The Fe${}_{1+y}$Te${}_{1-x}$Se${}_x$ series of materials is one of the prototype families of Fe-based superconductors. To provide further insight into these materials, we present systematic inelastic neutron scattering measurements of the low-energy spin excitations for $x=0.27$, 0.36, 0.40, and 0.49. These measurements show an evolution of incommensurate spin excitations towards the (1/2,1/2,0) wave vector with doping. Concentrations ($x=0.40$ and 0.49) which exhibit the most robust superconducting properties have spin excitations closest to (1/2,1/2,0) and also exhibit a strong spin resonance in the spin excitation spectrum below $T_c$. The resonance signal appears to be closer to (1/2,1/2,0) than the underlying spin excitations. We discuss the possible relationship between superconductivity and spin excitations at the (1/2,1/2,0) wave vector and the role that interstitial Fe may play.

In this work we compute subleading oscillating terms in the Renyi entropy of Fermi gases and liquids corresponding to $2k_F$-like oscillations. Our theoretical tools are the one-dimensional formulation of Fermi liquid entanglement familiar from discussions of the logarithmic violation of the area law and quantum Monte Carlo calculations. The main result is a formula for the oscillating term for any region geometry and a spherical Fermi surface in any dimension. Specializing to two dimensions, we compare this term to numerical calculations of Renyi entropies using the correlation function method and find excellent agreement. We also compare with quantum Monte Carlo data on interacting Fermi liquids where we also find agreement up to moderate interaction strengths.

The electronic structure of thin Ca/Sr alloy films epitaxially grown on the topological insulator Bi${}_2$Se${}_3$ is investigated by first-principles calculations. Despite a negligible spin-orbit coupling in Ca/Sr alloys and strict spin degeneracy in freestanding Ca/Sr films, the topological surface states on the pristine Bi${}_2$Se${}_3$ surface are spatially transferred, upon Ca/Sr overlayer growth, into the overlayer to form topological quantum well resonances characterized by a spin-polarized chiral Dirac cone. The physical origin of this effect and the implications regarding applications are discussed.

Features of a topological phase, and edge states in particular, may be obscured by overlapping in energy with a trivial conduction band. The topological nature of such a conductor, however, is revealed in its transport properties, especially in the presence of disorder. In this work, we explore the conductance behavior of such a system with disorder present, and contrast it with the quantized conductance in an ideal two-dimensional topological insulator. Our analysis relies on numerics on a lattice system and analytics on a simple toy model. Interestingly, we find that as disorder is increased from zero, the edge conductivity initially falls from its quantized value; yet, as disorder continues to increase, the conductivity recovers, and saturates at a value slightly below the quantized value of the clean system. We discuss how this effect can be understood from the tendency of the bulk states to localize, while the edge states remain delocalized.

I characterize the spatial optical solitons supported by arrays of metallic nanowires embedded in Kerr-type material. The array of nanowires is described using an effective medium model and is regarded as a continuous medium. I show that the conditions necessary for the formation of spatial solitons are radically different in presence of the nanowires. In particular, within the effective medium model, spatial solitons are allowed in the nanowire material only when the host material is a “self-defocusing” material. It is proven that the characteristic soliton beamwidth is related to the degree of hyperbolicity of the isofrequency surfaces of the photonic states and that a sufficiently strong electric field amplitude may enable subwavelength solitary waves.

We show that hyperbolic electromagnetic metamaterials, implemented as multilayers based on two material constituents arranged according to the Thue-Morse aperiodic sequence, may exhibit strong nonlocal effects, manifested as the appearance of additional extraordinary waves which are not predicted by standard effective-medium-theory (local) models. From the mathematical viewpoint, these effects can be associated with stationary points of the transfer-matrix trace and can be effectively parametrized via the trace-map formalism. We show that their onset is accompanied by a strong wave-vector dependence in the effective constitutive parameters. Despite the inherent periodicity enforced by the unavoidable (Bloch-type) supercell terminations, we show that such strong nonlocality is retained at any arbitrarily high-order iteration, i.e., approaching the actual aperiodic regime. Moreover, for certain parameter configurations, at a given wavelength and for two given material layers, these effects may be significantly less prominent when the same layers are arranged in a standard periodic fashion. Our findings indicate that the (aperiodic) positional order of the layers constitutes an effective and technologically inexpensive additional degree of freedom in the engineering of optical nonlocality.

