Band Insulator Research Articles

Alternative Structure Model of Correlated Charge Density Wave in Monolayer 1T-Nb(Ta)Se2.

The putative Mott charge density wave (CDW) phases of monolayer 1T-NbSe2 and 1T-TaSe2 have attracted a lot of recent interest due to the unexpected orbital texture of their Mott-Hubbard states and the superstructure related to an exotic possibility of a quantum spin liquid with a spinon Fermi surface. The origins of the orbital texture and the superstructure have been, however, elusive. We find by using density functional theory calculations that these CDW phases can have an alternative metastable structure, an anion (Se) centered cluster, in contrast to the prevailing model of a cation (Nb or Ta) centered David star cluster. This structure can be stabilized by the charge transfer from the bilayer graphene/SiC substrate used commonly in the experiments. The anion-centered structure has a similar electronic band structure of a charge transfer insulator to that of DS clusters but naturally explains the orbital texture of the upper Hubbard band from simply its atomic structure. Moreover, this band structure exhibits a Fermi surface nesting to possibly break the symmetry spontaneously into a -R30° superstructure observed experimentally. The resulting ground state of the superstructure is shown to be a trivial band insulator, in contrast to exotic proposals. This result emphasizes the huge structural flexibility of these heteroexpitaxial monolayers, for which careful studies on atomic structures and interactions with substrates are highly requested.

Effect of the spin–orbit interaction in nanotubes

In this work, we study the electronic properties of nanotubes with spin–orbit interaction that exhibit a spin Hall effect. Nanoribbons made of these materials are expected to have topologically protected states. The lack of edge states in a seamless tube eliminates the possibility of finding a topological edge state. The spin–orbit interaction breaks the degeneracy of Dirac’s cones and eliminates the chance of finding a metal nanotube. As a consequence, this makes all nanotubes with spin–orbit interaction trivial band insulators. We focus our attention on two features. First, we study the energy band gap as a function of the diameter of the nanotubes. Then, we concentrate on controlling the bandgap of a nanotube by applying an external radial electric field. Finally, we discuss the possibility of using one of these tubes as a field-effect transistor.

High-fidelity imaging of a band insulator in a three-dimensional optical lattice clock

High-fidelity imaging of a band insulator in a three-dimensional optical lattice clock

Light-matter interaction of the polar-polar interface LaVO3-KTaO3 (111)

KTaO3 (KTO) has emerged as a potential candidate in the spintronics block showing the emergent phenomena in its bulk form as well as thin films, especially in KTO-based heterostructures and interfaces. In this work, we have grown a thin film of polar Mott insulator LaVO3 (LVO) on polar band insulator KTO (111) using the pulsed laser deposition technique. This interfce shows the insulator-to-metal transition. We have studied the photoelectrical properties by using a blue laser light of wavelength 405 nm for illumination in temperatures ranging from 76 K and 300 K. Under blue light illumination, the interface shows enhancement in conductivity, for both 76 K to 300 K. This system has shown the persistent photoconductivity as well. Further, electrostatic gating has also been applied to tune the charge carrier density and the conductivity is increased by around 23 % under the effect of light and electrostatic gating.

Effective action approach to the filling anomaly in crystalline topological matter

In two dimensions, magnetic higher-order topological insulators (HOTIs) are characterized by excess boundary charge and a compensating bulk ``filling anomaly.'' At the same time, without additional noncrystalline symmetries, the boundaries of two-dimensional HOTIs are gapped and featureless at low energies, while the bulk of the system is predicted to have a topological response to the insertion of lattice (particularly disclination) defects. Until recently, a precise connection between these effects has remained elusive. In this work, we point the direction towards a unifying field-theoretic description for the bulk and boundary response of magnetic HOTIs. By focusing on the low-energy description of the gapped boundary of a two-dimensional magnetic HOTI with no time-reversing symmetries, we show that the boundary charge and filling anomaly arise from the gravitational ``Gromov-Jensen-Abanov'' (GJA) response action [A. Gromov et al., Phys. Rev. Lett. 116, 126802 (2016)] in the context of the quantum Hall effect. As in quantum Hall systems, the GJA action cancels apparent anomalies associated with the bulk response to disclinations, allowing us to derive a concrete connection between the bulk and boundary theories of HOTIs. We show how our results elucidate the connection between higher-order topology and geometric response in band insulators, and point towards a route to understanding interacting higher-order topological phases beyond the simple cases considered here.

Multiple Electronic Phases Coexisting under Inhomogeneous Strains in the Correlated Insulator.

