We report on the mass acquisition of Dirac fermions by doping Cr into the topmost quintuple layer or into the bulk of $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ topological insulator films. By careful investigation of the scanning tunneling microscopy/spectroscopy on the films, we find that the Landau level spectrum keeps a good quality even at a high Cr-doping level, enabling a demonstration of deviation of the zeroth Landau level, induced by the acquisition of a mass term in the surface states in the presence of surface or bulk magnetic doping. The magnitude of the mass term in the surface states increases with increasing Cr-doping level. Our observation suggests Cr-doped $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ is a promising candidate for the realization of the proposed magnetoelectric effects.

A prediction of how to experimentally distinguish excitations of extrinsic plasmarons from intrinsic plasmarons is presented. In surface systems where excitations of acoustic surface plasmons are possible, it is shown that the photoelectron yield in normal photoemission should decay according to an inverse square-root dependence with respect to the photon energy. A computational analysis of the system $p(2\times 2)$-K/graphite confirms this prediction.

We study transport properties of a quantum dot coupled to a Majorana zero mode and two normal leads. We investigate the full counting statistics of charge tunneling events and obtain complete information on current fluctuations through the dot. Using the Keldysh path-integral approach, we compute the cumulant generating function of the current. We first consider a spinless case and find that for the symmetric dot-lead couplings, the zero-frequency cumulants are independent of the microscopic parameters and exhibit a universal pattern described by Euler numbers. We then consider the spinful system and investigate the effect of both weak and strong Coulomb interactions. We show that cases with and without Majorana coupling exhibit qualitatively different full counting statistics of charge tunneling events despite the fact that differential linear conductance might have zero-bias features in both cases.

We derive a general theory of effective properties in metasolids based on phononic crystals with low frequency resonances. We demonstrate that in general these structures need to be described by means of a frequency-dependent and nonlocal anisotropic mass density, stiffness tensor and a third-rank coupling tensor, which shows that they behave like a nonlocal Willis medium. The effect of nonlocality and coupling tensor manifest themselves for some particular resonances, whereas they become negligible for other resonances. Considering the example of a two-dimensional phononic crystal, consisting of triangular arrangements of cylindrical shells in an elastic matrix, we show that its mass density tensor is strongly resonant and anisotropic presenting both positive and negative divergent values, while becoming scalar in the quasistatic limit. Moreover, it is found that the negative value of transverse component of the mass density is induced by a dipolar resonance, while that of the vertical component is induced by a monopolar one. Finally, the dispersion relation obtained by the effective parameters of the crystal is compared with the band structure, showing good agreement for the low-wave-number region, although the nonlocal effects are important given the existence of some resonant values of the wave number.

The equilibrium structure and functional properties exhibited by brownmillerite oxides, a family of perovskite-derived structures with alternating layers of $B{\mathrm{O}}_6$ octahedra and $B{\mathrm{O}}_4$ tetrahedra, viz., ordered arrangements of oxygen vacancies, is dependent on a variety of competing crystal-chemistry factors. We use electronic structure calculations to disentangle the complex interactions in two ferrates, ${\mathrm{Sr}}_2{\mathrm{Fe}}_2{\mathrm{O}}_5$ and ${\mathrm{Ca}}_2{\mathrm{Fe}}_2{\mathrm{O}}_5$, relating the stability of the equilibrium (strain-free) and thin film structures to both previously identified and herein newly proposed descriptors. We show that cation size and intralayer separation of the tetrahedral chains provide key contributions to the preferred ground state. We show the bulk ground-state structure is retained in the ferrates over a range of strain values; however, a change in the orientation of the tetrahedral chains, i.e., a perpendicular orientation of the vacancies relative to the substrate, is stabilized in the compressive region. The structure stability under strain is largely governed by maximizing the intraplane separation of the dipoles generated from rotations of the ${\mathrm{FeO}}_4$ tetrahedra. Lastly, we find that the electronic band gap is strongly influenced by strain, manifesting as an unanticipated asymmetric-vacancy alignment dependent response. This atomistic understanding establishes a practical route for the design of functional electronic materials in thin film geometries.

