration-free Haldane-pseudopotential–type Hamiltonians, which in particular stabilize Laughlin ground states. We present three applications of this formalism. We start demonstrating how to systematically construct all zero modes of Laughlin-type parent Hamiltonians in a framework that is free of first-quantized polynomial wave functions, and show that they are in one-to-one correspondence with dominance patterns. The starting point here is the pseudopotential Hamiltonian in “lattice form,” stripped of all information about the analytic structure of Landau levels (dynamical momenta). Second, as a by-product, we make contact with the bosonization method, and obtain an alternative proof for the equivalence between bosonic and fermionic Fock spaces. Finally, we explicitly derive the second-quantized version of Read's nonlocal (string) order parameter for the Laughlin state, extending an earlier description by Stone. Commutation relations between the local quasihole operator and the local electron operator are generalized to various geometries.