Monolayer transition metal dichalcogenides (TMDs) can host exotic phenomena such as correlated insulating and charge-density-wave (CDW) phases. Such properties are strongly dependent on the precise atomic arrangements. Strain, as an effective tuning parameter in atomic arrangements, has been widely used for tailoring material's structures and related properties, yet to date, a convincing demonstration of strain-induced dedicate phase transition at nanometer scale in monolayer TMDs has been lacking. Here, a strain engineering technique is developed to controllably introduce out-of-plane atomic deformations in monolayer CDW material 1T-NbSe2 . The scanning tunneling microscopy and spectroscopy (STM and STS) measurements, accompanied by first-principles calculations, demonstrate that the CDW phase of 1T-NbSe2 can survive under both tensile and compressive strains even up to 5%. Moreover, significant strain-induced phase transitions are observed, i.e., tensile (compressive) strains can drive 1T-NbSe2 from an intrinsic-correlated insulator into a band insulator (metal). Furthermore, experimental evidence of the multiple electronic phase coexistence at the nanoscale is provided. The results shed new lights on the strain engineering of correlated insulator and useful for design and development of strain-related nanodevices.

Topological phase transitions at finite temperature

The ground states of noninteracting fermions in one-dimension with chiral symmetry form a class of topological band insulators, described by a topological invariant that can be related to the Zak phase. Recently, a generalization of this quantity to open systems---known as the ensemble geometric phase (EGP)---has emerged as a robust way to describe topology at nonzero temperature. By using this quantity, we explore the nature of topology allowed for dissipation beyond a Lindblad description, to allow for coupling to external baths at finite temperatures. We introduce two main aspects to the theory of open-system topology. First, we discover topological phase transitions as a function of the temperature $T$, manifesting as changes in differences of the EGP accumulated over a closed loop in parameter space. We characterize the nature of these transitions and reveal that the corresponding nonequilibrium steady state can exhibit a nontrivial structure---contrary to previous studies where it was found to be in a fully mixed state. Second, we demonstrate that the EGP itself becomes quantized when key symmetries are present, allowing it to be viewed as a topological marker which can undergo equilibrium topological transitions at nonzero temperatures.

Stacking order and driving forces in the layered charge density wave phase of 1T-MX2 (M = Nb, Ta and X = S, Se)

The group-V transition metal dichalcogenides (TMD) have attracted a lot of research due to their unique structures and rich physical properties. In these materials, charge density waves (CDW) are still the subject worthy of in-depth research despite being a popular issue. Based on first principles, the stacking effect of the T- MX2 (M = Nb, Ta and X = S, Se) CDW phase are comprehensively explored, with the interplay of correlation effect and magnetic order. Without correlation effect, T- MX2 with one specific stacking order (AA_AC_AA) is most structurally stable and leads to a natural band insulator due to interlayer dimerization. In contrast, same materials with the other stacking orders (AA_AB_AA, AA, AB, AC) are metallic phase. In the presence of correlation effect, whether the systems are insulators or metal highly depends on the magnetic order. AA_AC_AA stacking T- MX2 with antiferromagnetic order end up with band insulator. T- MX2 with AA, AB, AC stacking are metal even in the presence of correlation effect, but with magnetic order, they become Mott insulator. The complication is that AA_AB_AA stacking is located at the intersection of the band insulator and Mott insulator. From this article, we can see T-MX2 with different stacking structure with/without correlation effect and magnetic order show different phases. We outline a rich landscape and systematically explain the causes of the insulating characteristic of the CDW phase and emphasizes the critical role of correlation effect and magnetic order, extending the underlying mechanism of metal-insulation transitions that previously relied only on Mott localization as a driving force.

Enhancement of chiral edge currents in ($d$+1)-dimensional atomic Mott-band hybrid insulators

We consider the effect of a local interatomic repulsion on synthetic heterostructures where a discrete synthetic dimension is created by Raman processes on top of SU(N)SU(N)-symmetric two-dimensional lattice systems. At a filling of one fermion per site, increasing the interaction strength, the system is driven towards a Mott state which is adiabatically connected to a band insulator. The chiral currents associated with the synthetic magnetic field increase all the way to the Mott transition, where they reach the maximum value, and they remain finite in the whole insulating state. The transition towards the Mott-band insulator is associated with the opening of a gap within the low-energy quasiparticle peak, while a mean-field picture is recovered deep in the insulating state.

Unconventional gapless semiconductor in an extended martini lattice in covalent honeycomb materials

We study characteristic electronic structures in an extended martini lattice model and propose its materialization in $\ensuremath{\pi}$-electron networks constructed by designated chemisorption on graphene and silicene. By investigating the minimal tight-binding model, we reveal rich electronic structures tuned by the ratio of hopping parameters, ranging from the band insulator to the unconventional gapless semiconductor. Remarkably, the unconventional gapless semiconductor is characterized by a flat band at the Fermi level. Further, the density functional theory calculations for candidate materials reveal that the characteristic electronic structures can be realized by designated chemisorption or chemical substitution on graphene and silicene, and that the electronic structure near the Fermi level is tunable by the choice of the atomic species of adsorbed atoms. Our results open the way to search exotic electronic structures and their functionalities induced by an extended martini lattice.

