Showing papers in "Physical Review B in 2023"
TL;DR: In this article , the diffusion process of negatively charged excitons (trions) in WSe2 transition metal dichalcogenide monolayer was investigated, and the authors measured time-resolved photoluminescence spatial profiles of these excitonic complexes which exhibit a non-linear diffusion process with an effective negative diffusion behavior.
Abstract: We investigate the diffusion process of negatively charged excitons (trions) in WSe2 transition metal dichalcogenide monolayer. We measure time-resolved photoluminescence spatial profiles of these excitonic complexes which exhibit a non-linear diffusion process with an effective negative diffusion behavior. Specifically, we examine the dynamics of the two negatively charged bright excitons (intervalley and intravalley trion) as well as the dark trion. The time evolution allows us to identify the interplay of different excitonic species: the trionic species appear after the neutral excitonic ones, consistent with a bimolecular formation mechanism. Using the experimental observations, we propose a phenomenological model suggesting the coexistence of two populations: a first one exhibiting a fast and efficient diffusion mechanism and a second one with a slower dynamics and a less efficient diffusion process. These two contributions could be attributed to hot and cold trion populations.
3 citations
TL;DR: In this article , the authors proposed entanglement measures, fractionalization, quantum correlations in quantum information, and spin chains for quantum information in 1-dimensional systems, and applied them to condensed matter, materials and applied physics.
Abstract: Received 2 February 2023DOI:https://doi.org/10.1103/PhysRevB.107.059902©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasEntanglement measuresFractionalizationQuantum correlations in quantum informationQuantum entanglementPhysical Systems1-dimensional systemsTechniquesInelastic neutron scatteringSpin chainsQuantum InformationCondensed Matter, Materials & Applied Physics
3 citations
TL;DR: In this paper , the first three coefficients for the longitudinal and transverse correlation functions for the uniaxial magnetocrystalline anisotropy of ferromagnetic nanoparticles with a longitudinal magnetic field are derived.
Abstract: The method of moments is developed and employed to analyze the equilibrium correlation functions of the magnetization of ferromagnetic nanoparticles in the case of inertial magnetization dynamics. The method is based on the Taylor series expansion of the correlation functions and the estimation of the expansion coefficients. This method significantly reduces the complexity of analysis of equilibrium correlation functions. Analytical expressions are derived for the first three coefficients for the longitudinal and transverse correlation functions for the uniaxial magnetocrystalline anisotropy of ferromagnetic nanoparticles with a longitudinal magnetic field. The limiting cases of very strong and negligibly weak external longitudinal fields are considered. The Gordon sum rule for inertial magnetization dynamics is discussed. In addition, we show that finite analytic series can be used as a simple and satisfactory approximation for the numerical calculation of correlation functions at short times.
2 citations
TL;DR: In this article , the authors investigated the twist-angle and gate dependence of the proximity-induced exchange coupling in the monolayer transition-metal dichalcogenides (TMDCs) MoSe$_2$ and WSe¼2$ due to the vdW coupling to the ferromagnetic semiconductor CrI$_3$, from first-principles calculations.
Abstract: We investigate the twist-angle and gate dependence of the proximity-induced exchange coupling in the monolayer transition-metal dichalcogenides (TMDCs) MoSe$_2$ and WSe$_2$ due to the vdW coupling to the ferromagnetic semiconductor CrI$_3$, from first-principles calculations. A model Hamiltonian, that captures the relevant band edges at the $K/K^{\prime}$ valleys of the proximitized TMDCs, is employed to quantify the proximity-induced exchange. Upon twisting from 0{\deg} to 30{\deg}, we find a transition of the TMDC valence band (VB) edge exchange splitting from about $-2$ to $2$ meV, while the conduction band (CB) edge exchange splitting remains nearly unchanged at around $-3$ meV. For the VB of WSe$_2$ (MoSe$_2$) on CrI$_3$, the exchange coupling changes sign at around 8{\deg} (16{\deg}). We find that even at the angles with almost zero spin splittings of the VB, the real-space spin polarization profile of holes at the band edge is highly non-uniform, with alternating spin up and spin down orbitals. Furthermore, a giant tunability of the proximity-induced exchange coupling is provided by a transverse electric field of a few V/nm. We complement our \textit{ab initio} results by calculating the excitonic valley splitting to provide experimentally verifiable optical signatures of the proximity exchange. Specifically, we predict that the valley splitting increases almost linearly as a function of the twist angle. Furthermore, the proximity exchange is highly tunable by gating, allowing to tailor the valley splitting in the range of 0 to 12 meV in WSe$_2$/CrI$_3$, which is equivalent to external magnetic fields of up to about 60 Tesla. Our results highlight the important impact of the twist angle and gating when employing magnetic vdW heterostructures in experimental geometries.
