We report an attractive critical point occurring in the Anderson localization scaling flow of symplectic models on fractals. The scaling theory of Anderson localization predicts that in disordered symplectic two-dimensional systems weak-antilocalization effects lead to a metal-insulator transition. This transition is characterized by a repulsive critical point above which the system becomes metallic. Fractals possess a noninteger scaling of conductance in the classical limit which can be continuously tuned by changing the fractal structure. We demonstrate that in disordered symplectic Hamiltonians defined on fractals with classical conductance scaling $g\sim L^{-\varepsilon }$, for $0<\varepsilon <{\beta }_{\max }\approx 0.15$, the metallic phase is replaced by a critical phase with a scale-invariant conductance dependent on the fractal dimensionality. Our results show that disordered fractals allow an explicit construction and verification of the $\varepsilon$ expansion.

We report magnetoresistance measurements of hydrogen-terminated (100)-oriented diamond surfaces wherein an ionic-liquid-gated field-effect-transistor technique was used to make hole carriers accumulate. Unexpectedly, the observed magnetoresistance is positive within the range of $2<T<10$ K and $-7<B<7$ T, in striking contrast to the negative magnetoresistance previously detected for similar devices with (111)-oriented diamond surfaces. Furthermore, we find that (1) the magnetoresistance is orders of magnitude larger than that of the classical orbital magnetoresistance; (2) the magnetoresistance is nearly independent of the direction of the applied magnetic field; and (3) for the in-plane field, the magnetoresistance ratio, defined as $\left[\rho \right(B)-\rho (0\left)\right]/\rho \left(0\right)$, follows a universal function of $B/T$. These results indicate that the spin degree of freedom of hole carriers plays an important role in the surface conductivity of hydrogen-terminated (100) diamond.

We analyze within quasiparticle theory a recent resonant inelastic x-ray scattering (RIXS) experiment on ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$ with the incoming photon energy detuned at several values from the resonance maximum [Minola et al., Phys. Rev. Lett. 114, 217003 (2015)]. Surprisingly, the data show a much weaker dependence on detuning than expected from recent measurements on a different cuprate superconductor, ${\text{Bi}}_2{\text{Sr}}_2{\text{CuO}}_{6+x}$ [Guarise et al., Nat. Commun. 5, 5760 (2014)]. We demonstrate here that this discrepancy, originally attributed to collective magnetic excitations, can be understood in terms of the differences between the band structures of these materials. We find good agreement between theory and experiment over a large range of dopings, both in the underdoped and overdoped regimes. Moreover, we demonstrate that the RIXS signal depends sensitively on excitations at energies well above the Fermi surface that are inaccessible to traditionally used band structure probes, such as angle-resolved photoemission spectroscopy. This makes RIXS a powerful probe of band structure, not suffering from surface preparation problems and small sample sizes, making it potentially applicable to a number of cuprate materials.

Numerical studies by Wójs, Yi, and Quinn have suggested that an unconventional fractional quantum Hall effect is plausible at filling factors $\nu =1/3$ and 1/5, provided the interparticle interaction has an unusual form for which the energy of two fermions in the relative angular momentum three channel dominates. The interaction between composite fermions in the second $\mathrm{\Lambda }$ level (composite fermion analog of the electronic Landau level) satisfies this property, and recent studies have supported unconventional fractional quantum Hall effect of composite fermions at ${\nu }^{*}=4/3$ and $5/3$, which manifests as fractional quantum Hall effect of electrons at $\nu =4/11$, 4/13, 5/13, and 5/17. I investigate in this article the nature of the fractional quantum Hall states at $\nu =4/5$, 5/7, 6/17, and 6/7, which correspond to composite fermions at ${\nu }^{*}=4/3$, 5/3, and 6/5, and find that all these fractional quantum Hall states are conventional. The underlying reason is that the interaction between composite fermions depends substantially on both the number and the direction of the vortices attached to the electrons. I also study in detail the states with different spin polarizations at 6/17 and 6/7 and predict the critical Zeeman energies for the spin phase transitions between them. I calculate the excitation gaps at 4/5, 5/7, 6/7, and 6/17 and compare them against recent experiments.

