Strong interactions between electrons in two dimensions can realize phases where their spins and charges separate. We capture this phenomenon within a dual formulation. Focusing on square lattices, we analyze the long-wavelength structure of vortices when the microscopic particles—electrons or spinful bosons—are near half-filling. These conditions lead to a compact gauge theory of spinons and chargons, which arise as the fundamental topological defects of the low-energy vortices. The gauge theory formulation is particularly suitable for studying numerous exotic phases and transitions. We support the general analysis by an exact implementation of the duality on a coupled-wire array. Finally, we demonstrate how the latter can be exploited to construct parent Hamiltonians for fractional phases and their transitions.

There remain a few unsolved problems when the ab initio approach was applied to study one of the most important binary shape memory alloys (SMAs), NiTi (nitinol), which is the starting material for alloying numerous multicomponent SMAs. This hinders the computational design of complex SMAs with desirable properties. In this Letter we report that including the electronic free energy in the Gibbs free energy nearly reduces by half the large errors in previously predicted martensitic transition temperatures, and the controversy on the ground state of NiTi is resolved by reaching the convergence in free-energy calculations. In addition, the present results demonstrate that the martensitic transition path in stoichiometric NiTi is directly from B2 to $\mathrm{B}{19}^{\prime }$ without intermediate phases.

We present a theoretical investigation of anisotropic superconducting spin transport at a magnetic interface between a $p$-wave superconductor and a ferromagnetic insulator. Our formulation describes the ferromagnetic resonance modulations due to spin current generation depending on spin-triplet Cooper pair, including the frequency shift and enhanced Gilbert damping, in a unified manner. We find that the Cooper pair symmetry is detectable from the qualitative behavior of the ferromagnetic resonance modulation. Our theory paves the way toward anisotropic superconducting spintronics.

The presence of chalcogen vacancies in monolayer transition metal dichalcogenides (TMDs) leads to excitons with mixed localized-delocalized character and to reduced valley selectivity. Recent experimental advances in defect design in TMDs allow for a close examination of such mixed exciton states as a function of their degree of circular polarization under external magnetic fields, revealing strongly varying defect-induced magnetic properties. A theoretical understanding of these observations and their physical origins demands a predictive, structure-sensitive theory. In this work, we study the effect of chalcogen vacancies on the exciton magnetic properties in monolayer ${\mathrm{MoS}}_2$. Using many-body perturbation theory, we show how the complex excitonic picture associated with the presence of defects—with reduced valley and spin selectivity due to hybridized electron-hole transitions—leads to a structurally controllable exciton magnetic response. We find a variety of $g$-factors with changing magnitudes and sign depending on the exciton energy and character. Our findings suggest a pathway to tune the nature of the excitons—and by that their magneto-optical properties—through defect architecture.

We demonstrate that the long-debated immunity of the microwave-induced resistance oscillations (MIRO) to the sense of circular polarization is in fact not an inherent property of this phenomenon in solid-state two-dimensional electron systems (2DES): We detect an up to 30 times larger MIRO signal for the cyclotron resonance (CR) active helicity, fully consistent with the concurrently measured transmission and the deduced CR shape of the Drude absorption. Using a 2DES as a sensitive sensor of the actual polarization state, we further study extrinsic near-field effects capable of producing an apparent immunity of the photoresponse.

Studying the cyclotron resonance (CR) induced photoconductivity in GaAs and HgTe two-dimensional electron structures under circularly polarized terahertz illumination, we observed an anomalous resonant photoresponse for the CR inactive helicity of almost the same magnitude as for the CR active helicity. This observation conflicts with simultaneous transmission measurements and fundamentally contradicts the conventional theory of CR. We provide a possible route to explain such a basic failure of the conventional description of light-matter interaction and discuss a modified electron dynamics near strong impurities that may provide a local near-field coupling of the two helicity modes. This should result in a CR enhanced absorption for both magnetic field polarities.