2 citations
TL;DR: In this paper , the authors map quantum Hall edge states along an electrostatically defined potential step in graphene using scanning tunneling microscopy, which enables investigating the edge states while avoiding tip-induced quantum dots within the sample.
Abstract: Edge states are the fingerprint of topological materials, specifically of the quantum Hall effect. The authors map quantum Hall edge states along an electrostatically defined potential step in graphene using scanning tunneling microscopy. Tight-binding simulations reproduce the measurements and allow for identifying a parameter range that minimizes the influence of the tip. This enables investigating the edge states while avoiding tip-induced quantum dots within the sample. The image shows a measured edge state featuring the expected antinodal patterns as it meanders along the interface.
2 citations
TL;DR: In this article , a double-band-folding strategy was proposed to achieve high-Q leaky modes in compound lattices, exemplified with a one-dimensional grating and a two-dimensional zigzag array of dielectric disks.
Abstract: Guided modes in photonic structures, with broadband infinite-quality factors, are inaccessible from the far field due to the momentum mismatch. They become leaky modes in the continuum when the mismatch is compensated for by introducing periodic perturbations to form photonic structures. However, the quality factors (Q factors) of such leaky modes deteriorate significantly in most regions of the $k$-space except at a few discrete high-symmetry points. It is an intriguing question as to whether guided modes can hop above the light cone and yet maintain high Q. Here, we propose a double-band-folding strategy to achieve high-Q leaky modes in compound lattices, exemplified with a one-dimensional grating and a two-dimensional zigzag array of dielectric disks. The Q factor of those leaky modes can be made ultrahigh at arbitrarily any incident angles, showing that such modes do not originate from bound states in the continuum (BICs) above the light cone. Our findings provide unique insight for elucidating the relations between guided modes, BICs, quasi-BICs, and radiation. They further provide a generalized recipe for numerous optical applications such as all-dielectric sensing, lasing, and nonlinear generation with multiple inputs.
2 citations
TL;DR: In this article , the edge phase diagram of non-Hermitian Chern insulators is analyzed using the notion of the generalized Brillouin zone (GBZ) and the asymptotic properties of block Toeplitz matrices.
Abstract: Non-Hermitian Chern insulators differ from their Hermitian cousins in one key aspect: their edge spectra are incredibly rich and confounding. For example, even in the simple case where the bulk spectrum consists of two bands with Chern number $\pm 1$, the edge spectrum in the slab geometry may have one or two edge states on both edges, or only at one of the edges, depending on the model parameters. This blatant violation of the familiar bulk-edge correspondence casts doubt on whether the bulk Chern number can still be a useful topological invariant, and demands a working theory that can predict and explain the myriad of edge spectra from the bulk Hamiltonian to restore the bulk-edge correspondence. We outline how such a theory can be set up to yield a thorough understanding of the edge phase diagram based on the notion of the generalized Brillouin zone (GBZ) and the asymptotic properties of block Toeplitz matrices. The procedure is illustrated by solving and comparing three non-Hermitian generalizations of the Qi-Wu-Zhang model, a canonical example of two-band Chern insulators. We find that, surprisingly, in many cases the phase boundaries and the number and location of the edge states can be obtained analytically. Our analysis also reveals a non-Hermitian semimetal phase whose energy-momentum spectrum forms a continuous membrane with the edge modes transversing the hole, or genus, of the membrane. Subtleties in defining the Chern number over GBZ, which in general is not a smooth manifold and may have singularities, are demonstrated using examples. The approach presented here can be generalized to more complicated models of non-Hermitian insulators or semimetals in two or three dimensions.
2 citations
TL;DR: In this article , the role of next-nearest-neighbor (NNN) hopping in the quasiperiodic many-body localization (MBL) model was investigated.
Abstract: We study many-body localization (MBL) in the quasiperiodic ${t}_{1}\text{\ensuremath{-}}{t}_{2}$ model, focusing on the role of next-nearest-neighbor (NNN) hopping ${t}_{2}$, which introduces a single-particle mobility edge. The calculated phase diagram can be divided into three distinct regimes, depending on the strength of the short-range interaction $U$. For weak interactions ($U\ensuremath{\ll}{t}_{1}$), this model is always nonthermal. For intermediate interactions ($U\ensuremath{\sim}{t}_{1}$), the thermal-MBL phase transition in this model is qualitatively the same as that of the Aubry-Andre (AA) model, which is consistent with existing experimental observations. For strong interactions $(U\ensuremath{\gg}{t}_{1})$, the NNN hopping produces qualitatively new physics because it breaks down the Hilbert space fragmentation present in the AA model. The NNN hopping is thus irrelevant when the interaction is intermediate but relevant for strong interactions.
