Quasiparticle tunnel current either between identical $d$-wave superconductors partially gapped by charge density waves (SCDWs) or between an SCDW and a normal metal was calculated. The cases of unidirectional and checkerboard CDWs were considered. The tunnel conductance was found in both cases to possess a number of peculiarities, which cannot be described by introducing a single combined gap. The results are in qualitative agreement with experimental data obtained for a number of cuprates by the scanning tunnel spectroscopy, intrinsic-tunneling, and break-junction measurements. The difference between the experiment and the theory seems to stem from the spread of gap values occurring due to the intrinsic spatial inhomogeneity of nonstoichiometric oxides and reflected in the cuprate tunnel spectra.

The sound velocity of Mo along the Hugoniot adiabat is calculated from first principles using density-functional theory based molecular dynamics. These data are compared to the sound velocity as measured in recent experiments. The theoretical and experimental Hugoniot and sound velocities are in very good agreement up to pressures of 210 GPa and temperatures of 3700 K on the Hugoniot. However, above that point the experiment and theory diverge. This implies that Mo undergoes a phase transition at about the same point. Considering that the melting point of Mo is likely much higher at that pressure, the related change in the sound velocity in experiment can be ascribed to a solid-solid transition.

We demonstrate significant improvements of the spin coherence time of a dense ensemble of nitrogen-vacancy (NV) centers in diamond through optimized dynamical decoupling (DD). Cooling the sample down to 77 K suppresses longitudinal spin relaxation $T_1$ effects and DD microwave pulses are used to increase the transverse coherence time $T_2$ from $\sim 0.7\mathrm{ms}$ up to $\sim 30\mathrm{ms}$. We extend previous work of single-axis (Carr-Purcell-Meiboom-Gill) DD towards the preservation of arbitrary spin states. Following a theoretical and experimental characterization of pulse and detuning errors, we compare the performance of various DD protocols. We identify that the optimal control scheme for preserving an arbitrary spin state is a recursive protocol, the concatenated version of the XY8 pulse sequence. The improved spin coherence might have an immediate impact on improvements of the sensitivities of ac magnetometry. Moreover, the protocol can be used on denser diamond samples to increase coherence times up to NV-NV interaction time scales, a major step towards the creation of quantum collective NV spin states.

We present a rigorous irreversible thermodynamics treatment of creep deformation of solid materials with interfaces described as geometric surfaces capable of vacancy generation and absorption and moving under the influence of local thermodynamic forces. The free energy dissipation rate derived in this work permits clear identification of thermodynamic driving forces for all stages of the creep process and formulation of kinetic equations of creep deformation and microstructure evolution. The theory incorporates capillary effects and reveals the different roles played by the interface free energy and interface stress. To describe the interaction of grain boundaries with stresses, we classify grain boundaries into coherent, incoherent and semicoherent, depending on their mechanical response to the stress. To prepare for future applications, we specialize the general equations to a particular case of a linear-elastic solid with a small concentration of vacancies. The proposed theory creates a thermodynamic framework for addressing more complex cases, such as creep in multicomponent alloys and cross-effects among vacancy generation/absorption and grain boundary motion and sliding.

We provide numerical evidence combined with an analytical understanding of the many-body mobility edge for the strongly anisotropic spin-$1/2 \mathit{XXZ}$ model in a random magnetic field. The system dynamics can be understood in terms of symmetry-constrained excitations about parent states with ferromagnetic and antiferromagnetic short range order. These two regimes yield vastly different dynamics producing an observable, tunable many-body mobility edge. We compute a set of diagnostic quantities that verify the presence of the mobility edge and discuss how weakly correlated disorder can tune the mobility edge further.

We have studied static and dynamic magnetic properties of a general asymmetric trilayer system using numerical simulations. For ferromagnetic, ${90}^{\circ }$, and antiferromagnetic coupling, the magnetizations of the two magnetic layers exhibit one, two, and three phases with increasing external field, respectively. The total magnetization and ferromagnetic resonance accordingly follow these phases of the magnetization vectors. The resonance condition is related to the interlayer coupling strength in such a way that a larger coupling constant yields a higher value of $f_{\mathrm{res}}/H$, where $f_{\mathrm{res}}$ is the resonance frequency at the external magnetic field $H$. Based on the simulation results, it is proposed that measurements of the acoustic mode resonance alone at unsaturated conditions provide a sensitive and accurate technique to extract the antiferromagnetic coupling strength. The technique is demonstrated experimentally with the broadband ferromagnetic resonance measurements of two trilayer films with weak and strong coupling strengths. The technique offers an efficient and sensitive method for antiferromagnetic coupling strength extraction, yielding coupling constant values with a precision of better than 0.03 erg/${\mathrm{cm}}^2$. Also, separation of the bilinear and biquadratic coupling contributions is possible with the technique.

