The existence of single-layer diamond or diamane, which could adopt the properties of its bulk counterpart, has been verified by previous calculations and experiments. Even though, carbon atoms on the top and bottom surfaces need to form dangling bonds with atoms and/or molecules to stabilize their ${\mathrm{sp}}^3$ hybridization. In this paper, diamane is substituted by N atoms assisting the diamondlike structure to be stabilize without any passivation studied by ab initio calculation. One-fourth of N substitution on diamane, vertically stacking as NCCC, is found to be stable by surface reconstruction forming a Pandey $\pi$-chain structure and has an antiferromagnetic property. Half nitrogen substitution on diamane, vertically stacking as NCCN, can be stable and prevails the diamane form. Its elastic constants yield high values, for example, its $C_{11}$ is as twice as diamond and its $C_{33}$ is only 21% lower than diamond. The NCCN phase is a metastable phase at 10 GPa compared with other layered carbon nitride phases showing the possible pathway to be created.

We show that a controllable dc magnetization is accumulated in a junction comprising a quantum dot coupled to nonmagnetic reservoirs if the junction is subjected to a time-dependent spin-orbit interaction. The latter is induced by an ac electric field generated by microwave irradiation of the gated junction. The magnetization is caused by inelastic spin-flip scattering of electrons that tunnel through the junction, and depends on the polarization of the electric field: a circularly polarized field leads to the maximal effect, while there is no effect in a linearly polarized field. Furthermore, the magnetization increases as a step function (smoothened by temperature) as the microwave photon energy becomes larger than the absolute value of the difference between the single energy level on the quantum dot and the common chemical potential in the leads.

Excess electron transport is explored at the reduced $\mathrm{Ti}{\mathrm{O}}_2$(110) rutile surface through high-resolution electron energy-loss spectroscopy. Changes in line shape, intensity, and energy position of elastic and phonon peaks are associated to changes in charge-carrier and band-gap state excitations. The dielectric modeling points to a defined inward charge profile and to distinct bulk and surface transport properties. The one-to-one correlation between band-gap state intensity and carrier density favors a dual polaronic behavior of charge carriers over the coexistence of trapped and free states.

We propose a simple variational wave function that captures the correct ground-state energy of the spin-1 Heisenberg chain model to within 0.04%. The wave function is written in the matrix product state (MPS) form with the bond dimension $D=8$ and is characterized by three fugacity parameters. The proposed MPS generalizes the Affleck-Kennedy-Lieb-Tasaki state by dressing it with dimers, trimers, and general $q$-mers. The fugacity parameters control the number and the average size of the $q$-mers. Furthermore, the $D=8$ variational MPS state captures the ground states of the entire family of the bilinear-biquadratic Hamiltonian belonging to the Haldane phase to high accuracy. The 2-4-2 degeneracy structure in the entanglement spectrum of our MPS state is found to match well the results of the density matrix renormalization group (DMRG) calculation, which is computationally much heavier. Spin-spin correlation functions also find an excellent fit with those obtained by DMRG.

We report on a comparative theoretical and experimental investigation of the electronic band structure of a family of three-dimensional topological insulators, $A^{IV}{\mathrm{Bi}}_4{\mathrm{Te}}_{7-x}{\mathrm{Se}}_x$ ($A^{IV}$= Sn, Pb; $x$ = 0, 1). We prove by means of density functional theory calculations and angle-resolved photoemission spectroscopy measurements that partial or total substitution of heavy atoms by lighter isoelectronic ones affects the electronic properties of topological insulators. In particular, we show that the modification of the Dirac cone position relative to the Fermi level and the bulk band gap size can be controlled by varying the stoichiometry of the compound. We also demonstrate that the investigated systems are inert to oxygen exposure.

