As opposed to random disorder, which localizes single-particle wave functions in one dimension (1D) at arbitrarily small disorder strengths, there is a localization-delocalization transition for quasiperiodic disorder in the 1D Aubry-André model at a finite disorder strength. On the single-particle level, many properties of the ground state at criticality have been revealed by applying a real-space renormalization-group scheme; the critical properties are determined solely by the continued-fraction expansion of the incommensurate frequency of the disorder. Here, we investigate the many-particle localization-delocalization transition in the Aubry-André model with and without interactions. In contrast to the single-particle case, we find that the critical exponents depend on a Diophantine equation relating the incommensurate frequency of the disorder and the filling fraction which generalizes the dependence, in the single-particle spectrum, on the continued-fraction expansion of the incommensurate frequency. This equation can be viewed as a generalization of the resonance condition in the commensurate case. When interactions are included, numerical evidence suggests that interactions may be irrelevant at at least some of these critical points, meaning that the critical exponent relations obtained from the Diophantine equation may actually survive in the interacting case.

We present magnetotransport data on the ferrimagnet ${\mathrm{GdMn}}_6{\mathrm{Sn}}_6$. From the temperature-dependent data we are able to extract a large intrinsic contribution to the anomalous Hall effect ${\sigma }_{xz}^{\mathrm{int}}\sim 32{\mathrm{\Omega }}^{-1}{\text{cm}}^{-1}$ and ${\sigma }_{xy}^{\mathrm{int}}\sim 223{\mathrm{\Omega }}^{-1}{\text{cm}}^{-1}$, which is comparable to values found in other systems also containing kagome nets of transition metals. From our transport anisotropy, as well as our density functional theory calculations, we argue that the system is electronically best described as a three-dimensional system. Thus we show that reduced dimensionality is not a strong requirement for obtaining large Berry phase contributions to transport properties. In addition, the coexistence of rare-earth and transition-metal magnetism makes the hexagonal ${\mathrm{MgFe}}_6{\mathrm{Ge}}_6$ structure type a promising system to tune the electronic and magnetic properties in future studies.

Systematic investigation of phase stability of the magnetic fcc Fe-Cr-Mn-Ni system—promising candidate structural materials to replace conventional austenitic steels—has been performed using a combination of spin-polarized density-functional theory, cluster expansion, and Monte Carlo simulations. The developed model was able to reproduce all known ground states (GSs) in the studied system and to predict new ones with strongly negative formation enthalpy—ternary ${\mathrm{CrMnNi}}_2$ and quaternary ${\mathrm{FeCr}}_2{\mathrm{MnNi}}_4$. Investigation of phase stability was done at 0 K and finite temperatures in the whole concentration range and allowed us to observe the important role of Ni and Mn. Ni is the only element in the system that increases the order-disorder transition (ODT) temperature, which means that the fcc alloys with decreased concentration of Ni will form solid solutions at lower temperatures. Analysis of the effect of the addition of Mn to Fe-Cr-Ni alloy confirms a general trend of statistical correlation between the averaged magnitude of magnetic moments and volume per atom found from the predicted stable structures in the quaternary system and underlying subsystems. This linear magneto-volume relationship trend is, however, weaker in Fe-Cr-Mn-Ni alloys in comparison with those in the Fe-Cr-Ni system. Furthermore, Ni and Mn form the most stable GS—$\text{L}{1}_0$-MnNi, which has one of the strongest tendencies to segregate in fcc Fe-Cr-Mn-Ni alloys evidenced by the strength of Mn-Ni short-range ordering (SRO). Mn-Ni SRO significantly increases ODT temperature in the vicinity of $\text{L}{1}_0$-MnNi and to the equiatomic region. The ODT of ${\mathrm{Cr}}_{18}{\mathrm{Fe}}_{27}{\mathrm{Mn}}_{27}{\mathrm{Ni}}_{28}$ alloy is found to be $1290\pm 150$ K, which supports the experimental observation of the disordered solid solution structure in ${\mathrm{Cr}}_{18}{\mathrm{Fe}}_{27}{\mathrm{Mn}}_{27}{\mathrm{Ni}}_{28}$ alloy at higher temperatures.