1 citations
TL;DR: In this paper , the authors theoretically propose hybrid two-dimensional semiconductors functionalized by organic molecules as prototypes of excitonic insulators, with the exemplary candidate F6TCNNQ.
Abstract: The excitonic insulator is an elusive electronic phase exhibiting a correlated excitonic ground state. Materials with such a phase are expected to have intriguing properties such as excitonic high-temperature superconductivity. However, compelling evidence on the experimental realization is still missing. Here, we theoretically propose hybrids of two-dimensional semiconductors functionalized by organic molecules as prototypes of excitonic insulators, with the exemplary candidate ${\mathrm{WS}}_{2}$-F6TCNNQ. This material system exhibits an excitonic insulating phase at room temperature with a ground state formed by a condensate of interlayer excitons. To address an experimentally relevant situation, we calculate the corresponding phase diagram for the important parameters: temperature, gap energy, and dielectric environment. Further, to guide future experimental detection, we show how to optically characterize the different excitonic phases via far-infrared to terahertz spectroscopy valid also for monolayer materials.
1 citations
TL;DR: In this article , an extended quadrupole topological insulator with couplings between adjacent atoms is proposed, where the couplings can also give rise to higher-order corner states.
Abstract: Non-Hermiticity can alter the topological phases of Hermitian lattices and induce gapped edge states and in-gap corner states for the topologically trivial structures in Hermitian cases. To date, however, the realizations of non-Hermiticity-induced higher-order topological phases have often needed intricate designs, where the non-Hermitian systems on a square lattice are comprised of 16 atoms per unit cell with staggered on-site non-Hermitian components at least. It is still necessary to further investigate their topological characteristics in both Hermitian and non-Hermitian systems. In this paper, we suggest an extended quadrupole topological insulator that contains eight atoms in each unit cell and whose topological phases in Hermitian cases are governed by the couplings between adjacent atoms. By introducing external losses into such a higher-order topological insulator, we show that therein the trivial lattices can also give rise to nontrivial quadrupole topology with higher-order corner states. Calculating the biorthogonal nested Wilson loop characterizes the topology for such a non-Hermitian system. Interestingly, the non-Hermitian system has four Wannier bands, with two near zero and two deviating from zero and distributing symmetrically in the negative and positive sections, implying a topology similar to the nontrivial phase in the Hermitian case. Furthermore, both the positive and negative Wannier bands exhibit a nontrivial polarization of 0.5 whereas the remain Wannier bands near zero carry a trivial polarization, indicating that the non-Hermitian system hosts a nontrivial quadrupole topology. We also propose a complete design with acoustic resonators to implement the homologous non-Hermitian topology in acoustic systems to validate our theory. The numerical results match the analytical solution perfectly. Our work introduces an alternative method for fabricating non-Hermitian-induced quadrupole topological insulators and paves the way for future research into the non-Hermitian higher-order topology.
1 citations
TL;DR: In this paper , the magnetization dynamics following short-pulse excitations is described in the exchange-only approximation for weak to moderate interactions, for stronger interactions and near transitions between magnetically ordered and frustrated phases, exchange and correlation torques tend to compensate each other.
Abstract: In spin-density-functional theory (SDFT) for noncollinear magnetic materials, the Kohn-Sham system features exchange-correlation (xc) scalar potentials and magnetic fields. The significance of the xc magnetic fields is not very well explored; in particular, they can give rise to local torques on the magnetization, which are absent in standard local and semilocal approximations. Exact benchmark solutions for a five-site extended Hubbard lattice at half filling and in the presence of spin-orbit coupling are compared with SDFT results obtained using orbital-dependent exchange-only approximations. The magnetization dynamics following short-pulse excitations is found to be reasonably well described in the exchange-only approximation for weak to moderate interactions. For stronger interactions and near transitions between magnetically ordered and frustrated phases, exchange and correlation torques tend to compensate each other and must both be accounted for.
TL;DR: In this article , a theoretical investigation of high-order harmonic generation (HHG) in silicon thin films was conducted to elucidate the effect of light propagation in reflected and transmitted waves, and it was found that the intensity of transmission HHG gradually decreases with the thickness, while the reflection HHG becomes constant from a certain thickness.