U(1)-symmetric Gaussian fermionic projected entangled paired states and their Gutzwiller projection

We develop a formalism for constructing particle-number-conserving Gaussian fermionic projected entangled pair states [U(1)-GfPEPS] and show that these states can describe ground states of band insulators and gapless fermions with band touching points. When using them as variational Ans\"{a}tze for two Dirac fermion systems ($\pi$-flux model on the square lattice and $[0,\pi]$-flux model on the kagome lattice), we find that the U(1)-GfPEPS, even with a relatively small bond dimension, can accurately approximate the Dirac Fermi sea ground states. By applying Gutzwiller projectors on top of these U(1)-GfPEPS, we obtain PEPS representation of U(1)-Dirac spin liquid states for spin-1/2 systems. With state-of-the-art tensor network numerics, the critical exponent in the spin-spin correlation function of the Gutzwiller-projected $\pi$-flux state is estimated to be $\eta \approx 1.7$.

Hyperbolic band topology with non-trivial second Chern numbers

Topological band theory establishes a standardized framework for classifying different types of topological matters. Recent investigations have shown that hyperbolic lattices in non-Euclidean space can also be characterized by hyperbolic Bloch theorem. This theory promotes the investigation of hyperbolic band topology, where hyperbolic topological band insulators protected by first Chern numbers have been proposed. Here, we report a new finding on the construction of hyperbolic topological band insulators with a vanished first Chern number but a non-trivial second Chern number. Our model possesses the non-abelian translational symmetry of {8,8} hyperbolic tiling. By engineering intercell couplings and onsite potentials of sublattices in each unit cell, the non-trivial bandgaps with quantized second Chern numbers can appear. In experiments, we fabricate two types of finite hyperbolic circuit networks with periodic boundary conditions and partially open boundary conditions to detect hyperbolic topological band insulators. Our work suggests a new way to engineer hyperbolic topological states with higher-order topological invariants.

Phase transitions in the Haldane-Hubbard model with ionic potentials

By employing the exact-diagonalization method, we revisit the ground-state phase diagram of the Haldane-Hubbard model on the honeycomb lattice with staggered sublattice potentials. The phase diagram includes the band insulator, Mott insulator, and two Chern insulator phases with Chern numbers C=2 and C=1, respectively. The character of transitions between different phases is studied by analyzing the lower-lying energy levels, excitation gaps, structure factors, and fidelity metric. We find that the C=1 phase can be continuously deformed into the C=2 phase without a gap closure in the periodic boundary condition, while a further analysis on the Berry curvatures indicates that the excitation gap closes at the phase boundary in a twisted boundary condition, accompanied by the discontinuities of structure factors. All the other phase transitions are found to be first-order ones as expected.

Multiple Quantum States Induced in 1T‐TaSe2 by Controlling the Stacking Order of Charge Density Waves

AbstractVan der Waals (vdW) materials afford unprecedented opportunities for control of electronic properties by utilizing the stacking degree of freedom. An intriguing frontier, largely unexplored, is the stacking of charge density wave (CDW) phases that is a broken‐symmetry state with periodically modulated charge density and the atomic lattice. Employing density functional theory, it is uncovered that the stacking order can play a significant role in the quantum phase transitions of layered 1T‐TaSe2 with a striking 2D CDW order. By controlling the vertical stacking order of CDWs, bulk 1T‐TaSe2 can host various electronic phases including quasi‐1D and 3D metals and band insulators. Particularly, the ground‐state stacking configuration shows 3D metallicity due to the enhanced intralayer and interlayer electron hopping, and the second lowest energy configuration shows band insulating behavior via interlayer dimerization, implying potential metal‐insulator transition. In ultrathin‐layer 1T‐TaSe2, not only the stacking order but also the thickness dictate the electronic properties. While the monolayer is a Mott insulator, the bilayer (trilayer) is a band insulator (metal). More interestingly, the four‐layer emerges as an insulator or a semimetal dependent on its stacking order. The wide‐tunable electronic properties of 1T‐TaSe2 CDW compound will open a new pathway for designing novel quantum devices.

Stability of the topological quantum critical point between multi-Weyl semimetal and band insulator

One could tune a topological double-Weyl semimetal or a topological triple-Weyl semimetal to become a topologically trivial insulator by opening a band gap. This kind of quantum phase transition is characterized by the change of certain topological invariant. A new gapless semimetallic state emerges at each topological quantum critical point. Here we perform a renormalization group analysis to investigate the stability of such critical points against perturbations induced by random scalar potential and random vector potential. We find that the quantum critical point between double-Weyl semimetal and band insulator is unstable and can be easily turned into a compressible diffusive metal by any type of weak disorder. The quantum critical point between triple-Weyl semimetal and band insulator flows to a stable strong-coupling fixed point if the system contains a random vector potential merely along the z-axis, but becomes a compressible diffusive metal when other types of disorders exist.