We investigate the behavior of qubits consisting of three electron spins in double and triple quantum dots subject to external electric fields. Our model includes two independent bias parameters, $\varepsilon$ and ${\varepsilon }_M$, which both couple to external electromagnetic fields and can be controlled by gate voltages applied to the quantum dot structures. By varying these parameters, one can switch the qubit type by shifting the energies in the single quantum dots, thus changing the electron occupancy in each dot. Starting from the asymmetric resonant exchange qubit with a (2,0,1) and (1,0,2) charge admixture, one can smoothly cross over to the resonant exchange qubit with a detuned (1,1,1) charge configuration, and to the exchange-only qubit with the same charge configuration but equal energy levels down to the hybrid qubits with (1,2,0) and (0,2,1) charge configurations. Here, $(l,m,n)$ describes a configuration with $l$ electrons in the left dot, $m$ electrons in the center dot, and $n$ electrons in the right dot. We first focus on random electromagnetic field fluctuations, i.e., “charge noise,” at each quantum dot resulting in dephasing of the qubit, and we provide a complete map of the resulting dephasing time as a function of the bias parameters. We pay special attention to the so-called sweet spots and double sweet spots of the system, which are least susceptible to noise. In the second part, we investigate the coupling of the qubit system to the coherent quantized electromagnetic field in a superconducting strip-line cavity, and we also provide a complete map of the coupling strength as a function of the bias parameters. We analyze the asymmetric qubit-cavity coupling via $\varepsilon$ and the symmetric coupling via ${\varepsilon }_M$.

We studied the migration behavior of mixed tilt and twist grain boundaries in the vicinity of a symmetric tilt $\langle 111\rangle \mathrm{\Sigma }7$ grain boundary in aluminum. We show that these grain boundaries fall into two main categories of stepped and kinked grain boundaries around the atomically flat symmetric tilt boundary. Using these structures together with size converged molecular dynamics simulations and investigating snapshots of the boundaries during migration, we obtain an intuitive and quantitative description of the kinetic and atomistic mechanisms of the migration of general mixed grain boundaries. This description is closely related to well-known concepts in surface growth such as step and kink-flow mechanisms and allows us to derive analytical kinetic models that explain the dependence of the migration barrier on the driving force. Using this insight we are able to extract energy barrier data for the experimentally relevant case of vanishing driving forces that are not accessible from direct molecular dynamics simulations and to classify arbitrary boundaries based on their mesoscopic structures.

Recently proposed classical analogs of topological insulators in phononic lattices have the advantage of much more accessible experimental realization compared to conventional materials. Drawn to their potential practical structural applications, we investigate how disorder, which is generically nonnegligible in macroscopic realization, can attenuate the topologically protected edge (TPE) modes that constitute robust transmitting channels at zero disorder. We simulate the transmission of phonon modes in a quasi-one-dimensional classical lattice waveguide with mass disorder and show that the TPE mode transmission remains highly robust ($\mathrm{\Xi }\sim 1$) in the presence of uncorrelated disorder but diminishes when disorder is spatially correlated. This reduction in transmittance is attributed to the Anderson localization of states within the mass disorder domains. By contrast, non-TPE channels exhibit qualitatively different behavior, with spatial correlation in the mass disorder leading to significant transmittance reduction (enhancement) at low (high) frequencies. Our results demonstrate how TPE modes drastically modify the effect of spatial correlation on mode localization.

We numerically study the dynamics of entanglement entropy, induced by an oscillating time periodic driving of the transverse field, $h\left(t\right)$, of a one-dimensional quantum Ising ring. We consider several realizations of $h\left(t\right)$, and we find a number of results in analogy with entanglement entropy dynamics induced by a sudden quantum quench. After a short-time relaxation, the dynamics of entanglement entropy synchronizes with $h\left(t\right)$, displaying an oscillatory behavior at the frequency of the driving. Synchronization in the dynamics of entanglement entropy is spoiled by the appearance of quasirevivals which fade out in the thermodynamic limit, and which we interpret using a quasiparticle picture adapted to periodic drivings. We show that the time-averaged entanglement entropy in the synchronized regime obeys a volume law scaling with the subsystem's size. Such result is reminiscent of a thermal state or a generalized Gibbs ensemble, although the system does not heat up towards infinite temperature as a consequence of the integrability of the model.

It is demonstrated that the magnetic diffraction pattern of the isotropic disordered maze pattern is well described utilizing a gamma distribution of domain sizes in a one-dimensional model. From the analysis, the mean domain size and the shape parameter of the distribution are obtained. The model reveals an average domain size that is significantly different from the value that is determined from the peak position of the structure factor in reciprocal space. As a proof of principle, a wedge-shaped $({\mathrm{Co}}_{t\AA }/{\mathrm{Pd}}_{10\AA }{)}_8$ multilayer film, that covers the thickness range of the spin-reorientation transition, has been used. By means of soft x-ray resonant magnetic scattering (XRMS) and imaging techniques the thickness-driven evolution of the magnetic properties of the cobalt layers is explored. It is shown that minute changes of the domain pattern concerning domain size and geometry can be investigated and analyzed due to the high sensitivity and lateral resolution of the XRMS technique. The latter allows for the determination of the magnetic anisotropies of the cobalt layers within a thickness range of a few angstroms.