Abstract: A theoretical investigation is conducted of high-order harmonic generation (HHG) in silicon thin films to elucidate the effect of light propagation in reflected and transmitted waves. The first-principles simulations are performed of the process in which an intense pulsed light irradiates silicon thin films up to $3\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ thickness. Our simulations are carried within the time-dependent density functional theory (TDDFT) with the account of coupled dynamics of the electromagnetic fields and the electronic motion. It was found that the intensity of transmission HHG gradually decreases with the thickness, while the reflection HHG becomes constant from a certain thickness. Detailed analyses show that transmission HHG has two origins: the HHG generated near the front edge and propagates to the back surface and the HHG generated near the back edge and emitted directly. The dominating mechanism of the transmission HHG is found to depend on the thickness of the thin film and the frequency of the HHG. At the film thickness of $1\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$, the transmission HHG with the frequency below 20 eV is generated near the back edge, while that with the frequency above 20 eV is generated near the front edge and propagates from there to the back surface.
TL;DR: In this paper , the authors considered a 2-impurity Kondo system with spin exchange coupling within the conduction band and showed that for strong intraband spin correlations the competition of these correlations with Kondo spin screening stabilizes a metallic spin-liquid phase of the localized spins without geometric frustration.
Abstract: We consider a 2-impurity Kondo system with spin-exchange coupling within the conduction band. Our numerical renormalization group calculations show that for strong intraband spin correlations the competition of these correlations with Kondo spin screening stabilizes a metallic spin-liquid phase of the localized spins without geometric frustration. For weak Kondo coupling the spin liquid and the Kondo singlet phase are separated by two quantum phase transitions and an intermediate RKKY spin-dimer phase, while beyond a critical coupling they are connected by a crossover. The results suggest how a quantum spin liquid may be realized in heavy-fermion systems near a spin-density wave instability.
TL;DR: In this paper , a tunable conversion of terahertz radiation into a dc current in spatially modulated bilayer graphene was shown to be related to the so-called ratchet effect.
Abstract: We report on a tunable - by magnetic field and gate voltage - conversion of terahertz radiation into a dc current in spatially modulated bilayer graphene. We experimentally demonstrate that the underlying physics is related to the so-called ratchet effect. Our key findings are the direct observation of a sharp cyclotron resonance in the photocurrent and the demonstration of two effects caused by electron-electron interaction: the plasmonic splitting of the resonance due to long-range Coulomb coupling and the partial suppression of its second harmonic due to fast interparticle collisions. We develop a theory which perfectly fits our data. We argue that the ratchet current is generated in the hydrodynamic regime of non-ideal electron liquid.
TL;DR: In this article , an analytical derivation of the electron spectrum and wave functions for rectangular derivatives of graphene rectangular derivatives is presented, along with an exact analytical description of extended and localized states, the transition between them, and a special case of a localized state when the wave function is nonzero only at the edge sites.
Abstract: Properties of bulk and boundaries of materials can, in general, be quite different, both for topological and nontopological reasons. One of the simplest boundary problems to pose is the tight-binding problem of noninteracting electrons on a finite honeycomb lattice. Despite its simplicity, the problem is quite rich and directly related to the physics of graphene. We revisit this long-studied problem and present an analytical derivation of the electron spectrum and wave functions for graphene rectangular derivatives. We provide an exact analytical description of extended and localized states, the transition between them, and a special case of a localized state when the wave function is nonzero only at the edge sites. The latter state has zero energy, and we discuss its existence in zigzag nanoribbons, zigzag nanotubes with a number of sites along a zigzag edge divisible by 4, and rectangular graphene nanoflakes with an odd number of sites along both zigzag and armchair edges.
TL;DR: In this paper , a class of Hamiltonians that embody a substantial steady-state photon-magnon entanglement enabled by a chiral coupling of the magnonic system to the cavity electric field is discussed.
Abstract: Electromagnonics is an emerging field with a focus on entangling magnonic excitations to the microwave cavity photon modes with the prospect for use in quantum information science. Here, we discuss a class of Hamiltonians that embody a substantial steady-state photon-magnon entanglement enabled by a chiral coupling of the magnonic system to the cavity electric field. It is demonstrated how the entanglement can be controlled via external parameters. As a realization, we study a layered system that hosts an interfacial Dzyaloshinskii-Moriya interaction whose strength varies linearly with the cavity electric field rendering the low-energy spin excitations susceptible to an electric field and resulting in nonlinear magnon-photon dynamics. Accounting for interactions with the environment, we derive from the stochastic quantum Langevin equations explicit expressions evidencing the existence of a finite, steady-state entanglement and detailing its dependencies on external probes. The results point to particular types of electromagnonic systems that are potentially useful for quantum information applications.
TL;DR: In this paper , the role of non-Hermiticity in the Chern insulating Haldane model on a dice lattice was investigated and a phase diagram was constructed to identify and locate the occurrence of these exceptional points in the parameter space.