The existence of various magnetic orders has recently been established in the Tsai-type 1/1 approximant Au-Al-Gd by variation of the electron-per-atom (e/a) ratio [Ishikawa et al. Phys. Rev. B 98, 220403(R) (2018)]. Here, we report ferromagnetic (FM) 2/1 quasicrystal approximants and show that the magnetic order of higher-order approximants can be tailored starting from known magnetic 1/1 approximants. The (Au,Cu)-(Al,In)-$R\left(R=\mathrm{Gd},\mathrm{Tb}\right)2$/1 approximants are synthesized by the simultaneous substitution of isovalent elements Cu and In for Au and Al, respectively, to the FM Au-Al-$R\left(R=\mathrm{Gd},\mathrm{Tb}\right)1$/1 approximants with $e/a=1.74$. Both the (Au,Cu)-(Al,In)-$R\left(R=\mathrm{Gd},\mathrm{Tb}\right)2$/1 approximants exhibit a FM transition at significantly high Curie temperatures of $T_{\mathrm{C}}=30.0$ and 15.3 K, respectively. Therefore, this method of isovalent substitution will also enable the realization of a variety of magnetic orders for the 2/1 approximant by variation of the e/a ratio. Such isovalent substitution may also stabilize magnetic quasicrystals for a given e/a ratio, which would lead to the realization of the long-range magnetic order in quasicrystals.

We present an approach based on a density matrix expansion to study thermodynamic properties of a quantum system strongly coupled to two or more baths. For slow external driving of the system, we identify the adiabatic and nonadiabatic contributions to thermodynamic quantities, and we show how the first and second laws of thermodynamics are manifested in the strong coupling regime. Particularly, we show that the entropy production is positive up to second order in the driving speed. The formulation can be applied both for bosonic and fermionic systems, and recovers previous results for the equilibrium case [Phys. Rev. B 98, 134306 (2018)]. The approach is then demonstrated for the driven resonant level model as well as the driven Anderson impurity model, where the hierarchical quantum master equation method is used to accurately simulate the nonequilibrium quantum dynamics.

Superconductivity results from an instability of the Fermi surface—contour of poles of the single-particle propagator—to an infinitesimally small attraction between electrons. Here, we instead discuss the analogous problem on a model Luttinger surface, or contour of zeros of the Green function. At zero temperature $\left(\beta \to \infty \right)$ and a critical interaction strength $\left(u_{c\infty }\right)$ characterized by the residue of self-energy pole, we find that the pair susceptibility diverges leading to a superconducting instability. We evaluate the pair fluctuation partition function and find that the spectral density in the normal state has an interaction-driven, power-law $\frac{1}{\sqrt{\omega }}$ type, van-Hove singularity (vHS), indicating non-Fermi liquid (NFL) physics. Crucially, in the strong coupling limit $\left(\beta u\gg 1\right)$, the leading-order fluctuation free-energy terms in the normal state of this NFL-SC transition resembles the equivalent $\left[O\left(1\right)\right]$ terms of the Sachdev-Ye-Kitaev (SYK) model. This free-energy contribution takes a simple form $-\beta F=\beta u_{c\infty }-\gamma \text{ln}\left(\beta u_{c\infty }\right),$ where $\gamma$ is a constant equal to $\frac{1}{2}$. Weak impurity scattering $\left(\tau \gg {\beta }^{-1}\right)$ leaves the low-energy spectral density unaffected but leads to an interaction-driven enhancement of superconductivity. Our results shed light on the role played by order-parameter fluctuations in providing the key missing link between Mott physics and strongly coupled toy models exhibiting gravity duals.

