$\mathrm{N}{\mathrm{i}}_{50\text{−}x}\mathrm{C}{\mathrm{o}}_x\mathrm{M}{\mathrm{n}}_{40}\mathrm{S}{\mathrm{n}}_{10}$ shape memory alloys in the approximate range $5\le x\le 8$ display desirable properties for applications as well as intriguing magnetism. These off-stoichiometric Heusler alloys undergo a martensitic phase transformation at a temperature $T_M$ of 300–400 K, from ferromagnetic (FM) to nonferromagnetic, with unusually low thermal hysteresis and a large change in magnetization. The low temperature magnetic structures in the martensitic phase of such alloys, which are distinctly inhomogeneous, are of great interest but are not well understood. Our present use of spin echo nuclear magnetic resonance in the large hyperfine fields at ${}^{55}\mathrm{Mn}$ sites provides compelling evidence that nanoscale magnetic phase separation into FM and antiferromagnetic (AFM) regions occurs below $T_M$ in alloys with $x$ in the range 0 to 7. At finite Co substitution, the FM regions are found to be of two distinct types, corresponding to high and low local concentrations of Co on Ni sites. Estimates of the size distributions of both the FM and AFM nanoregions have been made. At $x=7$, the AFM component is not long-range ordered, even below 4 K, and is quite different from the AFM component found at $x=0$; by $x=14$, the FM phase is completely dominant. Of particular interest, we find for $x=7$ that field cooling leads to dramatic changes in the AFM regions. These findings provide insight into the origins of magnetic phase separation and superparamagnetism in these complex alloys, particularly their intrinsic exchange bias, which is of considerable current interest.

We identify a large family of ground states of a topological $\mathbb{C}{P}^{N-1}$ skyrmion magnet whose classical degeneracy persists to all orders in a semiclassical expansion. This goes along with an exceptional robustness of the concomitant ground-state configurations, which are not at all dressed by quantum fluctuations. We trace these twin observations back to a common root: this class of topological ground states saturates a Bogomolny inequality. A similar phenomenology occurs in high-energy physics for some field theories exhibiting supersymmetry. We propose quantum Hall ferromagnets, where these skyrmions configurations arise naturally as ground states away from integer filling, as the best available laboratory realisations.

The development of the fundamental superconducting (SC) energy scales—the SC energy gap $\mathrm{\Delta }$ and the superfluid stiffness $J$—of granular aluminum, i.e., thin films composed of coupled nanograins, is studied by means of optical THz spectroscopy. Starting from well-coupled grains, $\mathrm{\Delta }$ grows as the grains are progressively decoupled, causing the unconventional increase of $T_c$ with sample resistivity. When the grain coupling is suppressed further, $\mathrm{\Delta }$ saturates while the critical temperature $T_c$ decreases, concomitantly with a sharp decline of $J$, delimiting a SC dome in the phase diagram. This crossover to a phase-driven SC transition is accompanied by an optical gap surviving into the normal state above $T_c$. We demonstrate that granular aluminum is an ideal testbed to understand the interplay between quantum confinement and global SC phase coherence due to nanoinhomogeneity.

The dynamics of glass is of paramount importance for understanding glass, while experimental studies of it covering broad time and temperature ranges are fraught with difficulty. We employ a method which can probe the extremely slow dynamics in various glassy states in metallic glass (MG). The flow dynamics of as-cast MG is found to follow a universal Arrhenius behavior in a wide temperature range, and aged MG follows a stretched exponential function with a “magic” exponent number of 3/7. Our observations have implications for understanding the structural evolution of the slow flow and the issue of finite temperature divergence in MGs.

We propose two approaches to cloak the spin waves (magnons) by investigating magnetization dynamics. One approach is based on a spatially inhomogeneous anisotropic magnetic moment tensor. The other mechanism is using a spatially inhomogeneous anisotropic gyromagnetic factor tensor and an inhomogeneous external magnetic field. For both approaches, the damping tensor is also inhomogeneous and anisotropic. The magnetic characteristic functions of the magnetic materials have been theoretically derived for both mechanisms. A nonmagnetic core, which prevents magnons from entering and consequently distorts the spin-wave propagation, can be cloaked by a structured magnetic shell to redirect the spin wave around the core using the above design mechanisms. We discuss the feasibility of the proposed mechanisms in an ensemble of quantum dot molecules and magnetic semiconductors. The proposed approaches shed light on transformation magnonics, and can be utilized for future spin-wave lenses, concentrators, low backscattering waveguides, and ultimately quantum computing.

We study the density of states in disordered $s$-wave superconductors with a small gap anisotropy. We consider disorder in the form of common nonmagnetic scatterers and pairing-potential impurities, which interact with electrons via an electric potential and a local distortion of the superconducting gap. Using quasiclassical Green functions, we determine the bound-state spectrum at a single impurity and the density of states at a finite concentration of impurities. We show that, if the gap is isotropic, an isolated impurity with suppressed pairing supports an infinite number of Andreev states. With growing impurity concentration, the energy-dependent density of states evolves from a sharp gap edge with an impurity band below it to a smeared BCS singularity in the so-called universal limit. Within one spin sector, pairing-potential impurities and weak spin-polarized magnetic impurities have essentially the same effect on the density of states. We note that, if a gap anisotropy is present, the density of states becomes sensitive to ordinary potential disorder, and the existence of Andreev states localized at pairing-potential impurities requires special conditions. An unusual feature related to the anisotropy is a nonmonotonic dependence of the gap edge smearing on impurity concentration.