Abstract: The interplay of topology and non-Hermiticity has led to diverse, exciting manifestations in a plethora of systems. In this work, we systematically investigate the role of non-Hermiticity in the Chern insulating Haldane model on a dice lattice. Due to the presence of a nondispersive flat band, the dice-Haldane model hosts a topologically rich phase diagram with the nontrivial phases accommodating Chern numbers $\ifmmode\pm\else\textpm\fi{}2$. We introduce non-Hermiticity into this model in two ways---through balanced non-Hermitian gain and loss, and by nonreciprocal hopping in one direction. Both these types of non-Hermiticity induce higher-order exceptional points of order three. Remarkably, the exceptional points at high-symmetry points occur at odd integer values of the non-Hermiticity strength in the case of balanced gain and loss, and at odd integer multiples of $1/\sqrt{2}$ for nonreciprocal hopping. We substantiate the presence and the order of these higher-order exceptional points using the phase rigidity and its scaling. Furthermore, we construct a phase diagram to identify and locate the occurrence of these exceptional points in the parameter space. Non-Hermiticity has yet more interesting consequences on a finite-sized lattice. Unlike for balanced gain and loss, in the case of nonreciprocal hopping, the nearest-neighbor lattice system under periodic boundary conditions accommodates a finite, nonzero spectral area in the complex plane. This manifests as the non-Hermitian skin effect when open boundary conditions are invoked. In the more general case of the dice-Haldane lattice model, the non-Hermitian skin effect can be caused by both gain and loss or nonreciprocity. Fascinatingly, the direction of localization of the eigenstates depends on the nature and strength of the non-Hermiticity. We establish the occurrence of the skin effect using the local density of states, inverse participation ratio, and the edge probability and demonstrate its robustness to disorder. Our results place the dice-Haldane model as an exciting platform to explore non-Hermitian physics.
TL;DR: In this article , the authors analyzed how various types of asymmetry affect this picture and showed that the phase transitions are robust against the coupling and particle-hole asymmetries, provided charge transfer is forbidden.
Abstract: In a related work [arXiv:2106.07519] we have shown that in the two-impurity Anderson (2iA) model with two hosts coupled by spin exchange in the most symmetric case there are either two phase transitions or none. The phases comprise the conventional Kondo and RKKY regimes and a novel one, interpreted as a Kondo-stabilized, metallic quantum spin liquid (QSL). Here we analyze how various types of asymmetry affect this picture. We demonstrate that the transitions are robust against the coupling and particle-hole asymmetries, provided charge transfer is forbidden. This holds true despite the scattering phase shift at each impurity taking non-universal values. Finally, for an extended model including charge transfer between the hosts and a small Coulomb interaction at the host sites directly coupled to impurities, we show that the presence of charge transfer changes the phase transitions into crossovers. Provided the inter-host hopping is sufficiently small, this leads to qualitatively the same physics at non-zero temperature. The relevance of this model for rare-earth atoms in a metallic host is discussed and potential experimental setups for observing our findings are proposed.
TL;DR: In this article , an extensive analysis of weak antilocalization in the quantum interference regime is used as a powerful tool to obtain the microscopic information of spin-orbit interaction and the coherence phase breaking scattering process among itinerant electrons.
Abstract: Platinum (Pt) has been very much used for spin-charge conversion in spintronics research due to its large intrinsic spin-orbit interaction. Magnetoconductance originating from weak antilocalization in the quantum interference regime is used as a powerful tool to obtain the microscopic information of spin-orbit interaction and the coherence phase breaking scattering process among itinerant electrons. To acquire the knowledge of different types of scattering processes, we have performed a magnetoconductance study on Pt thin films which manifests multiband (multichannel) conduction. An extensive analysis of quantum-interference-originated weak antilocalization reveals the existence of strong (weak) interband scattering between two similar (different) orbitals. Coherence phase breaking lengths $({l}_{\ensuremath{\phi}})$ and their temperature dependence are found to be significantly different for these two conducting bands. The observed effects are consistent with the theoretical prediction that there exist three Fermi sheets with one $s$ and two $d$ orbital character. This study provides the evidence of two independent nonsimilar conducting channels and the presence of anisotropic spin-orbit interaction along with $e\text{\ensuremath{-}}e$ correlation in Pt thin films.
TL;DR: In this article , the authors consider three prototype systems simulated by tight-binding models, namely, a Su-Schrieffer-Heeger/NM, a Kitaev/NM and a Chern insulator/NM.
