arXiv subjects

Keiji Nakatsugawa

Publications and source records attributed to Keiji Nakatsugawa.

9 recordsLinked to original sources

arXiv2026

Landau Theory for Commensurate Charge-Density Waves Coupled to Uniform Lattice Deformation

We formulate a minimal Landau theory for a charge-density wave (CDW) whose commensurability is defined with respect to a deformed lattice. The motivation is provided by recent observations on an isolated single NbS$_3$ chain, which exhibits a commensurate CDW state accompanied by a $6\%$ shrinkage of the lattice constant. A uniform stretch $a_0\to a_0(1+\varepsilon)$ changes the reciprocal lattice wave number to $G(\varepsilon)=G_0/(1+\varepsilon)$, so that an $N$-fold commensurate CDW has the wave number $Q_\mathrm{C}(\varepsilon)=G(\varepsilon)/N$, whereas the wave number $Q_\mathrm{IC}$ favored by the incommensurate instability remains fixed. We propose an amplitude-strain free energy for both $N=3$ and $N=4$, in which the CDW induces a finite uniform strain by relieving the mismatch between $Q_\mathrm{C}(\varepsilon)$ and $Q_\mathrm{IC}$. The mismatch is shared between the CDW and the lattice in a proportion set by their stiffness ratio; since the CDW stiffness grows with the CDW amplitude, the lattice takes up an increasing share of the mismatch as the CDW develops. Our results suggest a reexamination of lock-in theories and of strain-tuning experiments on density-wave systems.

cond-mat.other↗

arXiv2024

Revisitation of the lock-in transition in one-dimensional charge-density waves

We studied the lock-in transition of charge-density waves of one-dimensional conductors. Though this phenomenon has been known for decades, there are still discrepancies between the theories and the experiments. We focused on the pioneering study of this phenomenon by McMillan and revisited, in particular, his numerical calculations. We first reproduced his results by providing unwritten information in the article. The obtained critical point was in good agreement with the previous study, as well as the analytical solution proposed later. We found a simple power-law behavior of the free energy in the critical regime, leading to the power-law divergence of the specific heat. We also introduced the concept of stiffness, equivalent to helicity modulus, which also showed a power-law behavior. These results allowed us to propose a new perspective on the lock-in transition.

cond-mat.str-el↗

arXiv2023

Massive Klein Tunneling in Topological Photonic Crystals

Klein's paradox refers to the transmission of a relativistic particle through a high potential barrier. Although it has a simple resolution in terms of particle-to-antiparticle tunneling (Klein tunneling), debates on its physical meaning seem lasting partially due to the lack of direct experimental verification. In this article, we point out that honeycomb-type photonic crystals (PhCs) provide an ideal platform to investigate the nature of Klein tunneling, where the effective Dirac mass can be tuned in a relatively easy way from a positive value (trivial PhC) to a negative value (topological PhC) via a zero-mass case (PhC graphene). Especially, we show that analysis of the transmission between domains with opposite Dirac masses -- a case hardly be treated within the scheme available so far -- sheds new light on the understanding of the Klein tunneling.

cond-mat.mes-hall↗

arXiv2022

Conformality of Charge Density Wave

New quantum phenomena are continuously being discovered in 2D systems. In particular, the charge density wave (CDW) has the aspect of a quantum crystal with a macroscopic wave function (order parameter), so unlike quantum liquids (superconductivity, quantum Hall liquids $^3$He, $^4$He), new ground states such as supersolid and Moiré solids can be expected. However, it is difficult to describe these states because of their quantum aspect, hence there is still no theory that can explain CDW phases in a unified way. The best way to describe a quantum crystal seems to be a conformal transformation that allows local deformation (wave properties) and preserves local angles (crystal properties). Here, we propose a unifying conformal description of 2D CDW phases in the typical 2D CDW material transition metal dichalcogenides (MX$_2$). We discover that the discommensurate CDW phases in MX$_2$ can be explained beautifully by a discrete conformal transformation of CDW wavevectors. This conformality is due to commensurability of CDW with the MX$_2$ lattice. In other words, interference of harmonic wavefunction induces conformality. Using this new conformal formulation, we explain experimental nearly-commensurate/stripe/T CDW phases in 1$T$-TaS$_2$ ($\sqrt{13}\times\sqrt{13}$ structure), 2$H$-TaSe$_2$ ($\sqrt{9}\times\sqrt{9}$ structure), and explain the origin of a new experimental nearly-commensurate phase in TaSe$_2$ thin-film ($\sqrt{7}\times\sqrt{7}$ structure). This theory is very simple in the sense that it includes only discommensuration and comprises physics as rich as quantum Hall liquids. This new description will broaden our perspective of quantum crystals.