A new class of van der Waals heterostructures of ${\left(\mathrm{SnTe}\right)}_{\mathrm{m}}{\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)}_{\mathrm{n}}$ (with $m=1,2,..$ and $n=1,2,..$), consisting of a topological crystalline insulator SnTe and a topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ are emerging with exciting properties and applications, such as in thermoelectrics. Our study examines the stability of these heterostructures ($m$ = 1 and $n$ = 1,2) under pressure using Raman scattering, synchrotron x-ray diffraction, and density functional theory. Raman studies as a function of pressure carried out at room temperature reveal a phase transition in the pressure regime of 3–5 GPa for both the compounds, which is shown to be associated with an electronic topological transition involving change in the ${\mathrm{Z}}_2$ topological invariant. In addition to the electronic changes, our Raman experiments indicate that rhombohedral ($R\overline{3}m$) ${\mathrm{SnBi}}_2{\mathrm{Te}}_4$ undergoes structural transition at $\sim 6.0$ to a possible monoclinic phase and another transition at $\sim 12.0$ GPa. Raman and x-ray diffraction experiments on trigonal ($P\overline{3}m1$) ${\mathrm{SnBi}}_4{\mathrm{Te}}_7$ show two structural transitions at $\sim 9.5$ GPa to a monoclinic phase followed by one to cubic phase at $\sim 14.1$ GPa. Our analysis of electronic structure reveals that the phase transition at 9.5 GPa in ${\mathrm{SnBi}}_4{\mathrm{Te}}_7$ is accompanied by an insulator to semimetal transition.

Atomic ordering of ferromagnetic alloys, such as Heusler compounds, vastly influences their magnetic properties and overall usability. However, in some cases x-ray diffraction cannot unambiguously determine the correct atomic structure, which hampers further research. In this paper, we analyze the atomic ordering using spectroscopic ellipsometry—a simple tabletop method that researchers can use in situ during sample deposition. The ${\mathrm{Co}}_2\mathrm{Fe}({\mathrm{Ga}}_{0.5}{\mathrm{Ge}}_{0.5})$ Heusler compound was chosen for the analysis, as the atomic ordering greatly influences its spin-polarizing capabilities. The ellispometric results are confronted with x-ray diffraction and with the help of $\mathit{ab}$ initio calculations the qualitative changes in the optical response are linked to the changes in atomic ordering.

We present a vibrational dynamical mean-field theory (VDMFT) of the dynamics of atoms in solids with anharmonic interactions. Like other flavors of DMFT, VDMFT maps the dynamics of a periodic anharmonic lattice of atoms onto those of a self-consistently defined impurity problem with local anharmonicity and coupling to a bath of harmonic oscillators. VDMFT is exact in the harmonic and molecular limits, nonperturbative systematically improvable through its cluster extensions, usable with classical or quantum impurity solvers (depending on the importance of nuclear quantum effects) and can be combined with existing low-level diagrammatic theories of anharmonicity. When tested on models of anharmonic optical and acoustic phonons; we find that classical VDMFT gives good agreement with classical molecular dynamics, including the temperature dependence of phonon frequencies and lifetimes. Using a quantum impurity solver, signatures of nuclear quantum effects are observed at low temperatures. We test the description of nonlocal anharmonicity via cellular VDMFT and the combination with self-consistent phonon (SCPH) theory, yielding the powerful SCPH $+$ VDMFT approach.

The ${\mathrm{CuIr}}_{2-x}{\mathrm{Ru}}_x{\mathrm{Te}}_4$ superconductors (with a $T_c$ around 2.8 K) can host charge-density waves, whose onset and interplay with superconductivity are not well known at a microscopic level. Here, we report a comprehensive study of the $x=0$ and 0.05 cases, whose superconductivity was characterized via electrical-resistivity, magnetization, and heat-capacity measurements, while their microscopic superconducting properties were studied via muon-spin rotation and relaxation ($\mu \mathrm{SR}$). In ${\mathrm{CuIr}}_{2-x}{\mathrm{Ru}}_x{\mathrm{Te}}_4$, both the temperature-dependent electronic specific heat and the superfluid density (determined via transverse-field $\mu \mathrm{SR}$) are best described by a two-gap ($s+d$)-wave model, comprising a nodeless gap and a gap with nodes. The multigap superconductivity is also supported by the temperature dependence of the upper critical field $H_{\mathrm{c}2}\left(T\right)$. However, under applied pressure, a charge-density-wave order starts to develop and, as a consequence, the superconductivity of ${\mathrm{CuIr}}_2{\mathrm{Te}}_4$ achieves a more conventional $s$-wave character. Our series of experiments provides ample evidence that the ${\mathrm{CuIr}}_{2-x}{\mathrm{Ru}}_x{\mathrm{Te}}_4$ family belongs to the rare cases where an unconventional superconducting pairing is found near a charge-density-wave quantum critical point.