Whether or not a physical property can be enhanced in an inhomogeneous system compared with its homogeneous counterpart is an intriguing fundamental question. We provide a concrete example with a positive answer by uncovering a remarkable enhancement of both the antiferromagnetic structure factor and $d$-wave pairing tendency in a doped staggered periodic Anderson model (PAM) with two alternating inequivalent local moments. The common thread in these enhancements is found to originate from the generic “self-averaging” effect and nonmonotonic dependence of the corresponding physical quantity in homogeneous PAM. More strikingly, we provided evidence of the coexistence of these two enhancements via a tentative phase diagram. Our findings may imply the plausible generalization of enhancing physical properties in generic inhomogeneous systems.

The problem of a size quantization for charge carriers in a planar quantum well consisting of different monolayers of transition metal dichalcogenides is solved using the Dirac model and the four-band model. For excitons, bound states of electrons and holes at the size quantization levels in such a quantum well, the energy spectrum was found in two cases: the Bohr radius is much smaller than the width of the quantum well (dielectric permeability of a substrate is relatively small) and the Bohr radius is much larger than it (the case of a strong dielectric screening). It is shown that the energy spectra in these two cases are completely different. A method for the synthesis of the heterostructures under consideration is also proposed.

In this paper we develop the theory for two-dimensional to two-dimensional tunneling spectroscopy aided by magnetic or quantum-order excitations, and apply it to the description of van der Waals heterostructures of graphene or ultrathin $\alpha -{\mathrm{RuCl}}_3$. We study the behavior of both the differential conductance and the inelastic electron tunneling spectrum (IETS) of these heterostructures. The IETS in particular exhibits features such as the gap of continuum spinon excitations and Majorana bound states, the energies of which scale cubicly with the applied magnetic field. Such scaling, which exists for a relatively wide range of fields, is at odds with the linear one exhibited by conventional magnons and can be used to prove the existence of Kitaev quantum spin liquids.

The average time required for an open quantum system to reach a steady state (the steady-state time) is generally determined through a competition of coherent and incoherent (dissipative) dynamics. Here, we study this competition for a ubiquitous central-spin system, corresponding to a central-spin-1/2 coherently coupled to ancilla spins and undergoing dissipative spin relaxation. The ancilla system can describe $N$ spins-1/2 or, equivalently, a single large spin of length $I=N/2$. We find exact analytical expressions for the steady-state time in terms of the dissipation rate, resulting in a minimal (optimal) steady-state time at an optimal value of the dissipation rate, according to a universal curve. Due to a collective-enhancement effect, the optimized steady-state time grows only logarithmically with increasing $N=2I$, demonstrating that the system size can be grown substantially with only a moderate cost in steady-state time. This paper has direct applications to the rapid initialization of spin qubits in quantum dots or bound to donor impurities, to dynamic nuclear-spin polarization protocols, and may provide key intuition for the benefits of error-correction protocols in quantum annealing.

We present a study of the dielectric, structural, and magnetic properties of the multiferroic or linear magnetoelectric substitution series ${\left[{\left({\mathrm{NH}}_4\right)}_{1-x}{\mathrm{K}}_x\right]}_2\left[\mathrm{Fe}{\mathrm{Cl}}_5\right({\mathrm{H}}_2\mathrm{O}\left)\right]$. Pyroelectric currents, magnetic susceptibilities, and thermodynamic properties were examined on large single crystals of the erythrosiderite compounds and detailed magnetic-field versus temperature phase diagrams are derived for three different substitution levels. With increasing potassium concentration the material is tuned from a multiferroic ($x\le 0.06$) to a linear magnetoelectric ($x\ge 0.15$) ground state. In contrast to the respective pure parent compounds with $x=0$ or 1, however, the ferroelectric or linear magnetoelectric polarization in none of the substituted samples is switchable by external electric fields because these samples exhibit a significant electric polarization already above the magnetic ordering transition. The polarization arises at a higher-lying structural phase transition that is examined by THz spectroscopy, and, on a deuterated pure single crystal, by comprehensive neutron-diffraction experiments. The structural phase transition is attributed to an ordering of ${\mathrm{NH}}_4^{+}$ tetrahedra but does not break inversion symmetry in the pure material, while a finite K content causes pyroelectricity.

