Peierls-like Instability Research Articles

Selective Electron-Phonon Coupling in Dimerized 1T-TaS2 Revealed by Resonance Raman Spectroscopy.

The layered transition-metal dichalcogenide material 1T-TaS2 possesses successive phase transitions upon cooling, resulting in strong electron-electron correlation effects and the formation of charge density waves (CDWs). Recently, a dimerized double-layer stacking configuration was shown to form a Peierls-like instability in the electronic structure. To date, no direct evidence for this double-layer stacking configuration using optical techniques has been reported, in particular through Raman spectroscopy. Here, we employ a multiple excitation and polarized Raman spectroscopy to resolve the behavior of phonons and electron-phonon interactions in the commensurate CDW lattice phase of dimerized 1T-TaS2. We observe a distinct behavior from what is predicted for a single layer and probe a richer number of phonon modes that are compatible with the formation of double-layer units (layer dimerization). The multiple-excitation results show a selective coupling of each Raman-active phonon with specific electronic transitions hidden in the optical spectra of 1T-TaS2, suggesting that selectivity in the electron-phonon coupling must also play a role in the CDW order of 1T-TaS2.

The atomic arrangement and electronic interactions in vonsenite at 295, 100, and 90 K

Abstract Vonsenite, Fe22+Fe3+O2BO3, has been the subject of many studies in the materials science and condensed matter physics communities due to interest in the electronic and magnetic properties and ordering behavior of the phase. One such study, undertaken on synthetic material of end-member composition, reports X-ray diffraction structure refinements that indicate a phase transition from Pbam to Pbnm at or just below ~283 K, determined subsequently to arise from a Peierls-like instability. To compare the stability of the natural phase with that of synthetic material, we performed high-precision X-ray crystal-structure analyses at 295, 100, and 90 K (R1 = 0.0119, 0.0186, and 0.0183, respectively), Mössbauer spectroscopy at 295, 220, 150, 80, and 4.2 K, and wavelength-dispersive electron microprobe analysis on a vonsenite of near-end-member composition from Jayville, New York, U.S.A. The Pbnm structure is observed at 100 and 90 K, suggesting similar phase stability for the natural and synthetic phases. Comparison of Mössbauer data and X-ray site occupancies between the natural and synthetic phases suggests a reinterpretation of Mössbauer site assignments. We conclude that the Peierls-like instability underlying the reported transition from Pbam to Pbnm in synthetic material also occurs in our specimen of natural near-end-member vonsenite at temperatures between 295 and 100 K.

Origin of distorted 1T-phase ReS2: first-principles study

Group-VIIB transition metal dichalcogenides (TMDCs) are known to be stabilized solely in a distorted 1T phase termed as 1T″ phase, which is compared to many stable or metastable phases in other TMDCs. Using first-principles calculations, we study the structural origin of 1T″ phase group-VIIB TMDCs. We find that quasi 1D Peierls-like instability is responsible for the transition to the 1T″ phase ReS2 monolayer from the 1T′ phase, another distorted 1T phase. Two half-filled bands in 1T′-ReS2 make sharp peaks in the Lindhard function that prompt the charge density wave (CDW) phase with large band gap opening. Our calculations show that overlapping of the two bands in a broad energy range leads to robust CDW phase or stable 1T″ phase in group-VIIB TMDCs against compositional variation, which is in stark contrast to typical Peierls instability driven by a single band. Calculated total energy curve near the critical point exhibits the feature of the first-order Landau transition due to local chemical bonding. The structural stability of the 1T″ phase in group-VIIB TMDCs is thus guaranteed by two half-filled bands and local chemical bonding.

Superradiant Topological Peierls Insulator inside an Optical Cavity.