The search for macroscopic quantum effects, including superfluidity, in molecular hydrogen is mostly focused on its parahydrogen $\left(p-{\mathrm{H}}_2\right)$ nuclear spin modification because of weaker intermolecular interaction compared to orthohydrogen $\left(o-{\mathrm{H}}_2\right)$, both modifications being bosonic. In this work, mixed clusters of $o-{\mathrm{H}}_2$ and $p-{\mathrm{H}}_2$ containing $\sim {10}^4$ molecules are prepared by supersonic expansion with helium and studied by vibrational coherent anti-Stokes Raman scattering (CARS) spectroscopy. At similar experimental conditions the neat $p-{\mathrm{H}}_2$ clusters avoid freezing and remain fluid at 1–2 K, which is predicted to be the realm of their superfluid behavior [Phys. Rev. Lett. 101, 205301 (2008)]. Dependence of the vibrational frequencies and intensities of the main CARS peaks due to $o-{\mathrm{H}}_2$ and $p-{\mathrm{H}}_2$ versus the ratio of the $o-{\mathrm{H}}_2$ and $p-{\mathrm{H}}_2$ concentrations in the expanding gas suggests that $o-{\mathrm{H}}_2$ and $p-{\mathrm{H}}_2$ molecules are uniformly mixed in the interior of the clusters. A weak spectral feature at $4157\mathrm{c}{\mathrm{m}}^{-1}$ that appears independent of the concentration ratio is assigned to the outer shell of the clusters enriched with $p-{\mathrm{H}}_2$ molecules. Although the phase of the mixed clusters could not be unambiguously identified, the shift of the vibrational frequencies with respect to the bulk solid is consistent with the liquid state of the clusters.

The cuprates exhibit a prominent charge-density-wave (CDW) instability with a wave vector along [100], i.e., the Cu-O bond direction. Whereas CDW order is most prominent at moderate doping and low temperature, there exists increasing evidence for dynamic charge correlations throughout a large portion of the temperature-doping phase diagram. In particular, the signatures of incipient charge order have been observed as phonon softening and/or broadening near the CDW wave vector approximately halfway through the Brillouin zone. Most of this work is focused on moderately doped cuprates, for which the CDW order is robust, or on optimally doped samples, for which the superconducting transition temperature $\left(T_c\right)$ attains its maximum. Here we present a time-of-flight neutron scattering study of phonons in simple-tetragonal ${\text{HgBa}}_2{\text{CuO}}_{4+\delta }\left(T_c=55\mathrm{K}\right)$ at a low doping level where prior work showed the CDW order to be weak. We employ and showcase a new software-based technique that mines a large number of measured Brillouin zones for useful data in order to improve accuracy and counting statistics. Density-functional theory has not provided an accurate description of phonons in ${\text{HgBa}}_2{\text{CuO}}_{4+\delta }$, yet we find the right set of parameters to qualitatively reproduce the data. The notable exception is a dispersion minimum in the longitudinal Cu-O bond-stretching branch along [100]. This discrepancy suggests that, while CDW order is weak, there exist significant dynamic charge correlations in the optic phonon range at low doping, near the edge of the superconducting dome.

We report high-resolution dilatometry on high-quality single crystals of ${\mathrm{NiTiO}}_3$ grown by means of the optical floating-zone technique. The anisotropic magnetic phase diagram is constructed from thermal expansion and magnetostriction studies up to $B=15\mathrm{T}$ and magnetization studies in static (15-T) and pulsed (60-T) magnetic fields. Our data allow us to quantitatively study magnetoelastic coupling and to determine uniaxial pressure dependencies. While the entropy changes are found to be of magnetic nature, Grüneisen analysis implies only one relevant energy scale in the whole low-temperature regime. Thereby, our data suggest that the observed structural changes due to magnetoelastic coupling and previously reported magnetodielectric coupling [L. Balhorn, J. Hazi, M. C. Kemei, and R. Seshadri, Phys. Rev. B 93, 104404 (2016)] are driven by the same $magnetic$ degrees of freedom that lead to long-range magnetic order in ${\mathrm{NiTiO}}_3$.