We introduce the notion of a quantum version of a discontinuity critical point (DCP) in the context of a first-order quantum phase transition referred to as a quantum DCP (QDCP). We evaluate the critical exponents associated with such a QDCP using the appropriate scaling relations focusing mainly on a QDCP that is located at the boundary between a gapped phase and a gapless critical line as observed in a spin-$\frac{1}{2}$ XXZ chain while we also touch upon the conventional Ising case. We then study temporal and spatial quenches in the vicinity of the QDCP associated with the XXZ chain and propose the scaling relation for the defect density and the residual energy characterized by associated critical exponents. These predictions are numerically verified for quenches of the XXZ chain to the QDCP (or across it), thereby establishing the existence of a Kibble-Zurek scaling for such quenches given in terms of appropriate critical exponents. Finally, in the Appendix we consider a generic situation which reduces to the case when the QDCP separates two gapped phases and also to that occurring in the XXZ chain, and present detail analysis of the universality and scaling relations concerning quantum quenches.

We study the low-temperature isothermal magnetic hysteresis of cubical and spherical nanoparticles with ferromagnetic-core/antiferromagnetic-shell morphology, in order to elucidate the sensitivity of the exchange bias effect to the shape of the particles and the structural imperfections at the core-shell interface. We model the magnetic structure using a classical Heisenberg Hamiltonian with uniaxial anisotropy and simulate the hysteresis loop using the metropolis Monte Carlo algorithm. For nanoparticles with geometrically sharp interfaces, we find that cubes exhibit a higher coercivity and lower exchange bias field than spheres of the same size. With increasing interface roughness, the shape dependence of the characteristic fields gradually decays, and eventually, the distinction between cubical and spherical particles is lost for moderately rough interfaces. The sensitivity of the exchange bias field to the microstructural details of the interface is quantified by a scaling factor ($b$) relating the bias field to the net moment of the antiferromagnetic shell $(H_{\mathrm{eb}}=b{M}_{\mathrm{AF}}+H_o)$. Cubical particles exhibit a lower sensitivity to the dispersed values of the net interfacial moment.

By utilizing spin-polarized scanning tunneling microscopy (STM) and spectroscopy, we observe the coexistence of perpendicularly and in-plane magnetized cobalt nanoscale islands on an Ag(111) surface. The magnetization direction has the relationship with the observed moiré-corrugation amplitude on the islands; the islands with stronger moiré corrugation show perpendicular magnetization, and the ones with weaker moiré corrugation exhibit in-plane magnetization. We calculate the magnetic anisotropy energy for various stackings of a Co nanostructure based on density functional theory, and we reveal that perpendicular magnetic anisotropy is reduced drastically with increasing fcc stacking faults in the Co layer. Simulated STM images reproduce the moiré-corrugation difference observed experimentally when the stacking difference between perpendicularly and in-plane magnetized islands is considered. These theoretical analyses strongly suggest that the electronic and magnetic differences between the two types of islands are caused by the presence of fcc stacking faults in the intrinsic hcp stacking of Co.

We evaluate the microwave admittance of a one-dimensional chain of fluxonium qubits coupled by shared inductors. Despite its simplicity, this system exhibits a rich phase diagram. A critical applied magnetic flux separates a homogeneous ground state from a phase with a ground state exhibiting inhomogeneous persistent currents. Depending on the parameters of the array, the phase transition may be a conventional continuous one, or of a commensurate-incommensurate nature. Furthermore, quantum fluctuations affect the transition and possibly lead to the presence of gapless “floating phases.” The signatures of the soft modes accompanying the transitions appear as a characteristic frequency dependence of the dissipative part of admittance.