We have developed an efficient and accurate formalism which allows the simulation at the ab initio level of inelastic electron tunneling spectroscopy data under a scanning tunneling microscope setup. It exploits fully the tunneling regime by carrying out the structural optimization and vibrational mode calculations for surface and tip independently. The most relevant interactions in the inelastic current are identified as the inelastic tunneling terms, which are taken into account up to lowest order, while all other inelastic contributions are neglected. As long as the system is under tunneling regime conditions and there is no physisorbed molecule on the surface or tip apex, this lowest order in inelastic tunneling (LOIT) approach reduces drastically the computational cost compared to related approaches while maintaining a good accuracy. Adopting the wide-band limit for both tip and surface further reduces calculation times significantly, and is shown to give similar results to when the full energy dependence of the Green's functions is taken into account. The LOIT is applied to the Cu(111)+CO system probed by a clean and a CO contaminated tip to find good agreement with previous works. Different parameters involved in the calculations such as basis sets, $k$ sampling, tip-sample distance, or temperature, among others, are discussed in detail.

We show that the probability of electric-field-induced interband tunneling in solid-state systems is generically a nonmonotonic (oscillatory) function of the applied field. This unexpected behavior can be understood as arising due to a common path interference between two distinct tunneling solutions. The phenomenon is insensitive to magnetic field, and arises whenever the low-energy dispersion relation contains higher order terms in addition to the usual $p^2$ term. Such higher order terms are generically present, albeit with small coefficient, so that the oscillatory Zener tunneling is a universal phenomenon. However, the first “Zener oscillation” occurs at a transmission probability which is exponentially small when the coefficient of the higher order terms is small. This explains why this oscillatory aspect of Zener tunneling has been hitherto overlooked, despite its universality. The common path interference is also destroyed by the presence of odd powers of $p$ in the low-energy dispersion relation. Since odd powers of $p$ are strictly absent only when the tunneling barrier lies along an axis of mirror symmetry, it follows that the robustness of the oscillatory behavior depends on the orientation of the tunneling barrier. Bilayer graphene is identified as a particularly good material for observation of common path interference, due to its unusual nearly isotropic dispersion relation, where the $p^4$ term makes the leading contribution.

We present the results of high-field magnetization and muon-spin relaxation measurements on the coordination polymer CuF${}_2$(H${}_2$O)${}_2$(pyrazine) in pressures up to 22.5 kbar. We observe a transition from a quasi-two-dimensional to a quasi-one-dimensional antiferromagnetic phase at 9.1 kbar, driven by a rotation of the Jahn-Teller axis. Long-range antiferromagnetic ordering is seen in both regimes, as well as a phase separation in the critical pressure region. The magnetic dimensionality switching as pressure is increased is accompanied by a halving of the primary magnetic exchange energy $J$ and a fivefold decrease in the ordering temperature $T_{\text{N}}$. $J$ decreases gradually with pressure in the two-dimensional phase, and then increases in the one-dimensional regime. We relate both effects to the changes in the crystal structure with applied pressure.

Using the first-principles method and special quasirandom structure approach, we have studied the formation energies of two prototype defects in alloys, Ge${}_{\mathrm{As}}$ in Al${}_x$Ga${}_{1-x}$As and Cu${}_{\mathrm{Cd}}$ in CdS${}_x$Te${}_{1-x}$. We find that giant bowing effects for the defect formation energy can exist in semiconductor alloys. The bowing effect originates from the concentrated distribution of defects at low energy sites caused by the defect wave-function localization and the size-mismatch-induced strain effect. Because the bowing effect can drastically reduce the defect formation energy—even in dilute semiconductor alloys—it can have wide applications, such as alloy-enhanced defect solubility in semiconductors.

We investigate the dynamical and transport features of a Kondo dot side coupled to a topological superconductor (TS). The Majorana fermion states (MFSs) formed at the ends of the TS are found to be able to alter the Kondo physics profoundly: For an infinitely long wire where the MFSs do not overlap (${\epsilon }_m=0$) a finite dot-MFS coupling $\left({\Gamma }_m\right)$ reduces the unitary-limit value of the linear conductance by exactly a factor 3/4 in the weak-coupling regime $({\Gamma }_m<T_K)$, where $T_K$ is the Kondo temperature. In the strong-coupling regime (${\Gamma }_m>T_K)$, on the other hand, the spin-split Kondo resonance takes place due to the MFS-induced Zeeman splitting, which is a genuine many-body effect of the strong Coulomb interaction and the topological superconductivity. We find that the original Kondo resonance is fully restored once the MFSs are strongly hybridized (${\epsilon }_m>{\Gamma }_m$). This unusual interaction between the Kondo effect and the MFS can thus serve to detect the Majorana fermions unambiguously and quantify the degree of overlap between the MFSs in the TS.

