The nitrogen vacancy (NV) center in diamond coupled to addressable surrounding nuclear spins forms a versatile building block for future quantum technologies. While previous activities focused on sensing with only a single or very few spins in operation, recently multiqubit registers have been successfully implemented for quantum information processing. Further progress requires a detailed understanding of the performance of quantum protocols for consecutive gate operations and thus beyond established treatments for relaxation and dephasing. Here, we provide such a theoretical analysis for small spin registers with up to four spins built out of NV and environmental constituents in the presence of ensembles of interacting impurity spins. Adapting a cluster correlation expansion, we predict coherence properties as well as fidelities for Greenberger-Horne-Zeilinger- and Bell-gate operations also in the presence of decoupling pulses. The influence of the volume density and the geometry of the spin-bath consisting of the substitutional nitrogen or ${}^{13}\mathrm{C}$ atoms are also taken into account.

In topological photonics, artificial photonic structures are constructed for realizing nontrivial unidirectional propagation of photonic information. On the other hand, moiré superlattices are emerging as an important avenue for engineering quantum materials with novel properties. In this paper, we combine these two aspects and demonstrate theoretically that moiré superlattices of small-angle twisted monolayer-bilayer graphene provide a natural platform for valley-protected plasmons. Particularly, a complete plasmonic bandgap appears stemming from the distinct optical conductivities of the ABA and ABC stacked triangular domains. Moreover, the plasmonic crystals exhibit nonzero valley Chern numbers and unidirectional transport of plasmonic edge states protected from intervalley scattering is presented.

We investigate the valley relaxation due to intervalley coupling in a single-electron bilayer graphene quantum dot. The valley relaxation is assisted by both the emission of acoustic phonons via the deformation potential and bond-length change mechanisms and $1/f$ charge noise. In the perpendicular magnetic-field dependence of the valley relaxation time $T_1$, we predict a monotonic decrease of $T_1$ at higher fields due to electron-phonon coupling, which is in good agreement with recent experiments by Banszerus et al. (arXiv:2402.16691). We find that the dominant valley relaxation channel in the high-field regime is the electron-phonon coupling via the deformation potential. At lower fields, we predict that a peak in $T_1$ can arise from the competition between $1/f$ charge noise and electron-phonon scattering due to bond-length change. We also find that the interlayer hopping ${\gamma }_3$ opens a valley relaxation channel for electric charge noise for rotationally symmetric quantum dots in bilayer graphene.

We study the effects of relaxation/decoherence processes on quantum transport of noninteracting Fermi particles across the disordered tight-binding chain, where we distinguish between relaxation processes in the contacts (external decoherence) and those in the chain (internal decoherence). It is shown that external decoherence reduces conductance fluctuations but does not alter the Anderson localization length. This is in strong contrast with the effect of internal decoherence which is found to suppress Anderson localization. We also address quantum transport in chains with particle losses which are of considerable interest for laboratory experiments with cold atoms.

Two-dimensional (2D) Janus systems have garnered significant scientific interest owing to their novel properties and potential applications. The growing interest in these materials is driven by the idea that their structural asymmetry offers unprecedented opportunities for enhancing thermoelectric performance and unlocking groundbreaking advancements in energy conversion and waste heat utilization. In this context, we present a comprehensive study on the structural, vibrational, electronic, thermal, and thermoelectric properties of Janus ${\mathrm{Ge}}_2\mathrm{P}X(X=\mathrm{N},\mathrm{As},\mathrm{Sb},\mathrm{and}\mathrm{Bi})$ monolayers, using first-principles calculations combined with the Landauer formalism. The suggested configurations exhibit dynamical stability and retain structural integrity even at elevated temperatures. Electronic structure calculations employing hybrid functionals (HSE06) with spin-orbit coupling reveal that ${\mathrm{Ge}}_2\mathrm{PAs}$ and ${\mathrm{Ge}}_2\mathrm{PSb}$ monolayers exhibit anisotropic characteristics as indirect semiconductors, while ${\mathrm{Ge}}_2\mathrm{PN}$ and ${\mathrm{Ge}}_2\mathrm{PBi}$ exhibit metallic behavior. We also compare the thermal, electronic, and thermoelectric transport properties of these proposed monolayers to binary 2D GeP in the ballistic limit. Notably, both ${\mathrm{Ge}}_2\mathrm{PAs}$ and ${\mathrm{Ge}}_2\mathrm{PSb}$ exhibit $n$-type figure of merit ($ZT$) values exceeding 1 at 800 K, with their $n$-type $ZT$ values surpassing that of GeP at room temperature. Our analysis underscores the distinctive structural and electronic properties of ${\mathrm{Ge}}_2\mathrm{PAs}$ and ${\mathrm{Ge}}_2\mathrm{PSb}$ monolayers, accompanied by their highly promising thermoelectric performance. These findings position them as strong candidates for energy harvesting and conversion applications.