Abstract: When normal metals (NMs) are attached to topological insulators or topological superconductors, it is conceivable that the quantum states in these finite adjacent materials can intermix. In this case---and because the NM usually does not possess the same symmetry as the topological material---it is pertinent to ask whether zero-energy edge states in the topological layer are affected by the presence of the NM. To address this issue, we consider three prototype systems simulated by tight-binding models, namely a Su-Schrieffer-Heeger/NM, a Kitaev/NM, and a Chern insulator/NM. For all junctions investigated, we find that there exist trivial ``fine-tuned'' zero-energy states in the NM layer that can percolate into the topological region, thus mimicking a topological state. These zero-energy states are created by fine-tuning the NM chemical potential such that some of the NM states cross zero energy; they can occur even when the topological material is in the topologically trivial phase, and exist over a large region of the topological phase diagram. Interestingly, the true Majorana end modes of the Kitaev/NM model cannot be crossed by any NM state, as the NM metal layer in this case does not break particle-hole symmetry. On the other hand, when the chiral symmetry of the Su-Schrieffer-Heeger chain is broken by the attached NM, crossings are allowed. In addition, even in Chern insulators that do not preserve nonspatial symmetries, but the topological edge state self-generates a symmetry eigenvalue, such a fine-tuned zero-energy state can still occur. Our results indicate that when a topological material is attached to a metallic layer, one has to be cautious as to identify true topological edge states merely from their energy spectra and wave function profiles near the interface.
TL;DR: In this paper , the authors build an efficient microscopic framework to study electron-electron interaction driven superconductivities (SCs) in these extrinsic QCs, and find that their nature sits in between those of crystals and intrinsic QCs.
Abstract: The electron states in the quasicrystal (QC) are hot topics recently. While previous attentions were focused on such intrinsic QCs as the Penrose lattice, the recent ``twistronics'' provides us with a new type of QC, i.e., the extrinsic QC, including the ${30}^{\ensuremath{\circ}}$-twisted bilayer graphene and ${45}^{\ensuremath{\circ}}$-twisted bilayer cuprates as two synthesized examples, unifiedly dubbed as TB-QC. Here we build an efficient microscopic framework to study electron-electron interaction driven superconductivities (SCs) in these extrinsic QCs, and find that their nature sits in between those of crystals and intrinsic QCs. Remarkably, our microscopic calculations on the three exemplar TB-QCs reveal various novel topological SCs carrying high angular momenta and high Chern numbers protected by their unique QC symmetries, absent in conventional crystalline materials. The nature of SCs in these extrinsic QCs is also fundamentally different from those in intrinsic QCs in the aspect of pairing-symmetry classifications and topological properties.
TL;DR: In this article , it was shown that the motion of a single hole induces the nearest neighbor resonating-valence-bond ground state in the Hubbard model on a triangular cactus, a treelike variant of a kagome lattice.
Abstract: We prove that the motion of a single hole induces the nearest-neighbor resonating-valence-bond ground state in the $U=\ensuremath{\infty}$ Hubbard model on a triangular cactus---a treelike variant of a kagome lattice. The result can be easily generalized to $t\ensuremath{-}J$ models with antiferromagnetic interactions $J\ensuremath{\ge}0$ on the same graphs. This is a weak converse of Nagaoka's theorem of ferromagnetism on a bipartite lattice.
TL;DR: In this article , the authors extend the methods of topological band theory to photonic crystals formed by inclusions that are subject to a spacetime rotating-wave modulation that imitates a physical rotating motion.
Abstract: Topological photonics has recently emerged as a very general framework for the design of unidirectional edge waveguides immune to back-scattering and deformations, as well as other platforms that feature extreme nonreciprocal wave phenomena. While the topological classification of time invariant crystals has been widely discussed in the literature, the study of spacetime crystals formed by time-variant materials remains largely unexplored. Here, we extend the methods of topological band theory to photonic crystals formed by inclusions that are subject to a spacetime rotating-wave modulation that imitates a physical rotating motion. By resorting to an approximate nonhomogeneous effective description of the electromagnetic response of the inclusions, it is shown that they possess a bianisotropic response that breaks the time-reversal symmetry and may give rise to non-trivial topologies. In particular, we propose an implementation of the Haldane model in a spacetime modulated photonic crystal.
TL;DR: In this article , the authors studied the magnetic structure of compounds with antiferromagnetic ground states in the presence of mixed orbital manganese ions using powder neutron diffraction measurements supported by magnetization data.