In this paper, based on ab initio molecular dynamics and the exchange-interaction model, the magnetic order transition phenomenon of ${\mathrm{Na}}_2{\mathrm{Co}}_2\mathrm{Te}{\mathrm{O}}_6$ material under different temperatures is investigated. Our results exclude the effect of lattice distortion as a factor for magnetic susceptibility under different temperatures in ${\mathrm{Na}}_2{\mathrm{Co}}_2\mathrm{Te}{\mathrm{O}}_6$. Furthermore, we found that the charge transfer is not a main factor to effectively explain the magnetic phase transition. We proposed a method based on radial distribution function to analyze the space-charge density (SCD) to explore the thermal fluctuations. Our analysis of SCD implies that the thermal fluctuations induced by the variation of atomic vibration under different temperatures are responsible for the anomalies of magnetic susceptibility in ${\mathrm{Na}}_2{\mathrm{Co}}_2\mathrm{Te}{\mathrm{O}}_6$ below the Néel temperature.

We consider a clean layered magnetic superconductor in which a continuous magnetic transition takes place inside a superconducting state. We assume that the exchange interaction between superconducting and magnetic subsystems is weak so that superconductivity is not destroyed at the magnetic transition. A representative example of such material is $\mathrm{Rb}\mathrm{Eu}{\mathrm{Fe}}_4{\mathrm{As}}_4$. We investigate the suppression of the superconducting gap and superfluid density by correlated magnetic fluctuations in the vicinity of the magnetic transition. The influence of nonuniform exchange field on superconducting parameters is very sensitive to the relation between the magnetic correlation length ${\xi }_h$ and superconducting coherence length ${\xi }_s$ defining the ‘scattering’ (${\xi }_h<{\xi }_s$) and ‘smooth’ (${\xi }_h>{\xi }_s$) regimes. As a small uniform exchange field does not affect the superconducting gap and superfluid density at zero temperature, smoothening of the spatial variations of the exchange field reduces its effects on these parameters. We develop a quantitative description of this ‘scattering-to-smooth’ crossover for the case of quasi-two-dimensional magnetic fluctuations realized in $\mathrm{Rb}\mathrm{Eu}{\mathrm{Fe}}_4{\mathrm{As}}_4$. Since the magnetic-scattering energy scale is comparable with the gap in the crossover region, the standard quasiclassical approximation is not applicable and full microscopic treatment is required. We find that the corrections to both the gap and superfluid density increase proportionally to ${\xi }_h$ until it remains much smaller than ${\xi }_s$. In the opposite limit, when the correlation length exceeds the coherence length both parameters have much weaker dependence on ${\xi }_h$. Moreover, the gap correction may decrease with increasing of ${\xi }_h$ in the immediate vicinity of the magnetic transition if it is located at temperature much lower than the superconducting transition. We also find that the crossover between the two regimes is unexpectedly broad: The standard scattering approximation becomes sufficient only when ${\xi }_h$ is substantially smaller than ${\xi }_s$.

The internal structure of the superconducting state in ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$ remains elusive at present, and exhibits evidence for time-reversal symmetry breaking. Recent muon spin relaxation measurements under uniaxial strain have revealed an increasing splitting between the superconducting critical temperature $T_c$ and the onset of time-reversal symmetry breaking $T_{\mathrm{TRSB}}$ with applied strain (Grinenko et al., arXiv:2001.08152). In addition, static magnetic order is induced by the uniaxial strain beyond $\sim 1$ GPa, indicating that unstrained ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$ is close to a magnetic quantum critical point. Here we perform a theoretical study of the magnetic susceptibility and the associated pairing structure as a function of uniaxial strain. It is found that the recent muon relaxation data can be qualitatively explained from the perspective of spin-fluctuation mediated pairing and the associated strain dependence of accidentally degenerate pair states in unstrained ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$. In addition, while unstrained ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$ features mainly $(2\pi /3,2\pi /3)$ magnetic fluctuations, uniaxial strain promotes $(\pi ,\pm \pi /2)$ magnetic order.