cond-mat.str-el↗

arXiv2020

The Nakano-Nishijima-Gell-Mann Formula From Discrete Galois Fields

The well known Nakano-Nishijima-Gell-Mann (NNG) formula relates certain quantum numbers of elementary particles to their charge number. This equation, which phenomenologically introduces the quantum numbers $I_z$ (isospin), $S$ (strangeness), etc., is constructed using group theory with real numbers $\mathbb{R}$. But, using a discrete Galois field $\mathbb{F}_p$ instead of $\mathbb{R}$ and assuring the fundamental invariance laws such as unitarity, Lorentz invariance, and gauge invariance, we derive the NNG formula deductively from Meson (two quarks) and Baryon (three quarks) representations in a unified way. Moreover, we show that quark confinement ascribes to the inevitable fractionality caused by coprimeness between half-integer (1/2) of isospin and number of composite particles (e.g. three).

hep-ph↗

arXiv2019

Origin of Stripe and Quasi-Stripe CDW Structures in Monolayer MX$ _2$ compounds: Multivalley Free Energy Landscape

Ultrathin sheets of transition metal dichalcogenides (MX$ _2$) with charge density waves (CDWs) is increasingly gaining interest as a promising candidate for graphene-like devices. Although experimental data including stripe/quasi-stripe structure and hidden states have been reported, the ground state of ultrathin MX$ _2$ compounds and, in particular, the origin of anisotropic (stripe and quasi-stripe) CDW phases is a long-standing problem. Anisotropic CDW phases have been explained by Coulomb interaction between domain walls and inter-layer interaction. However, these models assume that anisotropic domain walls can exist in the first place. Here, we report that anisotropic CDW domain walls can appear naturally without assuming anisotropic interactions: We explain the origin of these phases by topological defect theory (line defects in a two-dimensional plane) and interference between harmonics of macroscopic CDW wave functions. We revisit the McMillan-Nakanishi-Shiba model for monolayer 1$T$-TaS$ _2$ and 2$H$-TaSe$ _2$ and show that CDWs with wave vectors that are separated by $120^\circ$ (i.e. the three-fold rotation symmetry of the underlying lattice) contain a free-energy landscape with many local minima. Then, we remove this $120^\circ$ constraint and show that free energy local minima corresponding to the stripe and quasi-stripe phase appear. Our results imply that Coulomb interaction between domain walls and inter-layer interaction may be secondary factors for the appearance of these phases. Furthermore, this model can predict new CDW phases, hence it may become the basis to study CDW further. We anticipate our results to be a starting point for further study in two-dimensional physics, such as explanation of "Hidden CDW states", study the interplay between supersolid symmetry and lattice symmetry, and application to other van der Waals structures.

cond-mat.mtrl-sci↗

arXiv2017

Incommensurate Charge Density Wave as Quantum Space-Time Crystal

A quantum time crystal (QTC) is a novel quantum mechanical ground state that was recently proposed by Wilczek. Although many QTC models have been proposed, it is not clear yet if such states are possible. We propose the idea that an incommensurate charge density wave (ICDW) forms a quantum space crystal (QSC), which is an extension of a QTC in the spatial domain. Consequently, a rotating ring-shaped ICDW naturally forms a quantum space-time crystal (QSTC), which combines the two concepts of QTC and QSC. The breaking of space-time translation symmetry is understood using time-dependent density matrices. Furthermore, we show that this model can be observed in real systems such as TaS$_3$ ring crystals at finite temperature. Our results suggest that QTC/QSTC can exist.

quant-ph↗

arXiv2017

Quantum Time Crystal By Decoherence: Proposal With Incommensurate Charge Density Wave Ring

We show that time translation symmetry of a ring system with a macroscopic quantum ground state is broken by decoherence. In particular, we consider a ring-shaped incommensurate charge density wave (ICDW ring) threaded by a fluctuating magnetic flux: the Caldeira-Leggett model is used to model the fluctuating flux as a bath of harmonic oscillators. We show that the charge density expectation value of a quantized ICDW ring coupled to its environment oscillates periodically. The Hamiltonians considered in this model are time independent unlike "Floquet time crystals" considered recently. Our model forms a metastable quantum time crystal with a finite length in space and in time.

quant-ph↗

arXiv2017

Direct Observation of Mono-, Bi-, and Tri-layer Charge Density Waves in 1T-TaS_2 by Transmission Electron Microscopy without a Substrate

Charge-density-waves (CDW) which occur mainly in low-dimensional systems have a macroscopic wave function similar to superfluids and superconductors. Kosterlitz-Thouless (KT) transition is observed in superfluids and superconductors, but the presence of KT transition in ultra-thin CDW systems has been an open problem. We report the direct real-space observation of CDWs with new order states in mono-, bi-, and tri-layer 1T-TaS_2 crystal by using a low voltage scanning-transmission-electron-microscope (STEM) without a substrate. This method is ideal to observe local atomic structures and possible defects. We clearly observed that the mono-layer crystal has a new triclinic stripe CDW order without the triple q condition q_1 + q_2 + q_3 = 0. A strong electron-phonon interaction gives rise to new crevasse (line) type defects instead of disclination (point) type defects due to the KT transition. These results reaffirm the importance of the electron-phonon interaction in mono-layer nanophysics.

cond-mat.mtrl-sci↗