Abstract: The ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ series of compounds with antiferromagnetic ground states ($x\ensuremath{\ge}1/2$) have been extensively studied due to the novel spin, orbital, and charge-ordering states observed when the calcium concentration is a simple fraction ($x=1/2,\phantom{\rule{0.28em}{0ex}}2/3$, and 3/4). The ground states of these compositions have been explained by the Goodenough charge, orbital, and spin ordering model. An important issue remaining is the elucidation of how the ground state changes when $x$ is not a simple number. Here we study the magnetic structure of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ for $0.51\ensuremath{\le}x\ensuremath{\le}0.69$ using powder neutron diffraction measurements supported by magnetization data. For compositions with $0.51\ensuremath{\le}x\ensuremath{\le}0.56$, the magnetic structure, which we term as an incommensurate charge exchange (CE) structure can be described by two propagation vectors ${\mathbf{k}}_{\mathrm{C}}=[1/2,0,1/2]$ and ${\mathbf{k}}_{\mathrm{E}}=[{\ensuremath{\varepsilon}}_{\mathrm{E}},0,1/2]$. In the second one, the component parallel to the ${\mathbf{a}}^{*}$ axis of the reciprocal lattice changes with the ${\mathrm{Mn}}^{4+}$ concentration $x$ as ${\ensuremath{\varepsilon}}_{\mathrm{E}}\ensuremath{\approx}x\ensuremath{-}1/2$ providing, thus, an unambiguous signature of the adoption of an incommensurate magnetic structure. As $x$ gradually increases, the diffraction data reveal that two magnetic phases---one adopting the incommensurate CE, and one adopting the commensurate ``2/3'' magnetic structure--co-exist in the concentration regime of $0.57\ensuremath{\le}x\ensuremath{\le}0.61$. Around the simple fraction $x=2/3$, the magnetic structure can be also described by three propagation vectors, the commensurate ${\mathbf{k}}_{\mathrm{E}}=[0,0,1/2]$, ${\mathbf{k}}_{\mathrm{C}}=[1/2,0,1/2]$, and an incommensurate ${\mathbf{k}}_{2/3}=[1/3+{\ensuremath{\varepsilon}}_{2/3},0,1/2]$ propagation vector with ${\ensuremath{\varepsilon}}_{2/3}$ taking negative/zero/positive values for $x$ smaller than/equal to/larger than 2/3, respectively. Our experimental results for $0.51\ensuremath{\le}x\ensuremath{\le}0.56$ are neither in favor of a stripe structure consisting of a fine mixture of $x=1/2$ and $x=2/3$ phases (phase separation) nor of a defect structure in which an appropriate amount of ${\mathrm{Mn}}^{3+}$-O sheets have been replaced by ${\mathrm{Mn}}^{4+}$-O sheets (defect structure). A sinusoidal modulated structure has been used as a possible candidate in explaining the experimental neutron diffraction magnetic Bragg peaks. This result may be linked to the presence of a mixed orbital state of the manganese ions.
TL;DR: In this paper , the authors identify anomalously strong excitonic effects in several vacancy-ordered double perovskites Cs2MX6 (M = Ti, Zr; X = I, Br).
Abstract: Using first-principles GW plus Bethe-Salpeter equation calculations, we identify anomalously strong excitonic effects in several vacancy-ordered double perovskites Cs2MX6 (M = Ti, Zr; X = I, Br). Giant exciton binding energies about 1 eV are found in these moderate-gap, inorganic bulk semiconductors, pushing the limit of our understanding of electron-hole (e-h) interaction and exciton formation in solids. Not only are the exciton binding energies extremely large compared with any other moderate-gap bulk semiconductors, but they are also larger than typical 2D semiconductors with comparable quasiparticle gaps. Our calculated lowest bright exciton energy agrees well with the experimental optical band gap. The low-energy excitons closely resemble the Frenkel excitons in molecular crystals, as they are highly localized in a single [MX6]2- octahedron and extended in the reciprocal space. The weak dielectric screening effects and the nearly flat frontier electronic bands, which are derived from the weakly bonded [MX6]2- units, together explain the significant excitonic effects. Spin-orbit coupling effects play a crucial role in red-shifting the lowest bright exciton by mixing up spin-singlet and spin-triplet excitons, while exciton-phonon coupling effects have minor impacts on the strong exciton binding energies.
TL;DR: In this paper , a high pressure yttrium allotrope was synthesized in a multi-anvil press at 20 GPa and 2000 K which is recoverable to ambient conditions.