Magnetic excitations in the optimally doped high-$T_{\mathrm{c}}$ superconductor ${\mathrm{Bi}}_{1.5}{\mathrm{Pb}}_{0.55}{\mathrm{Sr}}_{1.6}{\mathrm{La}}_{0.4}{\mathrm{CuO}}_{6+\delta }$ (OP-Bi2201, $T_{\mathrm{c}}\simeq 34$ K) are investigated by Cu $L_3$ edge resonant inelastic x-ray scattering (RIXS), below and above the pseudogap opening temperature. At both temperatures the broad spectral distribution disperses along the (1,0) direction up to $\sim 350$ meV at zone boundary, similar to other hole-doped cuprates. However, above $\sim 0.22$ reciprocal lattice units, we observe a concurrent intensity decrease for magnetic excitations and quasielastic signals with weak temperature dependence. This anomaly seems to indicate a coupling between magnetic, lattice, and charge modes in this compound. We also compare the magnetic excitation spectra near the antinodal zone boundary in the single layer OP-Bi2201 and in the bilayer optimally doped ${\mathrm{Bi}}_{1.5}{\mathrm{Pb}}_{0.6}{\mathrm{Sr}}_{1.54}{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ (OP-Bi2212, $T_{\mathrm{c}}\simeq 96$ K). The strong similarities in the paramagnon dispersion and in their energy at zone boundary indicate that the strength of the superexchange interaction and the short-range magnetic correlation cannot be directly related to $T_{\mathrm{c}}$, not even within the same family of cuprates.

We show that the zero point motion of a vortex in superconducting doped topological insulators leads to significant changes in the electronic spectrum at the topological phase transition in this system. This topological phase transition is tuned by the doping level, and the corresponding effects are manifest in the density of states at energies which are on the order of the vortex fluctuation frequency. Although the electronic energy gap in the spectrum generated by a stationary vortex is but a small fraction of the bulk superconducting gap, the vortex fluctuation frequency may be much larger. As a result, this quantum zero point motion can induce a discontinuous change in the spectral features of the system at the topological vortex phase transition to energies which are well within the resolution of scanning tunneling microscopy. This discontinuous change is exclusive to superconducting systems in which we have a topological phase transition. Moreover, the phenomena studied in this paper present effects of Magnus forces on the vortex spectrum which are not present in the ordinary $s\text{-wave}$ superconductors. Finally, we demonstrate explicitly that the vortex in this system is equivalent to a Kitaev chain. This allows for the mapping of the vortex fluctuating scenario in three dimensions into similar one-dimensional situations in which one may search for other novel signatures of topological phase transitions.

We establish a relation between the Raman response function in the $B_{1g}$ channel and the electronic contribution to the nematic susceptibility within the spin-driven approach to electron nematicity of the iron-based superconductors. The spin-driven nematic phase, characterized by the broken $C_4$ symmetry, but unbroken $O\left(3\right)$ spin-rotational symmetry, is generated by the presence of magnetic fluctuations associated with the striped phase. It occurs as a separate phase between $T_m$ and $T_s$ in systems where the structural and magnetic phase transitions are separated. Detecting the presence of nematic degrees of freedom in iron-based superconductors is a difficult task, since it involves measuring higher-order spin-correlation functions. We show that the nematic degrees of freedom manifest themselves in the experimentally measurable Raman response function. We calculate the Raman response function in the tetragonal phase in the large-$N$ limit by considering higher-order Aslamazov–Larkin type of diagrams. They are characterized by a series of inserted quartic paramagnon couplings mediated by electronic excitations that resemble the nematic coupling constant of the theory. These diagrams effectively account for collisions between spin fluctuations. By summing an infinite number of such higher-order diagrams, we demonstrate that the electronic Raman response function shows a clear maximum at the structural phase transition in the $B_{1g}$ channel. Hence, the Raman response function can be used to probe nematic degrees of freedom.

Anderson's localization on the edge of a two-dimensional time reversal topological insulator (TI) is studied. For the noninteracting case, the topological protection acts according to the ${\mathbb{Z}}_2$ classification, leading to conducting and insulating phases for odd and even fillings, respectively. In the presence of repulsive interaction, the phase diagram is notably changed. We show that for sufficiently strong values of the interaction, the zero-temperature fixed point of the TI is conducting, including the case of even fillings. We compute the boundaries of the conducting phase for various fillings and types of disorder.

To clarify the nature of correlations in Hund metals and its relationship with Mott physics we analyze the electronic correlations in multiorbital systems as a function of intraorbital interaction $U$, Hund's coupling $J_H$, and electronic filling $n$. We show that the main process behind the enhancement of correlations in Hund metals is the suppression of the double occupancy of a given orbital, as it also happens in the Mott insulator at half-filling. However, contrary to what happens in Mott correlated states the reduction of the quasiparticle weight $Z$ with $J_H$ can happen in spite of increasing charge fluctuations. Therefore, in Hund metals the quasiparticle weight and the mass enhancement are not good measurements of the charge localization. Using simple energetic arguments we explain why the spin polarization induced by Hund's coupling produces orbital decoupling. We also discuss how the behavior at moderate interactions, with correlations controlled by the atomic spin polarization, changes at large $U$ and $J_H$ due to the proximity to a Mott insulating state.