We have investigated the relation between structural transition with Ir-dimer formation and superconductivity in ${\mathrm{IrTe}}_2$ by combining the Se substitution and the Cu intercalation. Regardless of the structural transition temperature ($T_{\mathrm{s}}$), which increases with increasing the Se content ($y$) in ${\mathrm{IrTe}}_{2-y}{\mathrm{Se}}_y$, superconductivity emerges robustly by the Cu intercalation. As the Cu content $x$ in ${\mathrm{Cu}}_x{\mathrm{IrTe}}_{2-y}{\mathrm{Se}}_y$ increases, $T_{\mathrm{s}}$ tends to decrease, followed by the emergence of superconductivity with showing the highest critical temperature ($T_{\mathrm{c}}$) at the optimum Cu concentration ($x_{\mathrm{opt}}$) close to the structural phase boundary. Based on the transport and thermodynamic properties, the electron-phonon coupling constant is found to be enhanced near the structural phase boundary, which suggests an essential role of the structural instability for the superconductivity in doped ${\mathrm{IrTe}}_2$. With increasing $y$ from 0 to 0.5 in ${\mathrm{Cu}}_x{\mathrm{IrTe}}_{2-y}{\mathrm{Se}}_y, T_{\mathrm{s}}$ at $x=0$ increases by about 80%, whereas $T_{\mathrm{c}}$ at $x=x_{\mathrm{opt}}$ decreases by about 20%. This can be understood by the weakening of the interlayer hybridization upon the Se substitution, resulting in the weak but negative correlation between $T_{\mathrm{s}}$ and $T_{\mathrm{c}}$ through $y$.

We calculate the Drude weight in the superfluid (SF) and supersolid (SS) phases of the hard-core boson (HCB) model on a square lattice using stochastic series expansion (SSE). We demonstrate from our numerical calculations that the normal phase of HCBs in two dimensions can be an ideal conductor with dissipationless transport. In two dimensions, when the ground state is a SF, the superfluid stiffness drops to zero with a Kosterlitz-Thouless type transition at $T_{\mathrm{KT}}$. The Drude weight, though is equal to the stiffness below $T_{\mathrm{KT}}$, surprisingly, stays finite even for a range of temperatures above $T_{\mathrm{KT}}$ indicating the nondissipative transport in the normal state of this system. In contrast to this, in a three-dimensional SF phase, where the superfluid stiffness goes to zero continuously via a second-order phase transition at $T_c$, the Drude weight goes to zero at $T_c$, as expected. We also calculated the Drude weight in a two-dimensional SS phase, where the charge density wave (CDW) order coexists with superfluidity. For the SS phase we studied, superfluidity is lost via a Kosterlitz-Thouless transition at $T_{\mathrm{KT}}$ and the transition temperature for the CDW order is larger than $T_{\mathrm{KT}}$. In striped SS phase where the CDW order breaks the rotational symmetry of the lattice, the system behaves like an ideal conductor for a range of temperatures above $T_{\mathrm{KT}}$ along the lattice direction parallel to the stripes, while along the direction perpendicular to the stripes it behaves like an insulator for all $T>T_{\mathrm{KT}}$. In contrast to this, in the star-SS phase, the Drude weight along both lattice directions goes to zero along with the superfluid stiffness and for $T>T_{\mathrm{KT}}$ we have the finite temperature phase of a CDW insulator.

We have determined the superconducting and magnetic properties of a hydrothermally synthesized powder sample of tetragonal FeS using muon spin rotation ($\mu \mathrm{SR})$. The superconducting properties are entirely consistent with those of a recently published study, showing fully gapped behavior and giving a penetration depth of ${\lambda }_{ab}=204\left(3\right)$ nm. However, our zero-field $\mu \mathrm{SR}$ data are rather different and indicate the presence of a small, nonsuperconducting magnetic phase within the sample. These results highlight that sample-to-sample variations in magnetism can arise in hydrothermally prepared phases, but interestingly the superconducting behavior is remarkably insensitive to these variations.