We consider a spinless ultracold Fermi gas tightly trapped along the axis of an optical resonator and transversely illuminated by a laser closely tuned to a resonator mode. At a certain threshold pump intensity, the homogeneous gas density breaks a Z_{2} symmetry towards a spatially periodic order, which collectively scatters pump photons into the cavity. We show that this known self-ordering transition also occurs for low field seeking fermionic particles when the laser light is blue detuned to an atomic transition. The emergent superradiant optical lattice in this case is homopolar and possesses two distinct dimerizations. Depending on the spontaneously chosen dimerization, the resulting Bloch bands can have a nontrivial topological structure characterized by a nonvanishing Zak phase. In the case where the Fermi momentum is close to half of the cavity-mode wave number, a Peierls-like instability here creates a topological insulator with a gap at the Fermi surface, which hosts a pair of edge states. The topological features of the system can be nondestructively observed via the cavity output: the Zak phase of the bulk coincides with the relative phase between laser and cavity field, while the fingerprint of edge states can be observed as additional broadening in a well-defined frequency window of the cavity spectrum.

Perturbed vortex lattices and the stability of nucleated topological phases

We study the stability of nucleated topological phases that can emerge when interacting non-Abelian anyons form a regular array. The studies are carried out in the context of Kitaev's honeycomb model, where we consider three distinct types of perturbations in the presence of a lattice of Majorana mode binding vortices---spatial anisotropy of the vortices, dimerization of the vortex lattice, and local random disorder. While all the nucleated phases are stable with respect to weak perturbations of each kind, strong perturbations are found to result in very different behavior. Anisotropy of the vortices stabilizes the strong-pairing-like phases, while dimerization can recover the underlying non-Abelian phase. Local random disorder, on the other hand, can drive all the nucleated phases into a gapless thermal metal state. We show that all these distinct behaviors can be captured by an effective staggered tight-binding model for the Majorana modes. By studying the pairwise interactions between the vortices, i.e., the amplitudes for the Majorana modes to tunnel between vortex cores, the locations of phase transitions and the nature of the resulting states can be predicted. We also find that, due to oscillations in the Majorana tunneling amplitude, lattices of Majorana modes may exhibit a Peierls-like instability, where a dimerized configuration is favored over a uniform lattice. As the nature of the nucleated phases depends only on the Majorana tunneling, our results are expected to apply also to other system supporting localized Majorana mode arrays, such as Abrikosov lattices in $p$-wave superconductors, Wigner crystals in Moore-Read fractional quantum Hall states, or arrays of topological nanowires.

Competition between instabilities of Peierls transition and Mott transition in W-doped VO2thin films

The change in electronic structure of V${}_{1\ensuremath{-}x}$W${}_{x}$O${}_{2}$ thin films across a metal-insulator transition (MIT) is investigated in terms of hard x-ray photoemission spectroscopy and x-ray absorption spectroscopy. In the lower doping range (0 $\ensuremath{\le}$ $x$ $\ensuremath{\le}$ 0.08), the spectra exhibit the characteristic features for the dimerization of V ions in the monoclinic phase, indicating that Peierls-like instability predominately causes the MIT in this range. Conversely, in the higher doping range (0.1 $\ensuremath{\le}$ $x$), spectral weight transfer is observed from the coherent part at the Fermi level to the incoherent part, indicating that the ground state of V${}_{1\ensuremath{-}x}$W${}_{x}$O${}_{2}$ films in this range is a typical Mott insulator. The results suggest that the unusual phase diagram of the V${}_{1\ensuremath{-}x}$W${}_{x}$O${}_{2}$ thin films originates from the competition between the Peierls and Mott instabilities.

Electrochemical switching of the Peierls-like transition in metallic single-walled carbon nanotubes

The high-energy vibrational modes of metallic carbon nanotubes are believed to be softened compared to the semiconducting ones by a Peierls-like transition. The Raman modes, when excited with a red laser to enhance the metallic tubes, were found to exhibit an exceptionally high sensitivity to electrochemical doping. Our data may be interpreted as controlling the Peierls-like instability in metallic tubes with the applied potential. We also discuss the limits of applicability of the double-layer charging model and show Raman evidence for a hysteretic transition to a chemical doping regime.