We investigate the spectral and transport properties of a double quantum dot laterally attached to a topological superconducting nanowire, hosting the Majorana zero-energy modes. Specifically, we consider a geometry, in which the outer quantum dot is embedded between the external normal and superconducting leads, forming a circuit. First, we derive analytical expressions for the bound states in the case of an uncorrelated system and discuss their signatures in the tunneling spectroscopy. Then, we explore the case of strongly correlated quantum dots by performing the numerical renormalization group calculations, focusing on the interplay and relationship between the leaking Majorana mode and the Kondo states on both quantum dots. Finally, we discuss feasible means to experimentally probe the in-gap quasiparticles by using the Andreev spectroscopy based on the particle-to-hole scattering mechanism.

Two-dimensional (2D) ferromagnetic charge density waves (CDWs), an exotic quantum state for exploring the intertwining effect between correlated charge and spin orders in the 2D limit, have yet to be discovered under moderate temperatures. Here, we propose a feasible strategy to realize 2D ferromagnetic CDWs under external fields, which is demonstrated in monolayer ${\mathrm{VSe}}_2$ using first-principles calculations. Under external tensile strain, two ferromagnetic CDWs ($\sqrt{3}\times \sqrt{3}$ and $2\times 2\sqrt{3}$ CDWs) can be generated, accompanied by distinguishable lattice reconstructions of magnetic V atoms. Remarkably, because the driving forces for generating these two ferromagnetic CDWs are strongly spin dependent, fundamentally different from that in conventional CDWs, the $\sqrt{3}\times \sqrt{3}$ and $2\times 2\sqrt{3}$ CDWs can exhibit two dramatically different half-metallic phases under a large strain range, along with either a flat band or a Dirac cone around the Fermi level. Our proposed strategy and material demonstration may provide a feasible way to generate and manipulate the correlation effect between collective charge and spin orders via external fields.

Charge density waves (CDWs) with superconductivity, competing Fermi surface instabilities, and collective orders have captured much interest in two-dimensional van der Waals (vdW) materials. Understanding the CDW suppression mechanism, its connection to the emerging superconducting state, and electronic correlations provides opportunities for engineering the electronic properties of vdW heterostructures and thin-film devices. Using a combination of the thermal transport, x-ray photoemission spectroscopy, Raman measurements, and first-principles calculations, we observe an increase in electronic correlations of the conducting states as the CDW is suppressed in ${\mathrm{ZrTe}}_3$ with 5% Cu and Ni intercalation in the vdW gap. As superconductivity emerges, intercalation brings not only decoupling of quasi-one-dimensional conduction electrons with phonons as a consequence of intercalation-induced lattice expansion but also a drastic increase in ${\mathrm{Zr}}^{2+}$ at the expense of ${\mathrm{Zr}}^{4+}$ metal atoms. These observations not only demonstrate the potential of atomic intercalates in the vdW gap for ground-state tuning but also illustrate the crucial role of the Zr metal valence in the formation of collective electronic orders.

Compression of greenhouse gases still dominates the market of refrigeration devices. Although well established and efficient, this technology is neither safe for the environment nor able to be scaled down to nanoscale. Solid-state cooling technologies are being developed to overcome these limitations, including studies at nanoscale. Among them, the so-called elastocaloric effect (eC) consists of the thermal response $\mathrm{\Delta }T$ of a material under strain deformation. In this work, fully atomistic molecular dynamics simulations of the eC in carbon nanotubes (CNTs) are presented over a large temperature span. The efficiency of the CNTs as solid refrigerators is investigated by simulating their eC in a model of refrigerator machine running under Otto-like thermodynamic cycles (two adiabatic expansion/contraction plus two isostrain heat exchange processes) operating at temperatures $T_{\mathrm{O}}$ ranging 300–2000 K. The coefficient of performance (COP), defined as the ratio of heat removed from the cold region to the total work performed by the system per thermodynamic cycle, is calculated for each value of $T_{\mathrm{O}}$. Our results show a nonlinear dependence of $\mathrm{\Delta }T$ on $T_{\mathrm{O}}$, reaching a minimum value of about 30 K for $T_{\mathrm{O}}$ between 500 and 600 K, then growing and converging to a linear dependence on $T_{\mathrm{O}}$ for large temperatures. The COP of CNTs is shown to remain about the same and approximately equal to 8. These results are shown to be weakly depend on CNT diameter and chirality but not on length. The isothermal entropy change of the CNTs due to the eC is also estimated and shown to depend nonlinearly on $T_{\mathrm{O}}$ values. These results predict that CNTs can be considered versatile nanoscale solid refrigerators able to efficiently work over a large temperature span.