We consider the reduced density matrix (RDM) ${\rho }_A\left(t\right)$ for a finite subsystem $A$ after a global quantum quench in the infinite transverse-field Ising chain. It has been recently shown that the infinite time limit of ${\rho }_A\left(t\right)$ is described by the RDM ${\rho }_{\mathrm{GGE},A}$ of a generalized Gibbs ensemble. Here, we present some details on how to construct this ensemble in terms of local integrals of motion, and show its equivalence to the expression in terms of mode occupation numbers widely used in the literature. We then address the question of how ${\rho }_A\left(t\right)$ approaches ${\rho }_{\mathrm{GGE},A}$ as a function of time. To that end, we introduce a distance on the space of density matrices and show that it approaches zero as a universal power law $t^{-3/2}$ in time. As the RDM completely determines all local observables within $A$, this provides information on the relaxation of correlation functions of local operators. We then address the issue of how well a truncated generalized Gibbs ensemble with a finite number of local higher conservation laws describes a given subsystem at late times. We find that taking into account only local conservation laws with a range at most comparable to the subsystem size provides a good description. However, excluding even a single one of the most local conservation laws in general completely spoils this agreement.

Quantitative analysis of the x-ray photoelectron spectra structure in the binding energy (BE) range of 0 eV–∼35 eV for plutonium dioxide (PuO${}_2$) valence electrons was done. The BEs and structure of the core electronic shells (35 eV–1250 eV BE), as well as the relativistic discrete variation calculation results for the finite fragments of the PuO${}_2$ lattice and the data of other authors, were taken into account. The experimental data show that the many-body effects and the multiplet splitting contribute to the spectral structure much less than the outer (0 eV–∼15 eV) and the inner (∼15 eV–∼35 eV) valence molecular orbitals (OVMO and IVMO, respectively). The filled Pu 5$f$ electronic states were shown to form in the PuO${}_2$ valence band. The Pu 6$p$ electrons participate in the formation of both the IVMO and the OVMO (bands). The filled Pu 6$p_{3/2}$ and the O 2$s$ electronic shells were found to take maximum part in the IVMO formation. The MO composition and the sequence order in the BE range of 0 eV–∼35 eV in PuO${}_2$ were established. The experimental and theoretical data allowed a quantitative MO scheme for PuO${}_2$, which is fundamental for understanding both the chemical bond nature in plutonium dioxide and the interpretation of other x-ray spectra of PuO${}_2$.

Ambient operation poses a challenge to atomic force microscopy because in contrast to operation in vacuum or liquid environments, the cantilever dynamics change dramatically from oscillating in air to oscillating in a hydration layer when probing the sample. We demonstrate atomic resolution by imaging of the KBr(001) surface in ambient conditions by frequency-modulation atomic force microscopy with a cantilever based on a quartz tuning fork (qPlus sensor) and analyze both long- and short-range contributions to the damping. The thickness of the hydration layer increases with relative humidity; thus varying humidity enables us to study the influence of the hydration layer thickness on cantilever damping. Starting with measurements of damping versus amplitude, we analyzed the signal and the noise characteristics at the atomic scale. We then determined the optimal amplitude which enabled us to acquire high-quality atomically resolved images.

We consider the random-phase sine-Gordon model in two dimensions. It describes two-dimensional elastic systems with random periodic disorder, such as pinned flux-line arrays, random field $XY$ models, and surfaces of disordered crystals. The model exhibits a super-rough glass phase at low temperature $T<T_c$ with relative displacements growing with distance $r$ as $\overline{\langle {[\theta \left(r\right)-\theta \left(0\right)]}^2\rangle }\simeq A\left(\tau \right){\ln }^2(r/a)$, where $A\left(\tau \right)=2{\tau }^2-2{\tau }^3+\mathcal{O}\left({\tau }^4\right)$ near the transition and $\tau =1-T/T_c$. We calculate all higher cumulants and show that they grow as ${\overline{\langle {[\theta \left(r\right)-\theta \left(0\right)]}^{2n}\rangle }}_c\simeq [2{(-1)}^{n+1}\left(2n\right)!\zeta (2n-1){\tau }^2+\mathcal{O}\left({\tau }^3\right)]\ln (r/a)$, $n\ge 2$, where $\zeta$ is the Riemann $\zeta$ function. By summation, we obtain the decay of the exponential correlation function as $\overline{\langle e^{iq\left[\theta \left(r\right)-\theta \left(0\right)\right]}\rangle }\simeq {(a/r)}^{\eta \left(q\right)}\exp (-\frac{1}{2}\mathcal{A}\left(q\right){\ln }^2(r/a))$, where $\eta \left(q\right)$ and $\mathcal{A}\left(q\right)$ are obtained for arbitrary $q\le 1$ to leading order in $\tau$. The anomalous exponent is $\eta \left(q\right)=c{q}^2-{\tau }^2{q}^2[2{\gamma }_E+\psi \left(q\right)+\psi (-q)]$ in terms of the digamma function $\psi$, where $c$ is nonuniversal and ${\gamma }_E$ is the Euler constant. The correlation function shows a faster decay at $q=1$, corresponding to fermion operators in the dual picture, which should be visible in Bragg scattering experiments.