We investigate the thermodynamic and transport properties of silver-substituted $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ (122) crystals up to $\sim 4.5\%$. Similar to other transition-metal substitutions in 122, Ag diminishes the antiferromagnetic $\left(T_N\right)$ and structural $\left(T_S\right)$ transition temperatures, but unlike other electron-doped 122s, $T_N$ and $T_S$ coincide without splitting. Although magnetism drops precipitously to $T_N=84\mathrm{K}$ at doping $x=0.029$, it only weakly changes above this $x$, settling at $T_N=80\mathrm{K}$ at $x=0.045$. Compared to this persistent magnetism in Ag-122, doping other group 11 elements of either Cu or Au in 122 diminished $T_N$ and induced superconductivity near $T_c=2\mathrm{K}$ at $x=0.044$ or 0.031, respectively. Ag-122 crystals show reflective surfaces with surprising thicker cross sections for $x\ge 0.019$, the appearance that is in contrast to the typical thin stacked layered feature seen in all other flux-grown x122 and lower Ag-122. This physical trait may be a manifest of intrinsic weak changes in $c$ lattice and $T_N$. Our theoretical calculations suggest that Ag doping produces strong electronic scattering and yet a relatively small disruption of the magnetic state, both of which preclude superconductivity in this system.

A classical way for exploring the scattering behavior of a small sphere is to approximate Mie coefficients with a Taylor series expansion. This ansatz delivered a plethora of insightful results, mostly for small spheres supporting electric localized plasmonic resonances. However, many scattering aspects are still uncharted, especially with regards to magnetic resonances. Here, an alternative system ansatz is proposed based on the Padé approximants for the Mie coefficients. The results reveal the existence of a self-regulating radiative damping mechanism for the first magnetic resonance and general resonating aspects for the higher order multipoles. Hence, a systematic way of exploring the scattering response is introduced, sharpening our understanding of the sphere's scattering behavior and its emergent functionalities.

In this work, we illustrate the detrimental impact of the Cooper pair's spin structure on the thermodynamic and topological properties of a spin-triplet superconductor in an applied Zeeman field. We particularly focus on the paradigmatic one-dimensional case (Kitaev chain) for which we self-consistently retrieve the energetically preferred Cooper pair spin state in terms of the corresponding spin $\mathit{d}$ vector. The latter undergoes a substantial angular and amplitude reorganization upon the variation of the strength and the orientation of the field and results to a modification of the bulk topological phase diagram. Markedly, when addressing the open chain we find that the orientation of the $\mathit{d}$ vector varies spatially near the boundary, affecting in this manner the appearance of Majorana fermions at the edge or even altering the properties of the bulk region. Our analysis reveals the limitations and breakdown of the bulk-boundary correspondence in interacting topological systems.

The spin-$\frac{1}{2}$ Heisenberg orthogonal-dimer chain is considered within the perturbative strong-coupling approach, which is developed from the exactly solved spin-$\frac{1}{2}$ Ising-Heisenberg orthogonal-dimer chain with the Heisenberg intradimer and the Ising interdimer couplings. Although the spin-$\frac{1}{2}$ Ising-Heisenberg orthogonal-dimer chain exhibits just intermediate plateaus at zero, one-quarter, and one-half of the saturation magnetization, the perturbative treatment up to second order stemming from this exactly solvable model additionally corroborates the fractional one-third plateau as well as the gapless Luttinger spin-liquid phase. It is evidenced that the approximate results obtained from the strong-coupling approach are in an excellent agreement with the state-of-the-art numerical data obtained for the spin-$\frac{1}{2}$ Heisenberg orthogonal-dimer chain within the exact diagonalization and density-matrix renormalization group method. The nature of individual quantum ground states is comprehensively studied within the developed perturbation theory.

We explore the magnetic behavior of the kagome francisites ${\mathrm{Cu}}_3\mathrm{Bi}\left({\mathrm{SeO}}_3{)}_2{\mathrm{O}}_{2}X\right(X=\mathrm{Cl},\mathrm{Br})$ by using first-principles electronic structure calculations. To this end, we propose an approach based on the effective Hubbard model in the Wannier functions basis constructed on the level of local-density approximation. The ground-state spin configuration is determined by a mean-field Hartree-Fock solution of the Hubbard model both in zero magnetic field and in applied magnetic fields. Additionally, parameters of an effective spin Hamiltonian are obtained by taking into account hybridization effects and spin-orbit coupling. We show that only the former approach based on the Hartree-Fock approximation allows for a complete description of the anisotropic magnetization process. While our calculations confirm that the canted zero-field ground state arises from a competition between ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor couplings in the kagome planes, weaker anisotropic terms are crucial for fixing spin directions and for the strong anisotropy of the magnetization. We show that the Hartree-Fock solution of an electronic Hamiltonian is a viable alternative to the analysis of effective spin Hamiltonians when magnetic ground states and their evolution in external field are concerned.