Searcharxiv⌕ Search

arXiv subjects

Yuki Nakata

Publications and source records attributed to Yuki Nakata.

9 recordsLinked to original sources
arXiv2026

L-indistinguishability for covering groups of algebraic tori

A global packet may simultaneously contain an automorphic representation and a non-automorphic representation. The global $\mathcal S$-group is expected, and known in some cases, to specify the automorphic representations in each global packet. For a covering group of an algebraic torus, it is not obvious from the definition whether the analogue of a global $\mathcal S$-group has finite order. In this paper, we verify this finiteness for a Brylinski-Deligne covering group of a torus.

math.NT↗
arXiv2021

Fields whose Multiplicative Groups are Linear Spaces

The purpose of this paper is to study fields whose multiplicative groups admit the structure of linear spaces. We prove that the multiplicative group of a finite field is a linear space if and only if the order of the multiplicative group is 1, 2, or a Mersenne prime. We give necessary conditions for the multiplicative group of an infinite field to be a linear space over another field. We also construct an example of an infinite field whose multiplicative group is a linear space over $\mathbb{Q}$.

math.NT↗
arXiv2021

Robust charge-density wave strengthened by electron correlations in monolayer 1T-TaSe2 and 1T-NbSe2

Combination of low-dimensionality and electron correlation is vital for exotic quantum phenomena such as the Mott-insulating phase and high-temperature superconductivity. Transition-metal dichalcogenide (TMD) 1T-TaS2 has evoked great interest owing to its unique nonmagnetic Mott-insulator nature coupled with a charge-density-wave (CDW). To functionalize such a complex phase, it is essential to enhance the CDW-Mott transition temperature TCDW-Mott, whereas this was difficult for bulk TMDs with TCDW-Mott < 200 K. Here we report a strong-coupling 2D CDW-Mott phase with a transition temperature onset of ~530 K in monolayer 1T-TaSe2. Furthermore, the electron correlation derived lower Hubbard band survives under external perturbations such as carrier doping and photoexcitation, in contrast to the bulk counterpart. The enhanced Mott-Hubbard and CDW gaps for monolayer TaSe2 compared to NbSe2, originating in the lattice distortion assisted by strengthened correlations and disappearance of interlayer hopping, suggest stabilization of a likely nonmagnetic CDW-Mott insulator phase well above the room temperature. The present result lays the foundation for realizing monolayer CDW-Mott insulator based devices operating at room temperature.

cond-mat.str-el↗
arXiv2020

Observation of inverted band structure in topological Dirac-semimetal candidate CaAuAs

We have performed high-resolution angle-resolved photoemission spectroscopy of ternary pnictide CaAuAs which is predicted to be a three-dimensional topological Dirac semimetal (TDS). By accurately determining the bulk-band structure, we have revealed the coexistence of three-dimensional and quasi-two-dimensional Fermi surfaces with dominant hole carriers. The band structure around the Brillouin-zone center is characterized by an energy overlap between hole and electron pockets, in excellent agreement with first-principles band-structure calculations. This indicates the occurrence of bulk-band inversion, supporting the TDS state in CaAuAs. Because of the high tunability in the chemical composition besides the TDS nature, CaAuAs provides a precious opportunity for investigating the quantum phase transition from TDS to other exotic topological phases.

cond-mat.mes-hall↗
arXiv2019

Monolayer VTe2: Incommensurate Fermi surface nesting and suppression of charge density waves

We investigated the electronic structure of monolayer VTe2 grown on bilayer graphene by angle-resolved photoemission spectroscopy (ARPES). We found that monolayer VTe2 takes the octahedral 1T structure in contrast to the monoclinic one in the bulk, as evidenced by the good agreement in the Fermi-surface topology between ARPES results and first-principles band calculations for octahedral monolayer 1T-VTe2. We have revealed that monolayer 1T-VTe2 at low temperature is characterized by a metallic state whereas the nesting condition is better than that of isostructural monolayer VSe2 which undergoes a CDW transition to insulator at low temperature. The present result suggests an importance of Fermi-surface topology for characterizing the CDW properties of monolayer TMDs.

cond-mat.mtrl-sci↗
arXiv2018

Observation of Chiral Fermions with a Large Topological Charge and and Associated Fermi-Arc Surface States in CoSi

Topological semimetals materialize a new state of quantum matter where massless fermions protected by a specific crystal symmetry host exotic quantum phenomena. Distinct from well-known Dirac and Weyl fermions, structurally-chiral topological semimetals are predicted to host new types of massless fermions characterized by a large topological charge, whereas such exotic fermions are yet to be experimentally established. Here, by using angle-resolved photoemission spectroscopy, we experimentally demonstrate that a transition-metal silicide CoSi hosts two types of chiral topological fermions, spin-1 chiral fermion and double Weyl fermion, in the center and corner of the bulk Brillouin zone, respectively. Intriguingly, we found that the bulk Fermi surfaces are purely composed of the energy bands related to these fermions. We also find the surface states connecting the Fermi surfaces associated with these fermions, suggesting the existence of the predicted Fermi-arc surface states. Our result provides the first experimental evidence for the chiral topological fermions beyond Dirac and Weyl fermions in condensed-matter systems, and paves the pathway toward realizing exotic electronic properties associated with unconventional chiral fermions.

cond-mat.mes-hall↗
arXiv2018

Pseudogap, Fermi arc, and Peierls-insulating phase induced by 3D-2D crossover in monolayer VSe2

One of important challenges in condensed-matter physics is to realize new quantum states of matter by manipulating the dimensionality of materials, as represented by the discovery of high-temperature superconductivity in atomic-layer pnictides and room-temperature quantum Hall effect in graphene. Transition-metal dichalcogenides (TMDs) provide a fertile platform for exploring novel quantum phenomena accompanied by the dimensionality change, since they exhibit a variety of electronic/magnetic states owing to quantum confinement. Here we report an anomalous metal-insulator transition induced by 3D-2D crossover in monolayer 1T-VSe2 grown on bilayer graphene. We observed a complete insulating state with a finite energy gap on the entire Fermi surface in monolayer 1T-VSe2 at low temperatures, in sharp contrast to metallic nature of bulk. More surprisingly, monolayer 1T-VSe2 exhibits a pseudogap with Fermi arc at temperatures above the charge-density-wave temperature, showing a close resemblance to high-temperature cuprates. This similarity suggests a common underlying physics between two apparently different systems, pointing to the importance of charge/spin fluctuations to create the novel electronic states, such as pseudogap and Fermi arc, in these materials.

cond-mat.mtrl-sci↗
arXiv2018

Observation of band crossings protected by nonsymmorphic symmetry in the layered ternary telluride Ta3SiTe6

We have performed angle-resolved photoemission spectroscopy of layered ternary telluride Ta3SiTe6 which is predicted to host nodal lines associated with nonsymmorphic crystal symmetry. We found that the energy bands in the valence-band region show Dirac-like dispersions which present a band degeneracy at the R point of the bulk orthorhombic Brillouin zone. This band degeneracy extends one-dimensionally along the whole SR high-symmetry line, forming the nodal lines protected by the glide mirror symmetry of the crystal. We also observed a small band splitting near EF which supports the existence of hourglass-type dispersions predicted by the calculation. The present results provide an excellent opportunity to investigate the interplay between exotic nodal fermions and nonsymmorphic crystal symmetry.

cond-mat.mes-hall↗