Metrology of band topology via resonant inelastic x-ray scattering

Topology is a central notion in the classification of band insulators and the characterization of entangled many-body quantum states. In some cases, it manifests as quantized observables such as quantum Hall conductance. However, being inherently a global property depending on the entirety of the system, its direct measurement has remained elusive to local experimental probes in many cases. Here, we demonstrate that various topological band indices can be directly probed by resonant inelastic x-ray scattering. Specifically, we show that the crystalline symmetry eigenvalues at the high-symmetry momentum points, which determine the band topology, lead to distinct scattering intensity for particular momentum and energy. Our approach can be explicitly demonstrated in several examples such as 1D Su-Schrieffer-Heeger chain, 2D quadrupole insulator, 3D topological band insulator, and chiral hinge insulator. Our result establishes an incisive bulk probe for the measurement of band topology.

Mottness in two-dimensional van der Waals Nb3X8 monolayers (X=Cl,Br,andI)

We investigate strong electron-electron correlation effects on two-dimensional van der Waals materials ${\mathrm{Nb}}_{3}{X}_{8} (X=\mathrm{Cl},\mathrm{Br},\phantom{\rule{0.28em}{0ex}}\text{and}\phantom{\rule{0.28em}{0ex}}\mathrm{I})$. We find that the monolayers ${\mathrm{Nb}}_{3}{X}_{8}$ are ideal systems close to the strong correlation limit. They can be described by a half-filled single band Hubbard model in which the ratio between the Hubbard, $U$, and the bandwidth, $W, U/W\ensuremath{\approx}5--10$. Both Mott and magnetic transitions of the material are calculated by the slave boson mean-field theory. Doping the Mott state, a ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}+i{d}_{xy}$ superconducting pairing instability is found. We also construct a tunable bilayer Hubbard system for two sliding ${\mathrm{Nb}}_{3}{X}_{8}$ layers. The bilayer system displays a crossover between the band insulator and Mott insulator.

Delocalization of interacting directed polymers on a periodic substrate: Localization length and critical exponents from non-Hermitian spectra.

We study a classical model of thermally fluctuating polymers confined to two dimensions, experiencing a grooved periodic potential, and subject to pulling forces both along and transverse to the grooves. The equilibrium polymer conformations are described by a mapping to a quantum system with a non-Hermitian Hamiltonian and with fermionic statistics generated by noncrossing interactions among polymers. Using molecular dynamics simulations and analytical calculations, we identify a localized and a delocalized phase of the polymer conformations, separated by a delocalization transition which corresponds (in the quantum description) to the breakdown of a band insulator when driven by an imaginary vector potential. We calculate the average tilt of the many-body system, at arbitrary shear values and filling density of polymer chains, in terms of the complex-valued non-Hermitian band structure. We find the critical shear value, the localization length, and the critical exponent by which the shear modulus diverges in terms of the branch points (exceptional points) in the band structure at which the bandgap closes. We also investigate the combined effects of non-Hermitian delocalization and localization due to both periodicity and disorder, uncovering preliminary evidence that while disorder favors localization at high values, it encourages delocalization at lower values.

Octahedral rotations and defect-driven metallicity at the (001) surface of CaTiO3

Defect-free $\mathrm{Ca}\mathrm{Ti}{\mathrm{O}}_{3}$ is a band insulator, but angle-resolved photoemission spectroscopy experiments have demonstrated the existence of a nanometer-thin two-dimensional electron system (2DES) at the (001)-oriented surface of this compound. Our ab initio study finds that while oxygen defects are the driving mechanism for the metallicity, tiltings and rotations of the oxygen octahedra significantly impact the electronic structure of the 2DES and its response to externally applied strain deformations. The low-energy conduction subbands have a mixed ${t}_{2g}\text{\ensuremath{-}}{e}_{g}$ orbital character close to the center of the Brillouin zone. Good agreement with the experimental spectra is found for $\mathrm{Ti}{\mathrm{O}}_{2}$ surface divacancy configurations.

Merging of Dirac points through uniaxial modulation on an optical lattice

We analyze the scenario of modulating the potential strength of bound atoms in an optical honeycomb lattice patterned by an electric field to emulate uniaxial strain. This modulation can be achieved by a combination of the strength of the patterned electric field and gauge vector effects using the Floquet approach. We show that such a modulation allows one to follow through a topological transition between a semi-metal and a band insulator, when two non-equivalent K points merge as a function of the electric field strength. We explicitly compute the wavefunctions for the moving K points and the Chern numbers up to the transition. Anisotropic effective masses and the insulating gap are described close to the semimetal-insulator transition.