A combination of the density functional theory and the single-site dynamical mean-field theory is employed to study the electronic structures of various allotropes of elemental curium (Cm-I, Cm-II, and Cm-III). We find that the $5f$ valence electrons in the high-symmetry Cm-I and Cm-II phases remain localized, while they turn into itinerancy in the low-symmetry monoclinic Cm-III phase. In addition, conspicuous quasiparticle multiplets are identified in the $5f$ electronic density of states of the Cm-III phase. We believe that it is the many-body transition between $5f^7$ and $5f^8$ configurations that gives rise to these quasiparticle multiplets. Therefore, the Cm-III phase is probably a realization of the so-called Racah metal.

We use modified linear spin-wave theory (MLSWT) to study ground-state entanglement for a length-$L$ line subsystem in $L\times L$ square- and triangular-lattice quantum Heisenberg antiferromagnets with coplanar spiral magnetic order with ordering vector $\mathit{Q}=(q,q)$ and $N_G=3$ Goldstone modes, except if $q=\pi$ (collinear order, $N_G=2$). Generalizing earlier MLSWT results for $q=\pi$ to commensurate spiral order with $s\ge 3$ sublattices ($q=2\pi r/s$ with $r$ and $s$ coprime), we find analytically for large $L$ a universal and $n$-independent subleading term $(N_G/2)lnL$ in the Rényi entropy $S_n$, associated with $L^{1/2}$ scaling of ${\lambda }_0$ and ${\lambda }_{\pm q}$, with ${\lambda }_0\ne {\lambda }_{\pm q}$ for spiral order; here $\left\{{\lambda }_{k_y}\right\}$ are the $L$ mode occupation numbers of the entanglement Hamiltonian. The term $(3/2)lnL$ in $S_n$ agrees with a nonlinear sigma model study of $s=3$ spiral order ($q=2\pi /3$). These and other properties of $S_n$ and ${\lambda }_{k_y}$ are explored numerically for an anisotropic nearest-neighbor triangular-lattice model for which $q$ varies in the spiral phase.

We study energetics and the nature of both homogeneous and mixed spin (MS) states in ${\mathrm{LaCoO}}_3$ incorporating structural changes of the crystal volume expansion and the Co-O bond disproportionation (BD) during the spin-state transition using the density functional theory plus dynamical mean field theory ($\mathrm{DFT}+\mathrm{DMFT}$) method. $\mathrm{DFT}+\mathrm{DMFT}$ predicts that energetics of both excited spin states are almost the same while $\mathrm{DFT}+U$ calculations of the same structures energetically favor the MS states and produce various metastable solutions whose energetics depend sensitively on final spin states. Within $\mathrm{DFT}+\mathrm{DMFT}$, the homogeneous spin state in the expanded crystal volume shows the multiconfigurational nature with non-negligible occupancy probabilities of both high spin (HS) and low spin (LS) states along with $d^6$ and $d^7$ charge configurations indicating the dynamically fluctuating nature of spin and charge states due to the Co-O covalency. The nature of the MS state under the BD structure reveals that Co sites with the long Co-O bonds develop a Mott insulating state and favor HS with a $d^6$ configuration, while more covalent Co sites with the short Co-O bonds occupy more LS states with a $d^7$ configuration and behave as a band insulator, as a result, charge ordering is induced in the BD structure from the spin-state ordering. We also find that both energetics and electronic structure sensitively depend on the Co-O covalency effect, which can be tuned by changing the double counting potential and the resulting $d$ occupancy ($N_d$), and $N_d$ close to 6.7 is consistent with the nature of the spin-state transition. Our results show that structural changes during the spin-state transition can play an important role in understanding energetics and electronic structure of ${\mathrm{LaCoO}}_3$.