We study the element-specific magnetic depth profiles of a ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7/{\text{La}}_{2/3}{\text{Ca}}_{1/3}{\text{MnO}}_3$ (YBCO/LCMO) superlattice using soft x-ray resonant magnetic reflectometry (XRMR). This allows us to study the magnetic proximity effect (MPE) that is observed at the YBCO/LCMO interface, characterized by the occurrence of a depleted layer on the manganite side and an induced magnetization in the YBCO. Using the element-specific depth profiling capability of XRMR, we show that the Cu moments $(0.28{\mu }_B$ per interfacial Cu ion) reside on the YBCO side of the interface and originate from the $\mathrm{Cu}{\mathrm{O}}_2$ plane that is located at the interface. We also rule out the possibility that the Cu moments originate from the migration of Cu atoms into the LCMO layer (interdiffusion). Finally, we show that the suppression of the Mn moment on the LCMO side is only partial for the interface of LCMO deposited on YBCO $(0.62{\mu }_B$ per Mn ion), compared to the complete suppression for the interface of YBCO deposited on LCMO. These differences in the LCMO depleted layers indicate the strong variations in the MPE between the case of YBCO on LCMO and the case of LCMO on YBCO.

We present a detailed study of the gap symmetry and the quasiparticle wave function topology in two-dimensional superconductors without inversion center. The strong spin-orbit coupling of electrons with the crystal lattice makes it necessary to describe superconductivity in terms of one or more nondegenerate bands characterized by helicity. We develop a topological classification of the superconducting states using the integer-valued Maurer-Cartan invariants and the Bogoliubov Wilson loops, and also calculate the spectrum of fermionic boundary modes.

Current crowding in the bends of superconducting nanostructures not only restricts measurable critical current in such structures, but also redistributes local probabilities for the appearance of dark and light counts. Using structures in the form of a square spiral, where all bends have the same symmetry with respect to the directions of the bias current and external magnetic field, we have shown that areas around the bends largely contribute to the rate of dark counts and to the rate of light counts at small photon energies. The minimum in the rate of dark counts reproduces the asymmetry of the maximum in the critical current as a function of the magnetic field. Contrarily, the minimum in the rate of light counts demonstrates opposite asymmetry. The rate of light counts becomes symmetric at large currents and fields. Comparison of the computed local absorption probabilities for photons and the simulated local threshold detection current reveal the areas near bends that deliver the asymmetric rate of light counts. Asymmetry in count rates is absent in circular spirals without bends.

We study by numerical methods the phase dynamics in ballistic graphene-based short Josephson junctions. A superconductor-graphene-superconductor system exhibits superconductive quantum metastable states similar to those present in normal current-biased Josephson junctions. We investigate the effects of thermal and correlated fluctuations on the escape time from these metastable states, when the system is driven by an oscillating bias current in the presence of Gaussian white and colored noise sources. Varying the intensity and the correlation time of the noise source, it is possible to analyze the behavior of the escape time, or switching time, from a superconductive metastable state in different temperature regimes. Moreover, we are able to clearly distinguish dynamical regimes characterized by the dynamic resonant activation effect, in the absence of noise source, and the stochastic resonant activation phenomenon induced by the noise. For low initial values of the bias current, the dynamic resonant activation shows double-minimum structures, strongly dependent on the value of the damping parameter. Noise-enhanced stability is also observed in the system investigated. We analyze the probability density function (PDF) of the switching times. The PDFs for frequencies within the dynamic resonant activation minima are characterized by single peaks with exponential tails. The PDFs for noise intensities around the maxima of the switching time, peculiarity of the noise-enhanced stability phenomenon, are composed of regular sequences of two peaks for each period of the driving current, with exponentially decaying envelopes.

${\mathrm{Sc}}_2{\mathrm{Ga}}_2{\mathrm{CuO}}_7$ (SGCO) crystallizes in a hexagonal structure (space group: $P{6}_3/mmc)$, which can be seen as an alternating stacking of single and double triangular layers. Combining neutron, x-ray, and resonant x-ray diffraction, we establish that the single triangular layers are mainly populated by nonmagnetic ${\mathrm{Ga}}^{3+}$ ions (85% Ga and 15% Cu), while the bilayers have comparable population of ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Ga}}^{3+}$ ions (43% Cu and 57% Ga). Our susceptibility measurements in the temperature range 1.8–400 K give no indication of any spin-freezing or magnetic long-range order (LRO). We infer an effective paramagnetic moment ${\mu }_{\mathrm{eff}}=1.79\pm 0.09{\mu }_B$ and a Curie-Weiss temperature ${\theta }_{\mathrm{CW}}$ of about $-44$ K, suggesting antiferromagnetic interactions between the ${\mathrm{Cu}}^{2+}(S=1/2)$ ions. Low-temperature neutron powder diffraction data showed no evidence for LRO down to 1.5 K. In our specific heat data as well, no anomalies were found down to 0.35 K, in the field range 0–140 kOe. The magnetic specific heat $C_m$, exhibits a broad maximum at around 2.5 K followed by a nearly power law $C_m\propto T^{\alpha }$ behavior at lower temperatures, with $\alpha$ increasing from 0.3 to 1.9 as a function of field for fields up to 90 kOe and then remaining at 1.9 for fields up to 140 kOe. Our results point to a disordered ground state in SGCO.