Capitalizing on recent work, which clarifies the consistent use of bosonization-debosonization methods to study Kondo-type quantum impurity models even in nonequilibrium settings, we revisit the compactification procedure of the two-channel Kondo model (by which it is rewritten more “compactly” using the single-channel version of the model and exploiting separation and duality between spin and charge) and uncover some hidden approximations that could limit its range of validity. This complements and extends, for two or any even number of channels, beyond previous work on the Toulouse limit of these models, and reinforces the need for the use of an extended framework in these calculations. We carry out a number of exact comparisons between the different models and show that keeping track of the so-called consistency factors leads to full agreement between the compactified and original versions of the model.

Following up on the systematic compactification of the two-channel Kondo model [and its multichannel extensions; see Ljepoja et al., Phys. Rev. B 110, 045108 (2024)] and the demonstration of its validity over the past proposal of compactification, we resort to a study of scaling using Anderson's simple poor man's procedure to carry out a comparative study of these two and the original model. By doing so we unveil a universal agreement among the three models in how they flow upon scaling, and suggest the general limits of such a concordance. In this way we further elucidate the conditions under which the standard simplifications implicit in many bosonization-based mappings (particularly of quantum impurity models) can be used reliably, and when the consistent bosonization-debosonization approach is needed.

We carry out a field-theoretical renormalization group procedure based on the Callan-Symanzik equation to calculate the detailed flow for the (multi) two-channel Kondo model and its compactified versions. In doing so, we go beyond the universal terms in the beta function we obtained using poor man's scaling [see Phys. Rev. B 110, 045109 (2024)] and culminate our analysis of how the compactified versions of the model fare against the original one. Among other results, we explore the large-channel-number limit and extend our considerations to the finite temperature crossover region. Moreover, we gain insights into the contradistinction between the consistent vs conventional bosonization-debosonization formalisms, thereby advancing our understanding on multiple fronts. In particular, we make use of renormalization-flow arguments to further justify the consistent refermionization of the parallel Kondo interaction we presented earlier [see Phys. Rev. B 110, 045108 (2024)].

The recent studies on the anomalous Hall effect (AHE) have revealed an intrinsic relationship between the topological band structure and the experimentally observed transverse conductivity. Consequently, this has led to a heightened focus on examining the topological aspects of AHE. Here, an in-depth study of temperature driven sign reversal of anomalous Hall conductivity in the single crystalline MnPtGa (space group: $P{6}_3/mmc$) has been presented. From the interdependence of the linear and anomalous Hall resistivity, the origin of AHE is confirmed to be of intrinsic type. By systematically studying the electronic band structure and Berry curvature of MnPtGa using first principle calculations supported by temperature and magnetic field dependent magnetization measurements, it is concluded that the temperature dependent complex magnetic structure plays a significant role and leads to the sign reversal of anomalous Hall conductivity. Supported by the $ab initio$ calculations, a continuous evolution of the magnetic structure is proposed, which is consistent with the experimental data. This study has established that the critical temperature ($\approx 110$ K), where the sign reversal appears is strongly associated with the magnetic structure and the magnitude of Mn moments.

Most nonperturbative numerical studies of electron-phonon interactions focus on model Hamiltonians where the electrons interact with a phonon branch via a single type of microscopic mechanism. Two commonly explored couplings in this context are the Holstein and Su-Schrieffer-Heeger (SSH) interactions, which describe phonons modulating the on-site energy and intersite electron hopping, respectively. Many materials, however, have multiple phonon branches that can each interact with electronic degrees of freedom in different ways. We present here a determinant quantum Monte Carlo study of the half-filled two-dimensional (bond) SSH-Holstein Hamiltonian, where electrons couple to different phonon branches via either the Holstein or SSH mechanism. We map the model's phase diagram and determine the nature of the transitions between charge-density wave, bond-order wave, and antiferromagnetic order.

We discuss the charge polarization in a generalized Rice-Mele model at arbitrary particle filling per site as a model of charge-ordered systems in one dimension. The model possesses neither the conventional bond-centered inversion symmetry nor the one site translation symmetry alone but has combinations of these symmetries. We show that the charge polarization in the ground state is quantized by the combined symmetry and is characterized solely by the filling. Especially, the polarization can be $1/2$ (mod 1) in the zero filling limit. Under the open boundary condition, there exist excess charges accumulated near edges of the system irrespective of existence or absence of edge modes. Correspondingly, we decompose the polarization into a bulk contribution and an edge contribution and numerically demonstrate that the polarization is dominated by the former (latter) when the energy gap is small (large). We also discuss a simple generalization of our model and examine absence/existence of a gapless edge mode protected by the inversion symmetry by introducing intra unit cell and inter unit cell contributions of the charge polarization.