Topologically protected qubits based on nanostructures hosting Majorana bound states (MBSs) hold great promise for fault-tolerant quantum computing. We study the transport properties of nanowire networks hosting MBSs with a focus on the effects of the charging energy and the overlap between neighboring MBSs in short mesoscopic samples. In particular, we investigate structures hosting four MBSs such as $\mathsf{T}$ junctions and Majorana boxes. Using a master equation in the Markovian approximation, we discuss the leading transport processes mediated by the MBSs. Single-electron tunneling and processes involving creation and annihilation of Cooper pairs dominate in the sequential-tunneling limit. In the cotunneling regime the charge in the MBSs is fixed and transport is governed by transitions via virtual intermediate states. Our results show that four-terminal measurements in the $\mathsf{T}$ junction and Majorana box geometries can be useful tools for the characterization of the properties of MBSs with finite overlaps and charging energy.

In the topological phase of spin-orbit coupled nanowires Majorana bound states are known to localize at the nanowire edges and to exhibit a spin density orthogonal to both the magnetic field and the spin-orbit field. By investigating a nanowire exposed to a uniform magnetic field with an interface between regions with different spin-orbit couplings, we find that the orthogonal spin density is pinned at the interface even when both interface sides are in the topologically trivial phase, and even when no bound state is present at all. A trivial bound state may additionally appear at the interface, especially if the spin-orbit coupling takes opposite signs across the interface. However, it can be destroyed by a smoothening of the spin-orbit profile or by a magnetic field component parallel to the spin-orbit field. In contrast, the orthogonal spin density persists in various and realistic parameter ranges. We also show that, while the measurement of bulk equilibrium spin currents has been elusive so far, such robust orthogonal spin density peak may provide a way to detect spin current variations across interfaces.

We establish an important duality correspondence between topological order in quantum many-body systems and criticality in ferromagnetic classical spin systems. We show how such a correspondence leads to a classical and simple procedure for characterization of topological order in an important set of quantum entangled states, namely the Calderbank-Shor-Steane (CSS) states. To this end, we introduce a particular quantum Hamiltonian which allows us to consider the existence of a topological phase transition from quantum CSS states to a magnetized state. We study the ground state fidelity in order to find nonanalyticity in the wave function as a signature of a topological phase transition. Since hypergraphs can be used to map any arbitrary CSS state to a classical spin model, we show that fidelity of the quantum model defined on a hypergraph $H$ is mapped to the heat capacity of the classical spin model defined on dual hypergraph $\tilde{H}$. Consequently, we show that a ferromagnetic-paramagnetic phase transition in a classical model is mapped to a topological phase transition in the corresponding quantum model. We also show that magnetization does not behave as a local order parameter at the transition point while the classical order parameter is mapped to a nonlocal measure on the quantum side, further indicating the nonlocal nature of the transition. Our procedure not only opens the door for identification of topological phases via the existence of a local and classical quantity, i.e., critical point, but also offers the potential to classify various topological phases through the concept of universality in phase transitions.

Half-metallic materials allow electrons with one spin orientation to conduct through, while completely blocking electrons with the other spin orientation. Here we use a multiscale approach to predict that ${\mathrm{MnSiTe}}_3$ within the transition metal trichalcogenide (TMT) family is an intrinsic layered half-metal both in bulk form and at the ultrathin two-dimensional (2D) limit. We first use first-principles calculations to demonstrate that the system is a van der Waals crystal with weak interlayer coupling, allowing ready separation of a monolayer. Next we show that the half-metallicity of the bulk crystal is preserved also for a freestanding monolayer, with highly desirable ferromagnetic interlayer coupling. The ferromagnetic order of the monolayer is further shown to originate from the interplay of the exchange interaction and Ruderman-Kittel-Kasuya-Yosida coupling mediated by the itinerant carriers, while the singly spin-polarized and long-range nature of the latter is largely responsible for the unusually large next-next-nearest ferromagnetic coupling among the Mn ions. These findings, together with the latest developments surrounding ferromagnetism in 2D TMTs, may offer immense application potentials in nanomagnetic and spintronic devices.