While the emission of localized plasmons is often treated in the real-space picture, working in wavevector space makes it possible to resolve the contributions of various plasmonic Fourier components. By calculating the rate of plasmon emission by interlayer excitons in twisted ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ van der Waals heterostructures as a function of plasmon localization, twist angle, and temperature, we show that a much wider range of exciton states can radiate into subwavelength plasmon modes than into free space. Most of the emission in this case takes place via plasmonic Fourier components that exceed the maximum photon wavevector in free space. We demonstrate that this provides an avenue for continued Purcell scaling of the emission rate with mode size. Furthermore, the scaling is well in excess of the Purcell trend for sufficiently large twist angles and strong plasmonic localization.

The recent surge of optics of hyperbolic metamaterials (HMMs) has been fueled by their fascinating optical properties, one of the most intriguing being their strong optical nonlocality. In this work we demonstrate that in metal nanorod-based HMMs the nonlocality results in fast and slow light effects in the propagation of femtosecond laser pulses in close spectral vicinity of the HMM epsilon-near-zero regime. These effects are switchable via the angle of incidence and light wavelength. We elucidate that revealed dynamical phenomena stem from the zero-transmission points of HMM and related phase singularities caused by the destructive interference of the main optical wave and the additional wave mediated by the spatial dispersion of light in the HMMs.

We investigate magnetotransport on Bernal-stacked tetralayer graphene whose band structure consists of two massive subbands with different effective masses. Under a finite displacement field, we observe the valley splitting of Landau levels (LLs) only in the light-mass subband, consistent with a tight-binding model. At low density, we find unexpected magnetoconductance oscillations in bulk gaps which originate from a series of hybridizations between electronlike and holelike LLs due to band inversion in tetralayer graphene. In contrast to a trivial LL quantization gap, these inverted hybridization gaps can lead to a change in the number of edge states which explains the observed oscillations.

To minimize their electrostatic energy, insulating ferroelectric films tend to break up into nanoscale “Kittel” domains of opposite polarization that are separated by uncharged ${180}^{\circ }$ domain walls. Here I report on self-consistent solutions of coupled Landau-Ginzburg-Devonshire and Schrödinger equations for an electron-doped ferroelectric thin film. The model is based on ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interfaces in which the ${\mathrm{SrTiO}}_3$ substrate is made ferroelectric by cation substitution or strain. I find that electron doping destabilizes the Kittel domains. As the two-dimensional electron density $n_{2\mathrm{D}}$ increases, there is a smooth crossover to a zigzag domain wall configuration. The domain wall is positively charged but is compensated by the electron gas, which attaches itself to the domain wall and screens depolarizing fields. The domain wall approaches a flat head-to-head configuration in the limit of perfect screening. The polarization profile may be manipulated by an external bias voltage and the electron gas may be switched between surfaces of the ferroelectric film.

Multiple subcontacts formation/rupture and puckering the evolution of contact quality play crucial roles in modulating friction on two-dimensional lamellar materials but have up to now been studied separately in the field of nanotribology. In this work, by using variable temperature friction force microscopy in ultrahigh-vacuum conditions, we investigate friction on graphite surfaces with different numbers of free edges as a function of temperature from 295.0 to 100.0 K. We introduce subcontacts formation/rupture simply by changing temperature and tune the out-of-plane deformation compliance by changing the number of free edges. It is found that friction on a plane with large deformation compliance is constantly larger than that with small deformation compliance at cryogenic temperatures. While appearing to be influenced secondly by deformation compliance, the coefficients of friction increase, however, linearly with temperature. By performing systematic stick-slip measurements, the aforementioned separate friction mechanisms have been linked, where the temperature-induced increase of subcontacts rupture force can give rise to the frictional contact quality which additionally becomes more prominent on a plane with large deformation compliance at cryogenic temperatures. These results emphasize a way to vary the contact quality via temperature and demonstrate that the subcontacts formation/rupture and the contact quality must be correlatively accounted for.