An experiment in moiré ${\mathrm{MoTe}}_2$ bilayers reported the observation of a topologically ordered state with zero Hall conductivity and half of the edge conductance of a standard time-reversal invariant quantum spin Hall insulator [K. Kang et al., Nature (London) 628, 522 (2024)]. This state is believed to emerge at total filling one of a pair of bands with Chern numbers $C=\pm 1$ related by time-reversal symmetry. By viewing these bands as a pair of Landau levels with opposite magnetic fields, and starting from a parent magnet with one filled band, we demonstrate that a class of Halperin states constructed by adding particles to the empty Chern band and holes to the occupied Chern band have all the properties observed in ${\mathrm{MoTe}}_2$. Remarkably, these states break time-reversal symmetry but have exactly zero Hall conductivity and helical edge conductance of $e^2/2h$. These states also feature a spinless composite fermion with the same charge as the electron but split equally between both valleys. In a standard Halperin 331 state, this particle would be a neutral Bogoliubov composite fermion. However, in our context this composite fermion is charged but remains itinerant because it is split into the two valleys that effectively experience opposite magnetic fields. The existence of such charged itinerant particles is a key difference between Landau levels with opposite magnetic fields and standard multicomponent Landau levels, where all the itinerant particles are charge neutral, such as the magnetoroton of the Laughlin state or the Bogoliubov composite fermion of the Moore-Read state. When the electron density changes away from the ideal filling and these itinerant charged particles are added to the parent state, the disorder potential is less efficient at localizing them as compared to standard Landau levels. This can explain why the state reported in K. Kang et al. [Nature (London) 628, 522 (2024)] did not display a robust Hall plateau upon changing the electron density.

Recently, a quantum anomalous Hall (QAH) state was observed in AB-stacked moiré ${\mathrm{MoTe}}_2/{\mathrm{WSe}}_2$ heterobilayers at half filling. More recent layer-resolved magnetic circular dichroism measurements revealed that spin-polarized moiré bands from both the ${\mathrm{MoTe}}_2$ and the ${\mathrm{WSe}}_2$ layers are involved at the formation of the QAH state. This scenario is not expected by existing theories. In this work, we suggest that the observed QAH state is a new state of matter, namely, a topological $p_x+i{p}_y$ intervalley coherent state. We point out that the massive Dirac spectrum of the ${\mathrm{MoTe}}_2$ moiré bands, together with the Hund's interaction and the Coulomb interactions, gives rise to this novel QAH state. Through a self-consistent Hartree-Fock analysis, we study how this $p_x+i{p}_y$-pairing phase could appear. Besides explaining several key features of the experiments, our theory predicts that the order parameter would involve the pairing of electrons and holes with a definite momentum mismatch such that the pairing would generate a new unit cell which is three times the size of the original moiré unit cell, due to the order parameter modulations.

Electromagnetic absorbers serve as fundamental components for a wide range of applications, encompassing energy and heat management, sensing, and communications. In this study, we explore several complex permittivity combinations for lithography-free material-reflector and material-spacer-reflector configurations that lead to perfect absorption peaks across distinct permittivity regimes and varying thicknesses. We provide an extensive analysis of angle and polarization dependencies, specifically using silicon carbide as an illustrative example. Our findings reveal the potential for harnessing different absorption regimes within a single device, thus enabling the realization of multiband absorption capabilities. Furthermore, we demonstrate perfect absorption linked with extreme values of permittivity, and we find the conditions to get perfect absorption in ultrathin slabs. In addition, we carry out an analysis about the response at oblique incidence, and we identify a particular mode in the negative permittivity region and its hybridization with epsilon-near-zero modes at oblique incidence. This investigation serves as a valuable standardization of absorber design, offering insights to develop perfect absorbers for infrared applications such as thermal emission, communications, and sensing.

The low-symmetry and anistropic lattice of $1{\mathrm{T}}^{\prime }{\mathrm{WTe}}_2$ is responsible for the existence of parallel one-dimensional channels in the moiré patterns of twisted bilayers. This gives the opportunity to explore moiré physics of a different nature to that widely observed in twisted bilayers of materials with hexagonal symmetries. Here, we combine plane-wave and linear-scaling density functional theory calculations to describe the electronic properties of twisted bilayer $1{\mathrm{T}}^{\prime }{\mathrm{WTe}}_2$. We investigate the strain dependence of both aligned bilayer configurations across a range of stacking geometries, and of the twisted bilayer itself. For a small change in the lattice parameters of the constituent $1{\mathrm{T}}^{\prime }{\mathrm{WTe}}_2$ monolayers, we find a very substantial increase in the moiré-induced striped electrostatic potential landscape in the twisted bilayer. We measure a peak-to-trough magnitude $>200$ meV between parallel linear channels, sufficient to induce Luttinger liquid behavior.