We study numerically the fractional Shapiro step response in low-frequency driven Josephson junctions which have a nonsinusoidal current-phase relation. We perform this study within the resistively shunted Josephson junction model. We demonstrate that fractional steps, as a fingerprint of a skewed current phase relation, will manifest themselves only for higher values of the reduced frequency. We compare the theoretical observations with experimental measurements in an anisotropic Josephson junction array containing over 500 superconductor-normal-superconductor junctions having a skewed current phase relation. We demonstrate that changing the critical current by applying a magnetic field is a robust method to modify the reduced frequency over a broad range of values. The presence of fractional Shapiro steps at high values of the reduced frequency directly confirm the theoretical results.

Superconductivity occurs in metals when attractive interactions between electrons promote the formation of Cooper pairs which get locked in a phase-coherent state, leading to superfluid behavior. In situations where the phase stiffness falls below the pairing energy, the superconducting transition temperature $T_c$ is driven by phase fluctuations and pairing can continue to survive above $T_c$. Such a behavior has long been thought of as the hallmark of unconventional superconductivity. Here we combine sub-K scanning tunneling spectroscopy, magnetic penetration depth measurements, and magnetotransport measurements to show that in ultrathin amorphous MoGe ($a$-MoGe) films the superfluid density is strongly suppressed by quantum phase fluctuations at low temperatures for thickness below 5 nm. This is associated with a rapid decrease in the superconducting transition temperature $T_c$ and the emergence of a pronounced pseudogap above $T_c$. These observations suggest that even in conventional superconductors strong disorder and low dimensionality will ultimately trigger a Bosonic route for the destruction of superconductivity.

Mixed ${}^3\mathrm{He}-{}^4\mathrm{He}$ droplets may acquire angular momentum during their passage through the nozzle of the experimental apparatus, cooling down and undergoing isotopic segregation, developing a ${}^3\mathrm{He}$ crust surrounding a superfluid ${}^4\mathrm{He}$ core. Within density functional theory, we investigate their stability and the relations between their angular momenta and their shapes. We uncover a variety of behaviors where the interplay between the superfluid ${}^4\mathrm{He}$ core and the normal-fluid ${}^3\mathrm{He}$ coating leads to a scenario that both bears analogies with viscous rotating drops and displays new features such as configurations with a fissioned or three-lobed ${}^4\mathrm{He}$ core, or with multiply charged vortices.

Degrees of freedom like charge, spin, and valley in condensed matter have drawn much attention in the past decades. Recently, several kinds of nodal line semimetals have been discovered for which the conduction and valence bands cross along one-dimensional lines, usually forming loops. These distinct loops, similar to valleys, can be used as a possible carrier of information. Here we investigate the loop polarization during quantum transports, i.e., barrier tunneling as well as disorder scattering, in Hopf semimetals with two disconnected loops. We find that in both cases, the loop polarization is mostly dominated by the performance of normal incidence, due to the mirror symmetry of two loop planes and the finite size effect along one direction. After scattered by nonmagnetic disordered potentials, the opposite polarizations can intrinsically reveal the topological structure of two loops, linked or unlinked. The effects from smooth barriers and disorders are also investigated. We find that a long range smooth barrier leads to an even clearer and purer picture of scattering and loop polarization, due to the suppression of large momentum transfer. These effects can be used as a controllable loop filter and detector.

The combination of strong disorder and interactions in closed quantum systems can lead to many-body localization (MBL). However, this quantum phase is not stable when the system is coupled to a thermal environment. We investigate how MBL is destroyed in systems that are weakly coupled to a dephasive Markovian environment by focusing on their entanglement dynamics. We numerically study the third Rényi negativity $R_3$, a recently proposed entanglement proxy based on the negativity that captures the unbounded logarithmic growth in the closed case and that can be computed efficiently with tensor networks. We also show that the decay of $R_3$ follows a stretched exponential law, similarly to the imbalance, with, however, a smaller stretching exponent.