We report on terahertz time-domain spectroscopy on superconducting and metallic iron chalcogenides ${\mathrm{Rb}}_{0.75}{\mathrm{Fe}}_{1.6}{\mathrm{Se}}_{2-z}{\mathrm{S}}_z$. The superconducting transition is reduced from $T_c=32$ K $\left(z=0\right)$ to 22 K $\left(z=1.0\right)$, and finally suppressed $\left(z=1.4\right)$ by isoelectronic substitution of Se with S. Dielectric constant and optical conductivity exhibit a metal-to-insulator transition associated with an orbital-selective Mott phase. This orbital-selective Mott transition appears at higher temperature $T_{\mathrm{met}}$ with increasing sulfur content, identifying sulfur substitution as an efficient parameter to tune orbital-dependent correlation effects in iron-chalcogenide superconductors. The reduced correlations of the $d_{xy}$ charge carriers can account for the suppression of the superconductivity and the pseudogaplike feature between $T_c$ and $T_{\mathrm{met}}$ that was observed for $z=0$.

${\mathrm{HoTe}}_3$, a member of the rare-earth tritelluride ($R{\mathrm{Te}}_3$) family, and its Pd-intercalated compounds, ${\mathrm{Pd}}_x{\mathrm{HoTe}}_3$, where superconductivity (SC) sets in as the charge-density wave (CDW) transition is suppressed by the intercalation of a small amount of Pd, are investigated using angle-resolved photoemission spectroscopy (ARPES) and electrical resistivity. Two incommensurate CDWs with perpendicular nesting vectors are observed in ${\mathrm{HoTe}}_3$ at low temperatures. With a slight Pd intercalation ($x=0.01$), the large CDW gap decreases and the small one increases. The momentum dependence of the gaps along the inner Fermi surface (FS) evolves from orthorhombicity to near tetragonality, manifesting the competition between two CDW orders. At $x=0.02$, both CDW gaps decrease with the emergence of SC. Further increasing the content of Pd for $x=0.04$ will completely suppress the CDW instabilities and give rise to the maximal SC order. The evolution of the electronic structures and electron-phonon couplings (EPCs) of the multiple CDWs upon Pd intercalation are carefully scrutinized. We discuss the interplay between multiple CDW orders, and the competition between CDW and SC in detail.

The intense femtosecond-scale pulses from x-ray free electron lasers (XFELs) are able to create and interrogate interesting states of matter characterized by long-lived nonequilibrium semicore or core electron occupancies or by the heating of dense phases via the relaxation cascade initiated by the photoelectric effect. We address here the latter case of “warm dense matter” (WDM) and investigate the observable consequences of x-ray heating of the electronic degrees of freedom in crystalline systems. We report temperature-dependent density functional theory calculations for the x-ray diffraction from crystalline LiF, graphite, diamond, and Be. We find testable, strong signatures of condensed-phase effects that emphasize the importance of wide-angle scattering to study nonequilibrium states. These results also suggest that the reorganization of the valence electron density at eV-scale temperatures presents a confounding factor to achieving atomic resolution in macromolecular serial femtosecond crystallography (SFX) studies at XFELs, as performed under the “diffract before destroy” paradigm.

Quantum disordering a discrete-symmetry-breaking state by condensing domain walls can lead to a trivial symmetric insulator state. In this work, we show that if we bind a one-dimensional representation of the symmetry (such as a charge) to the intersection point of several domain walls, condensing such modified domain walls can lead to a nontrivial symmetry-protected topological (SPT) state. This result is obtained by showing that the modified domain-wall condensed state has a nontrivial SPT invariant, the symmetry-twist-dependent partition function. We propose two different kinds of field theories that can describe the above-mentioned SPT states. The first one is a Ginzburg-Landau–type nonlinear sigma model theory, but with an additional multikink domain-wall topological term. Such theory has an anomalous $U^k\left(1\right)$ symmetry but an anomaly-free $Z_N^k$ symmetry. The second one is a gauge theory, which is beyond Abelian Chern-Simons/BF gauge theories. We argue that the two field theories are equivalent at low energies. After coupling to the symmetry twists, both theories produce the desired SPT invariant.

We derive an effective Hamiltonian for a (001)-confined quasi-two-dimensional hole gas in a strained zinc-blende semiconductor heterostructure including both Rashba and Dresselhaus spin-orbit coupling. In the presence of uniaxial strain along the $〈110〉$ axes, we find a conserved spin quantity in the vicinity of the Fermi contours in the lowest valence subband. In contrast to previous works, this quantity meets realistic requirements for the Luttinger parameters. For more restrictive conditions, we even find a conserved spin quantity for vanishing strain, restricted to the vicinity of the Fermi surface.