We investigate a way to suppress high-frequency coupling between a gate and low-dimensional electron systems in the gigahertz range by measuring the velocity of edge magnetoplasmons (EMPs) in InAs quantum Hall systems. We compare the EMP velocity in three samples with different electromagnetic environments―one has a highly resistive zinc oxide (ZnO) top gate, another has a normal metal (Ti/Au) top gate, and the other does not have a gate. The measured EMP velocity in the ZnO gate sample is one order of magnitude larger than that in the Ti/Au gate sample and almost the same as that in the ungated sample. As is well known, the smaller velocity in the Ti/Au gate sample is due to the screening of the electric field in EMPs. The suppression of the gate screening effect in the ZnO gate sample allows us to measure the velocity of unscreened EMPs while changing the electron density. It also offers a way to avoid unwanted high-frequency coupling between quantum Hall edge channels and gate electrodes.

We have simulated the time evolution of the photon number distribution in a semiconductor quantum-dot–microcavity system driven by chirped laser pulses and compare with unchirped results. When phonon interactions with the dot are disregarded—thus corresponding to the limit of atomic cavity systems—chirped pulses generate photon number distributions that change their shape drastically in the course of time. Phonons have a strong and qualitative impact on the photon statistics. The asymmetry between phonon absorption and emission destroys the symmetry of the photon distributions obtained for positive and negative chirps. While for negative chirps transient distributions resembling thermal ones are observed, for positive chirps the photon number distribution still resembles its phonon-free counterpart but with overall smoother shapes. In sharp contrast, using unchirped pulses of the same pulse area and duration wave packets are found that move up and down the Jaynes-Cummings ladder with a bell shape that changes little in time. For shorter pulses and lower driving strength Rabi-like oscillations occur between low photon number states. For all considered excitation conditions transitions between sub- and super-Poissonian statistics are found at certain times. For resonant driving with low intensity the Mandel parameter oscillates and is mostly negative, which indicates a nonclassical state in the cavity field. Finally, we show that it is possible that the Mandel parameter dynamically approaches zero and still the photon distribution exhibits two maxima and thus is far from being a Poissonian.

Magnetoelectric dipole coupling effects in all-dielectric metasurfaces composed of particles with bianisotropic electromagnetic response are investigated. This bianisotropic response is associated with the trapped mode excitation. Maintaining the trapped mode resonant conditions allows one to sufficiently increase the quality factor and reduce radiation losses in all-dielectric nanostructures (metasurfaces). An analytical model accounting for the contributions of both electric and magnetic dipole moments induced in particles by external electromagnetic fields is proposed. We show how bianisotropy can lead to the excitation of the trapped mode in metasurfaces. This mode corresponds to the electromagnetic coupling between the out-of-plane particle dipole moments, which do not radiate collectively from the metasurface plane resulting in the enhanced storage of electromagnetic energy. Our approach reveals a physical mechanism of the trapped mode excitation and demonstrates that the specially initiated bianisotropy of particles enables the energy flow between external electromagnetic waves and the trapped mode. Due to this bianisotropy, one can control the process of light-matter interaction and energy storage in all-dielectric metasurfaces via excitation of trapped modes.

Nanoparticles levitated in vacuum can be set to spin at ultimate frequencies, limited only by the tensile strength of the material. At such high frequencies, drastic changes to the dynamics of solid-state quantum excitations are to be expected. Here, we theoretically describe the interaction between acoustic phonons and the rotation of a nanoparticle around its own axis and model how the acoustic and optical properties of the nanoparticle change when it rotates at a fixed frequency. As an example, we analytically predict the scaling of the shape, the acoustic eigenmode spectrum, the permittivity, and the polarizability of a spinning dielectric nanosphere. We find that the changes to these properties at frequencies of a few gigahertz achieved in current experiments should be measurable with presents technology. Our work aims at exploring solid-state quantum excitations in mesoscopic matter under extreme rotation, a regime that is now becoming accessible with the advent of precision control over highly isolated levitated nanoparticles.