In two-dimensional (2D) electron systems, Wigner crystals (WC) and fractional quantum Hall effect (FQHE) liquids are competing ground states under low temperatures $\left(T\right)$ and high magnetic fields $\left(B\right)$. Here we report differential conductivity results demonstrating the reentrant insulating phase around $\nu =1$/5 in a 2D hole system in AlGaAs/GaAs quantum wells and unexpected features in the solid-liquid phase transition between WC and FQHE liquids in ultrahigh magnetic fields up to 45 T. Remarkably, the electric field $\left(E\right)$ plays an equivalent role as the temperature does in our phase diagram. From the $E-T$ “duality” analysis, a characteristic length of 450 nm is derived, which can be understood as the phase-coherent domain size of WC. Moreover, evidence shows that with weak disorder the insulating phase and composite fermion liquid could be coexisting around $\nu =$ 1/5, pointing to the possibility that the insulating phase is the four flux quantum Wigner crystal, as proposed by theories.

The energetics and thermodynamic properties of two-dimensional binary graphene-like alloys made from graphene, silicene, or germanene are investigated by combining first-principles total energy calculations, and a statistical approach to account for disorder and composition effects. For the electronic properties the calculations are performed within the GGA-1/2 approach for an approximate quasiparticle bands. We derive lattice constants, first-neighbor distances, and buckling parameters as a function of composition $x$. The ${\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x$ system is the only stable random alloy at usual growth temperatures. For ${\mathrm{Ge}}_{1-x}{\mathrm{C}}_x$, we observe strong distortions of the lattice making the random configurations less favorable and leading to a pronounced tendency for phase separation. The situation for ${\mathrm{Si}}_{1-x}{\mathrm{C}}_x$ alloys is completely different. An ordered structure with composition $x=0.5$ is stable up to $T\approx 1000$ K, while intermediate compositions are mainly realized by silicongraphene and graphene or silicene. The ordering and decomposition effects have a strong influence on the average fundamental energy gap versus composition. Whereas large gaps appear for ${\mathrm{Si}}_{1-x}{\mathrm{C}}_x$ systems they almost vanish for ${\mathrm{Ge}}_{1-x}{\mathrm{Si}}_x$ and ${\mathrm{Ge}}_{1-x}{\mathrm{C}}_x$. Moreover, the dependence of the ${\mathrm{Si}}_{1-x}{\mathrm{C}}_x$ energy gap on growth temperature is also obtained. The results can be very useful for chemical vapor deposition growth of these materials.

Two-dimensional (2D) topological insulators (TIs) have been recognized as a new class of quantum state of matter. They are distinguished from normal 2D insulators with their nontrivial band-structure topology identified by the $Z_2$ number as protected by time-reversal symmetry (TRS). Two-dimensional TIs have intriguing spin-velocity locked conducting edge states and insulating properties in the bulk. In the edge states, the electrons with opposite spins propagate in opposite directions and the backscattering is fully prohibited when the TRS is conserved. This leads to a quantized dissipationless “two-lane highway” for charge and spin transportation and promises potential applications. Up to now, only very few 2D systems have been discovered to possess this property. The lack of suitable material obstructs further study and application. Here, by using first-principles calculations, we propose that functionalized MXenes with oxygen, ${\mathrm{M}}_2{\mathrm{CO}}_2$ (M=W, Mo, and Cr), are 2D TIs with the largest gap of 0.194 eV in the W case. They are dynamically stable and natively antioxidant. Most importantly, they are very likely to be easily synthesized by recently developed selective chemical etching of transition-metal carbides (the ${\mathrm{M}}_{n+1}{\mathrm{AX}}_n$ phase). This will pave the way to tremendous applications of 2D TIs, such as “ideal” conducting wire, multifunctional spintronic devices, and the realization of topological superconductivity and Majorana modes for quantum computing.

In this work we introduce a low-energy Hamiltonian for single-layer and bilayer black phosphorus that describes the electronic states at the vicinity of the $\mathrm{\Gamma }$ point. The model is based on a recently proposed tight-binding description for electron and hole bands close to the Fermi level. We calculate expressions for the Landau-level spectrum as function of magnetic field, and in the case of bilayer black phosphorus we investigate the effect of an external bias on the electronic band gap. The results showcase the highly anisotropic character of black phosphorus, and in particular for bilayer BP, the presence of bias allows for a field-induced semiconductor-metal transition.