We show that the electromagnetic forces generated by the excitations of a mode in graphene-based optomechanical systems are highly tunable by varying the graphene chemical potential, and orders of magnitude stronger than usual non-graphene-based devices, in both attractive and repulsive regimes. We analyze coupled waveguides made of two parallel graphene sheets, either suspended or supported by dielectric slabs, and study the interplay between the light-induced force and the Casimir-Lifshitz interaction. These findings pave the way to advanced possibilities of control and fast modulation for optomechanical devices and sensors at the nano- and microscales.

Recent progress in the design and fabrication of artificial two-dimensional (2D) materials paves the way for the experimental realization of electron systems moving on complex geometries, such as plane fractals. In this work, we calculate the quantum conductance of a 2D electron gas roaming on a Sierpinski carpet (SC), i.e., a plane fractal with Hausdorff dimension intermediate between 1 and 2. We find that the fluctuations of the quantum conductance are a function of energy with a fractal graph, whose dimension can be chosen by changing the geometry of the SC. This behavior is independent of the underlying lattice geometry.

Periodically driven (Floquet) crystals are described by their quasienergy spectrum. Their topological properties are characterized by invariants attached to the gaps of this spectrum. In this article, we define such invariants in all space dimensions, both in the case where no symmetry is present and in the case where the unitary chiral symmetry is present. When no symmetry is present, a $\mathbb{Z}$-valued invariant can be defined in each gap in all even space dimensions. This invariant does not capture all the properties of a system where chiral symmetry is present. In even space dimension, chiral symmetry puts constraints on its values in different gaps. In odd space dimension, chiral symmetry also enables to define a $\mathbb{Z}$-valued invariant, only in the chiral gaps 0 and $\pi$. We relate both gap invariants to the standard invariants characterizing the quasienergy bands of the system. Examples in one and three dimensions are discussed.

We show that the correlations of electrons with a fixed energy in metals have very anomalous time and space dependencies. Due to soft modes that exist in any Fermi liquid, combined with the incomplete screening of the Coulomb interaction at finite frequencies, the correlations in two-dimensional systems grow as the square of a time scale. In the presence of disorder, the spatial correlations grow as a distance squared. Similar, but in general weaker, effects are present in three-dimensional systems and in the absence of quenched disorder. We propose ways to experimentally measure these anomalous correlations.

We consider nonequilibrium phenomena in a very simple model that displays a zero-temperature first-order phase transition. The quantum Ising model with a four-spin exchange is adopted as a general representative of first-order quantum phase transitions that belong to the Ising universality class, such as for instance the order-disorder ferroelectric transitions, and possibly first-order $T=0$ Mott transitions. In particular, we address quantum quenches in the exactly solvable limit of infinite connectivity and show that, within the coexistence region around the transition, the system can remain trapped in a metastable phase, as long as it is spatially homogeneous so that nucleation can be ignored. Motivated by the physics of nucleation, we then study in the same model static but inhomogeneous phenomena that take place at surfaces and interfaces. The first-order nature implies that both phases remain locally stable across the transition, and with that the possibility of a metastable wetting layer showing up at the surface of the stable phase, even at $T=0$. We use mean-field theory plus quantum fluctuations in the harmonic approximation to study quantum surface wetting.

We demonstrate through electrolyte gating measurements of a single nanobeam that the rultile phase of ${\mathrm{VO}}_2$ is electrochemically more reactive than the monoclinic phase. Our results show that the complete suppression of the metal-insulator transition and stabilization of the metallic phase is possible when gate voltage is applied in the rutile metallic phase. The results are discussed based on the formation of oxygen vacancies wherein accommodation of a high concentration of vacancies in the rutile phase selectively stabilizes it by disrupting dimerization of adjacent V-V pairs required for a transition to the monoclinic phase. The creation of oxygen vacancies is proposed to proceed through the oxidation of the electrolyte. Raman spectroscopy data suggest surface metallization upon electrolyte gating with an initial coexistence of insulating monoclinic and metallic domains. The selective electrochemical reactivity of the rutile phase and the resulting defect-induced stabilization of this phase across a vastly expanded temperature window suggest a facile defect engineering route to tune electronic phase transitions.

In this paper we study the phase diagram of a disordered, spin-orbit coupled superconductor with $s$-wave or $d+id$-wave pairing symmetry in symmetry class $D$. We analyze the topological phase transitions by applying three different methods, which include a disorder averaged entanglement entropy, a disorder averaged real-space Chern number, and an evaluation of the momentum space Chern number in a disorder averaged effective model. We find evidence for a disorder-induced topological state. While in the clean limit there is a single phase transition from a trivial phase with a Chern number $C=4$ to a topological phase with $C=1$, in the disordered system there is an intermediate phase with $C=3$. The phase transition from the trivial $C=4$ phase into the intermediate phase with $C=3$ is seen in the real-space calculation of the Chern number. In spite of this, this phase transition is not detectable in the entanglement entropy. A second phase transition from the disorder-induced $C=3$ into the $C=1$ phase is seen in all three quantities.