We present a multiphase-field theory for describing pattern formation in multidomain and/or multicomponent systems. The construction of the free energy functional and the dynamic equations is based on criteria that ensure mathematical and physical consistency. We first analyze previous multiphase-field theories and identify their advantageous and disadvantageous features. On the basis of this analysis, we introduce a way of constructing the free energy surface and derive a generalized multiphase description for arbitrary number of phases (or domains). The presented approach retains the variational formalism, reduces (or extends) naturally to lower (or higher) number of fields on the level of both the free energy functional and the dynamic equations, enables the use of arbitrary pairwise equilibrium interfacial properties, penalizes multiple junctions increasingly with the number of phases, ensures non-negative entropy production and the convergence of the dynamic solutions to the equilibrium solutions, and avoids the appearance of spurious phases on binary interfaces. The approach is tested for multicomponent phase separation and grain coarsening.

The classical cubic dimer model has a columnar ordering transition that is continuous and described by a critical Anderson-Higgs theory containing an $\mathrm{SU}\left(2\right)$-symmetric complex field minimally coupled to a noncompact $\mathrm{U}\left(1\right)$ gauge theory. Defects in the dimer constraints correspond to monopoles of the gauge theory, with charge determined by the deviation from unity of the dimer occupancy. By introducing such defects into Monte Carlo simulations of the dimer model at its critical point, we determine the scaling dimensions $y_2=1.48\pm 0.07$ and $y_3=0.20\pm 0.03$ for the operators corresponding to defects of charge $q=2$ and 3, respectively. These results, which constitute the first direct determination of the scaling dimensions, shed light on the deconfined critical point of spin-$\frac{1}{2}$ quantum antiferromagnets, thought to belong to the same universality class. In particular, the positive value of $y_3$ implies that the transition in the $JQ$ model on the honeycomb lattice is of first order.

We report x-ray total scattering and pair distribution function (PDF) studies of the structural relaxation around interstitial manganese (${\mathrm{Mn}}_i$) in ferromagnetic ${\mathrm{Mn}}_{1+\delta }\mathrm{Sb}$ ($0.03\le \delta \le 0.23$) alloys, guided by density functional theory (DFT). Refinements to the experimental PDF using a crystallographically constrained structural model indicate an expansion in the equatorial plane of the ${\mathrm{Mn}}_i{\mathrm{Sb}}_5$ trigonal bipyramidal site, which introduces significant positional disorder in addition to the nominally random occupation of interstitial voids. Observation of a weak diffuse signal near the symmetry-forbidden (001) reflection position is indicative of correlated disorder from the clustering of ${\mathrm{Mn}}_i$. Density functional relaxation of supercells approximating the $\delta =0.08$, 0.15, and 0.23 compositions provides improved models that accurately describe the short-range structural distortions captured in the PDFs. Such structural relaxation increases the DFT calculated moment on ${\mathrm{Mn}}_i$, which aligns antiparallel to the primary Mn moments, but leads to insubstantial changes in the average Mn and Sb moments and moments of Mn and Sb proximal to interstitials, thus providing a more accurate description of the observed bulk magnetic properties.

We present a general first-principles approach to treat various linear response phenomena relevant for spintronics. It is based on a Kubo-Bastin formalism and implemented within the multiple-scattering Korringa-Kohn-Rostoker (KKR) Green's function method with the underlying electronic structure determined by density functional theory. The symmetric (e.g., longitudinal electronic transport) as well as the antisymmetric (e.g., transverse transport) parts of the response tensor are determined, including both the so-called Fermi-sea and the Fermi-surface contributions. To describe spin-orbit-induced phenomena, such as the anomalous and spin Hall effects, a fully relativistic description is employed. Exploiting the adopted Green's function method substitutional disorder in the full concentration range of alloys is treated within the coherent potential approximation, taking full account of occurring vertex corrections in the averaging procedure for the linear response quantities. Extrinsic (scattering related, e.g., side-jump and skew scattering) and intrinsic (band structure–related) contributions to the transport tensors are treated on equal footing. Other phenomena, such as Gilbert damping and spin-orbit torques, are particular cases of the general framework and their determination is briefly addressed. The versatility of the method is demonstrated by presenting results for the anomalous and spin Hall conductivities for elemental transition metals and their alloys.