The metal-insulator transitions of VO2: A band theoretical approach

The results of first principles electronic structure calculations for the metallic rutile and the insulating monoclinic M1 phase of vanadium dioxide are presented. In addition, the insulating M2 phase is investigated for the first time. The density functional calculations allow for a consistent understanding of all three phases. In the rutile phase metallic conductivity is carried by metal t_2g orbitals, which fall into the one-dimensional d_parallel band, and the isotropically dispersing e_g^pi bands. Hybridization of both types of bands is weak. In the M1 phase splitting of the d_parallel band due to metal-metal dimerization and upshift of the e_g^pi bands due to increased p-d overlap lead to an effective separation of both types of bands. Despite incomplete opening of the optical band gap due to the shortcomings of the local density approximation, the metal-insulator transition can be understood as a Peierls-like instability of the d_parallel band in an embedding background of e_g^pi electrons. In the M2 phase, the metal-insulator transition arises as a combined embedded Peierls-like and antiferromagnetic instability. The results for VO2 fit into the general scenario of an instability of the rutile-type transition-metal dioxides at the beginning of the d-series towards dimerization or antiferromagnetic ordering within the characteristic metal chains. This scenario was successfully applied before to MoO2 and NbO2. In the d^1 compounds, the d_parallel and e_g^pi bands can be completely separated, which leads to the observed metal-insulator transitions.

Distortions from the Tetragonal HfCuSi2-Type Structures to the Orthorhombic GdCuAs1.15P0.85-Type Structures in REMX2 (RE = Rare Earth; M = Cu, Ag; X = P, As, Sb): Mismatch between the Sizes of the M and X Atoms

The RECuAs2-xPx phases undergo symmetry-breaking transitions from tetragonal HfCuSi2-type structures (P4/nmm space group) to orthorhombic GdCuAs1.15P0.85-type structures (Pmmn space group) upon substitution of arsenic by phosphorus. According to Landau theory, the distortions correspond to the B1g irreducible representation and can be continuous. The P4/nmm → Pmmn transitions are not caused by a Peierls-like instability of the square As/P nets but result from a structural adaptation for the smaller P atoms. The stability of the tetragonal RECuAs2-xPx and other REMX2 structures with respect to the orthorhombic distortion can be characterized by a relative interatomic distance in the As/P or X layer. Although the tetragonal phases have small relative As/P−As/P or X−X interatomic distances, the orthorhombic phases have larger distances.

Evolution of dynamic localization regimes in coupled minibands

We consider the behaviour of two coupled symmetric electronic minibands in the presence of an AC intense field. Symmetric coupled minibands are realized as dimerization procedures. The minibands are emulated by a tight-binding finite chain, whereas the interaction with an AC field is described by a time-independent Hamiltonian. We follow the evolution of dressed minibands, non perturbatively, in a wide frequency range. For one dimerization procedure, a transition between both limits can be systematically followed and a dynamic breakdown is unambigously identified. The most striking effect that may occur is a field-induced analog of an insulator-metal transition due to the suppression of a Peierls-like instability.

Metal-insulator transition in disordered VO 2

Disordered VO 2 films were prepared by electrochemical anodic oxidation of vanadium. The crystallinity, stoichiometry, conductivity, Hall effect, ESR, and optical adsorption data in disordered VO 2 films have been studied. The main result of this work is that metal-insulator phase transition (MIT) has been preserved in the absence of long-range crystallographic order. A Peierls-like instability which is required for the realization of long-range crystallographic order is therefore not the driving force of the MIT in VO 2. The properties of the disordered VO 2 are compared to those of crystalline VO 2 in metallic and semiconductor phases. The essential role of electron localization in influencing the behavior of the disordered VO 2 is shown. Possible mechanisms of electron localization in the semiconductor and metallic phases are discussed.