Abstract: A high-pressure (HP) yttrium allotrope, $hP3$-Y (space group $P6/mmm$), was synthesized in a multi-anvil press at 20 GPa and 2000 K which is recoverable to ambient conditions. Its relative stability and electronic properties were investigated using density functional theory calculations. A $hP3$-Y derivative hydride, $hP3\ensuremath{-}\mathrm{YH}x$, with a variable hydrogen content ($x=2.8$, 3, 2.4), was synthesized in diamond anvil cells by the direct reaction of yttrium with paraffin oil, hydrogen gas, and ammonia borane upon laser heating to \ensuremath{\sim}3000 K at 51, 45 and 38 GPa, respectively. Room-temperature decompression leads to gradual reduction and eventually the complete loss of hydrogen at ambient conditions. Isostructural $hP3\ensuremath{-}\mathrm{NdH}x$ and $hP3\ensuremath{-}\mathrm{GdH}x$ hydrides were synthesized from Nd and Gd metals and paraffin oil, suggesting that the $hP3$-Y structure type may be common for rare-earth elements. Our results expand the list of allotropes of trivalent lanthanides and their hydrides and suggest that they should be considered in the context of studies of HP behavior and properties of this broad class of materials.
TL;DR: In this paper , it was shown that even for a spin chain of four qubits, rich regime structures can be established by observing signatures of the discrete time crystal and the Floquet symmetry-protected topological regime both distinct from the thermal regime.
Abstract: We demonstrate that exciting possible realizations of quantum Floquet matter are within reach for modern silicon spin qubits based in quantum dots, most notably the discrete time crystal (DTC). This is significant given that spin qubits have fallen behind other qubit architectures in terms of size and control. However, silicon spin qubits are especially well suited to this task as the charge noise that usually foils gate operations can now be leveraged as an asset in this time-crystal realization. We illustrate differences between prethermal phenomena and true time-crystalline spatiotemporal order. We demonstrate that even for a spin chain of four qubits, rich regime structures can be established by observing signatures of the discrete time crystal and the Floquet symmetry-protected topological regime both distinct from the thermal regime. We also analyze the persistence of these signatures at longer chain lengths, showing that the DTC lifetime grows exponentially with the system length and that these signatures may even be detectable for chains as small as three qubits. We also discuss the effects of longer pulse durations and the effectiveness of pulse sequences for converting the exchange interaction to an Ising model. Our theoretical predictions are well suited for immediate experimental implementations using currently existing quantum dot spin qubit systems.
TL;DR: In this paper , the authors adopt an atomistic method by combining the tight-binding method with the semiclassical molecular dynamics to investigate the electronic structures of twisted trilayer graphene (TTG) with two independent twist angles.
Abstract: Twisted graphene multilayers have been recently demonstrated to share several correlation-driven behaviors with twisted bilayer graphene. In general, the van Hove singularities (VHSs) can be used as a proxy of the tendency for correlated behaviors. In this paper, we adopt an atomistic method by combining the tight-binding method with the semiclassical molecular dynamics to investigate the electronic structures of twisted trilayer graphene (TTG) with two independent twist angles. The two independent twist angles can lead to the interference of the moir\'e patterns forming a variety of commensurate/incommensurate complex supermoir\'e patterns. In particular, the lattice relaxation, twist angle and angle disorder effects on the VHS are discussed. We find that the lattice relaxation significantly influences the position and magnitude of the VHSs. In the supermoir\'e TTG, the moir\'e interference provides constructive or destructive effects depending on the relative twist angle. By modulating the two independent twist angles, novel superstructures, for instance, the Kagome-like lattice, could be constructed via the moir\'e pattern. Moreover, we demonstrate that a slight change in twist angles (angle disorder) provides a significant suppression of the peak of the VHSs. Apart from the moir\'e length, the evolution of the VHSs and the LDOS mapping in real space could be used to identify the twist angles in the complicated TTG. In practice, our work could provide a guide for exploring the flat band behaviors in the supermoir\'e TTG experimentally.
TL;DR: In this paper , the authors present several interesting properties related to flat-band ferromagnetic degeneration in the Hubbard model, and show a pattern of ferromagnetism which appears in small pentagonal and hexagonal plaquettes at filling factors of roughly 3/10 and 1/4.
Abstract: We present several interesting phenomena related to flat-band ferromagnetism in the Hubbard model. The first is a mathematical theorem stating certain conditions under which a flat-band ferromagnetic must necessarily be degenerate with a nonferromagnetic state. This theorem is generally applicable and geometry independent but holds only for a small number of holes in an otherwise filled band. The second phenomenon is a peculiar example where the intuition fails that particles prefer to doubly occupy low-energy states before filling higher-energy states. Lastly, we show a pattern of ferromagnetism which appears in small pentagonal and hexagonal plaquettes at filling factors of roughly 3/10 and 1/4. These examples require only a small number of lattice sites and may be observable in quantum dot arrays currently available as laboratory spin qubit arrays.