Using spatially and temporally resolved microphotoluminescence, the effect of a strong magnetic field on the hydrodynamic properties of an electron-hole fluid is studied. The recombination rate of photogenerated holes is related to the rate of their diffusion. In this way, the electronic Venturi effect is observed in a mesoscopic variable-width GaAs channel and is used as a tool to probe the hydrodynamic response. It is shown that a quantizing magnetic field leads to the disappearance of the Venturi effect, which means that electron-hole fluid becomes nonhydrodynamic. The observed transition from the hydrodynamic regime to the nonhydrodynamic one is due to the magnetic field–induced decrease in the electron-electron scattering rate and is similar in nature to viscous negative magnetoresistance.

Two-dimensional transition metal dichalcogenide heterostructures provide a unique opportunity for quantum engineering of electronic and excitonic states at the nanoscale. Critical optical properties of interlayer excitons, including transition energy, optical selectivity, and quantum yield, are strongly correlated to the stacking orders. However, these optical properties could vary from sample to sample, setting an obstacle to extracting the intrinsic stacking order dependence experimentally. We report an effective method to fabricate heterobilayers with both stacking orders obtained on a single device. The sharp difference of interlayer excitons induced by the stacking orders was unambiguously identified, including emission wavelength, valley polarization, and temperature dependence of quantum yield. This method provides a flexible platform to study stacking order dependence of heterobilayer excitons, and can be readily applied to explore the layer hybridization, strong correlations, and exciton diffusion that are sensitive to stacking order.

The three-dimensional (3D) quantum Hall effect (QHE) has been intensely studied in Weyl semimetals and Dirac semimetals in recent years. Here, we study the 3D QHE in nodal-line semimetal through calculating the Hall conductivity numerically in a nodal-line semimetal slab. It is found that the Hall conductivity in nodal-line semimetal is anisotropic with respect to the magnetic field, Fermi energy, and sample thickness when the magnetic field is perpendicular to or parallel to the nodal line. The Hall conductivity is symmetric with respect to the energy of the nodal line in both cases. When the Fermi energy deviates from the nodal line, a Hall plateau appears and the plateau is wider and higher for the magnetic field parallel to the nodal line than perpendicular to the nodal line. For magnetic field dependence, the Hall conductivity follows $1/B$ dependence when the magnetic field is perpendicular to the nodal line, while it is independent on the magnetic field parallel to the nodal line. Moreover, the Hall conductivity as a function of sample thickness is independent of the sample thickness for the magnetic field perpendicular to the nodal line, while it is a increasing function for the magnetic field parallel to the nodal line. We give a possible explanation for these anisotropies from the energy spectrum of Landau bands under the magnetic field. This unique anisotropic 3D QHE can be a transport feature to recognize the topological nodal-line semimetals.

Since the seminal works in the '50s, it has been known that a bath of noninteracting electrons mediates an interaction between local moments (such as nuclear spins or magnetic impurities) coupled to distant sites of the lattice. Recent efforts in simultaneous control of multiqubit arrays rely on defining quantum dots in environments which likewise contain electrons capable of mediating effective interactions. It is therefore interesting to analyze systems with more than two impurities in higher orders of perturbation theory. Here we derive and discuss the significance of the effective spin interactions between four impurities in fourth order of the coupling between the impurities and the substrate, mediated by conduction electrons of a two-dimensional square lattice. This effective interaction resembles the ring exchange—a coupling of four spins appearing in perturbative treatment of the Hubbard model. Despite it being a higher-order effect, it can dramatically influence the magnetic ordering. We show that as an effective interaction between impurity spins, the ring exchange can also compete with the more familiar two-spin interactions.

We analyze the transport properties of a Cooper pair splitter device composed of two-point electrodes in contact with a ferromagnetic/superconductor (F/S) junction constructed on the surface of a topological insulator (TI). For the pair potential in the S region, we consider $s$- and $d$-wave symmetries, while for the F region, we focus on a magnetization vector normal to the TI surface. Nonlocal transport along the F/S interface is mediated by chiral Majorana edge states, with chirality controlled by the polarization of the magnetization vector. We demonstrate that crossed Andreev reflections slowly decay with the separation of the electrodes in standard clean samples. Our system exhibits a maximum Cooper-pair-splitting efficiency of 80% for a symmetrical voltage configuration, even in high-temperature superconductor devices.