Ultrafast magnetization dynamics induced by femtosecond laser pulses have been measured in ferrimagnetic Co${}_{0.8}$Gd${}_{0.2}$, Co${}_{0.74}$Tb${}_{0.26}$, and Co${}_{0.86}$Tb${}_{0.14}$ alloys. Using element sensitivity of x-ray magnetic circular dichroism at the Co $L_3$, Tb $M_5$, and Gd $M_5$ edges, we see that the demagnetization dynamics is element dependent. We show that a thermalization time as fast as 280 $\pm$ 30 fs is observed for the rare earth in the alloy when the excited-state temperature is below the compensation temperature. It is limited to 500 $\pm$ 100 fs when the excited-state temperature is below the Curie temperature ($T_C$). Therefore, for transition-metal rare-earth alloys, we propose that critical spin fluctuations in the vicinity of $T_C$ reduce the demagnetization rates of the 4$f$ electrons, whereas far from $T_C$ the limited demagnetization rates should be avoided.

We present scanning tunneling microscopy and spectroscopy studies around an individual excess Fe atom, working as a local perturbation, in the parent material of the iron-chalcogenide superconductor Fe${}_{1+\delta }$Te. Spectroscopic imaging reveals an isosceles triangular electronic structure around the excess Fe atoms. Its spatial symmetry reflects the underlying bicollinear antiferromagnetic spin state and the structural monoclinic symmetry. These findings provide important clues to understanding the role of the excess Fe atoms, which complicate the understanding of the phenomena occurring in iron-chalcogenide materials.

We report the structural and superconducting evolution of the solid solution of ${\mathrm{Ir}}_{0.95-x}{\mathrm{Rh}}_x{\mathrm{Te}}_2$ ($0\le x\le 0.52$) and compare the results with those of analogous selenides. The structural analysis revealed that the unit cell unexpectedly expands and the separation of the Te${}_2$${}^{-2}$ dimer exhibits a maximal value of around $x=0.2$ with increasing Rh content. However, unlike the domed $T_c$ in ${\mathrm{Ir}}_{0.94-x}{\mathrm{Rh}}_x{\mathrm{Te}}_2$, the present system exhibits a monotonous decrease in $T_c$, which is independent of the instability of the Te${}_2$${}^{-2}$ dimer. As the analysis of resistivity and heat capacity, it is suggested that electron-electron/phonon correlation strength is a considerable factor leading to the different evolution of $T_c$ in the vicinity of the structural instability state of pyrite superconductors.

In this paper, we report an adaptation of the Harbola-Sahni (HS) exchange potential to the tight-binding linear muffin-tin orbital (TB-LMTO) method to determine band gaps (BGs) of solids accurately. We show that the electrostatic basis of derivation of the Harbola-Sahni potential allows this nonvariational approach to improve substantially over local-density approximation derived BGs, bringing them very close to experimental values. That the accuracy of the HS potential is directly responsible for the determination of correct BGs is demonstrated by performing similar calculations with an accurate model potential that too leads to BGs close to their experimental values. Moreover, ground-state properties like equilibrium lattice parameters and bulk moduli (BM) for various semiconductors like C, Si, AlN, AlP, BP, and 3C-SiC calculated with the HS approach are in close agreement with the experiments. The clear physical interpretation of HS potential leads us to suggest exploring its use for calculating various properties of solids.

Superconducting transition under pressure ($P$) has been investigated for optimum-doped $R$FeAsO${}_{1-x}$F${}_x$ ($R=\text{La}$, Ce–Sm) by dc magnetization measurements. For $R=\text{La}$, $T_c$ is found to be pressure independent up to $P\sim 3.0$ GPa and then shows a monotonic decrease with increasing pressure. The plateau width decreases for the system with smaller lattice constants and shrinks to almost zero for $R=\text{Sm}$. From the $T_c$($P$) data, we construct the $T_c$ evolution map on the height of the As atom from the Fe-plane $h_{\mathrm{As}}$ versus lattice-constant $a$ or $c$ plane. It is shown that the characteristic $T_c$ variations under pressure for all compounds and the rapid decrease in $T_c$ induced by changing the $R$ element from Sm to La can be described by a common $T_c$ ($h_{\mathrm{As}}$, $a$ or $c$) surface, suggesting that $T_c$ is determined by $h_{\mathrm{As}}$ and lattice constant. It is also suggested that there exists an upper limit of the lattice constant $a_{\mathrm{ulm}}$ (or $c_{\mathrm{ulm}}$), above which $d{T}_c$$/$$da$ (or $d{T}_c$$/$$dc$) changes the sign from positive to negative.