We identify and discuss the ground state of a quantum magnet on a triangular lattice with bond-dependent Ising-type spin couplings, that is, a triangular analog of the Kitaev honeycomb model. The classical ground-state manifold of the model is spanned by decoupled Ising-type chains, and its accidental degeneracy is due to the frustrated nature of the anisotropic spin couplings. We show how this subextensive degeneracy is lifted by a quantum order-by-disorder mechanism and study the quantum selection of the ground state by treating short-wavelength fluctuations within the linked cluster expansion and by using the complementary spin-wave theory. We find that quantum fluctuations couple next-nearest-neighbor chains through an emergent four-spin interaction, while nearest-neighbor chains remain decoupled. The remaining discrete degeneracy of the ground state is shown to be protected by a hidden symmetry of the model.

The damping $\alpha$ of the spin-wave resonances in 75, 120, and $200\mathrm{n}\mathrm{m}$ thick permalloy films is measured via vector-network-analyzer ferromagnetic resonance (VNA-FMR) in the out-of-plane geometry. Inductive coupling between the sample and the waveguide leads to an additional radiative damping term. The radiative contribution to the over-all damping is determined by measuring perpendicular standing spin waves (PSSWs) in the permalloy films, and the results are compared to a simple analytical model. The damping of the PSSWs can be fully explained by three contributions to the damping: the intrinsic damping, the eddy-current damping, and the radiative damping. It was not necessary to invoke any additional damping contributions to explain the data. Furthermore, a method to determine the radiative damping in FMR measurements with a single resonance is suggested.

We calculate angle-dependent magnetoresistance oscillations (AMRO) for interlayer transport of cuprate superconductors in the presence of ($\pi ,\pi$) order. The order reconstructs the Fermi surface, creating magnetic breakdown junctions; we show how such magnetic breakdown effects can be incorporated into calculations of interlayer conductivity for this system. We successfully fit experimental data from an overdoped cuprate using our model, showing that behavior previously attributed to anisotropic scattering in this material may in fact be due to ($\pi ,\pi$) ordering. This work paves the way for the use of AMRO as a tool to distinguish ordered states that have different ordering wave vectors.

We use the two fluid model to determine the conditions under which the nuclear spin-lattice lattice relaxation rate $T_1$ of candidate heavy quantum critical superconductors can exhibit scaling behavior and find that it can occur if and only if their “hidden” quantum critical spin fluctuations give rise to a temperature-independent intrinsic heavy electron spin-lattice relaxation rate. The resulting scaling of $T_1$ with the strength of the heavy electron component and the coherence temperature $T^{*}$ provides a simple test for their presence at pressures at which the superconducting transition temperature $T_c$ is maximum and is proportional to $T^{*}$. These findings support the previously noted partial scaling of the spin-lattice relaxation rate with $T_c$ in a number of important heavy electron materials and provide additional evidence that in these materials their optimal superconductivity originates in the quantum critical spin fluctuations associated with a nearby phase transition from partially localized to fully itinerant quasiparticles.

We present an ab initio analysis of a continuous Hamiltonian that maps into the celebrated Haldane model. The tunneling coefficients of the tight-binding model are computed by means of two independent methods—one based on the maximally localized Wannier functions, and the other through analytic expressions in terms of gauge-invariant properties of the spectrum—that provide a remarkable agreement and allow one to accurately reproduce the exact spectrum of the continuous Hamiltonian. By combining these results with the numerical calculation of the Chern number, we are able to draw the phase diagram in terms of the physical parameters of the microscopic model. Remarkably, we find that only a small fraction of the original phase diagram of the Haldane model can be accessed, and that the topological insulator phase is suppressed in the deep tight-binding regime.

Tuning the spin-orbit coupling strength via foreign element doping and modifying bonding strength via strain engineering are the major routes to convert normal insulators to topological insulators. We here propose an alternative strategy to realize topological phase transition by tuning the orbital level. Following this strategy, our first-principles calculations demonstrate that a topological phase transition in the cubic perovskite-type compounds ${\mathrm{CsGeBr}}_3$ and ${\mathrm{CsSnBr}}_3$ could be facilitated by carbon substitutional doping. Such a unique topological phase transition predominantly results from the lower orbital energy of the carbon dopant, which can pull down the conduction bands and even induce band inversion. Beyond conventional approaches, our finding of tuning the orbital level may greatly expand the range of topologically nontrivial materials.