arXiv subjects

D. Nakamura

Publications and source records attributed to D. Nakamura.

7 recordsLinked to original sources

arXiv2023

Signatures of a magnetic superstructure phase induced by ultrahigh magnetic fields in a breathing pyrochlore antiferromagnet

The mutual coupling of spin and lattice degrees of freedom is ubiquitous in magnetic materials and potentially creates exotic magnetic states in response to the external magnetic field. Particularly, geometrically frustrated magnets serve as a fertile playground for realizing magnetic superstructure phases. Here, we observe an unconventional two-step magnetostructural transition prior to a half-magnetization plateau in a breathing pyrochlore chromium spinel by means of state-of-the-art magnetization and magnetostriction measurements in ultrahigh magnetic fields available up to 600 T. Considering a microscopic magnetoelastic theory, the intermediate-field phase can be assigned to a magnetic superstructure with a three-dimensional periodic array of 3-up-1-down and canted 2-up-2-down spin molecules. We attribute the emergence of the magnetic superstructure to a unique combination of the strong spin-lattice coupling and large breathing anisotropy.

cond-mat.mtrl-sci↗

arXiv2019

A series of magnon crystals appearing under ultrahigh magnetic fields in a kagomé antiferromagnet

Search for a new quantum state of matter emerging in a crystal is one of recent trends in condensed matter physics. For magnetic materials, geometrical frustration and high magnetic field are two key ingredients to realize it: a conventional magnetic order is possibly destroyed by competing interactions (frustration) and is replaced by an exotic state that is characterized in terms of quasiparticles, that are magnons, and the magnetic field can control the density and chemical potential of the magnons. Here we show that a synthetic copper mineral, Cd-kapellasite, comprising a kagome lattice made of corner-sharing triangles of Cu2+ ions carrying spin-1/2 exhibits an unprecedented series of fractional magnetization plateaux in ultrahigh magnetic fields up to 160 T, which may be interpreted as crystallizations of emergent magnons localized on the hexagon of the kagome lattice. Our observation reveals a novel type of particle physics realized in a highly frustrated magnet.

cond-mat.str-el↗

arXiv2017

Tomonaga-Luttinger spin liquid in the spin-1/2 inequilateral diamond-chain compound K$_3$Cu$_3$AlO$_2$(SO$_4$)$_4$

K$_3$Cu$_3$AlO$_2$(SO$_4$)$_4$ is a highly one-dimensional spin-1/2 inequilateral diamond-chain antiferromagnet. Spinon continuum and spin-singlet dimer excitations are observed in the inelastic neutron scattering spectra, which is in excellent agreement with a theoretical prediction: a dimer-monomer composite structure, where the dimer is caused by strong antiferromagnetic (AFM) coupling and the monomer forms an almost isolated quantum AFM chain controlling low-energy excitations. Moreover, muon spin rotation/relaxation spectroscopy shows no long-range ordering down to 90~mK, which is roughly three orders of magnitude lower than the exchange interaction of the quantum AFM chain. K$_3$Cu$_3$AlO$_2$(SO$_4$)$_4$ is, thus, regarded as a compound that exhibits a Tomonaga-Luttinger spin liquid behavior at low temperatures close to the ground state.

cond-mat.str-el↗

arXiv2015

Exciton splitting in semiconducting carbon nanotubes in ultrahigh magnetic fields above 300 T

In high magnetic fields, the exciton absorption spectrum of a semiconducting single-walled carbon nanotube splits as a result of Aharonov-Bohm magnetic flux. A magnetic field of 370 T, generated by the electro-magnetic flux compression destructive pulsed magnet-coil technique, was applied to single-chirality semiconducting carbon nanotubes. Using streak spectroscopy, we demonstrated the separation of the independent band-edge exciton states at the K and K' points of the Brillouin zone after the mixing of the dark and bright states above 150 T. These results enable a quantitative discussion of the whole picture of the Aharonov-Bohm effect in single-walled carbon nanotubes.

cond-mat.mtrl-sci↗

arXiv2013

Precise Measurement of a Magnetic Field Generated by the Electromagnetic Flux Compression Technique

The precision of the values of a magnetic field generated by electromagnetic flux compression was investigated in ultra-high magnetic fields of up to 700 T. In an attempt to calibrate the magnetic field measured by pickup coils, precise Faraday rotation (FR) measurements were conducted on optical (quartz and crown) glasses. A discernible "turn-around" phenomenon was observed in the FR signal as well as the pickup coils before the end of a liner implosion. We found that the magnetic field measured by pickup coils should be corrected by taking into account the high-frequency response of the signal transmission line. Near the peak magnetic field, however, the pickup coils failed to provide reliable values, leaving the FR measurement as the only method to precisely measure an extremely high magnetic fields.

physics.ins-det↗

arXiv2010

Terahertz conductivity spectroscopy of Co-doped BaFe$_2$As$_2$ Thin Film

We investigated the complex conductivity spectrum of a Co-doped BaFe$_2$As$_2$ epitaxial thin film in the THz region. In the normal state, the complex conductivity shows a Drude-type frequency dependence, while in the superconducting state, the frequency dependence of the complex conductivity changes to that of a typical superconducting materials. We estimated the magnetic penetration depth at absolute zero to be 710 nm and the superconducting gap energy to be 2.8 meV, which is considered to be the superconducting gap opened at the electron-type Fermi surface near the M point. We succeeded in obtaining the low-energy elementary excitation of a Fe-based superconductor using the electromagnetic method without invoking the Kramers-Kronig transformation.

cond-mat.supr-con↗

arXiv2009

Static Friction as a Function of Waiting Time Probed by Dynamics of Driven Vortices in La2-xSrxCuO4 Thin Films

We investigated the dynamics of driven vortices in high-$T_c$ superconductor as an ideal model system to study the physics of friction. The waiting-time dependence of the maximum static friction force, $F_s(t_w)$, was measured in La$_{2-x}$Sr$_x$CuO$_4$ thin films with different structures, sample sizes and pinning force. We found various kinds of $F_s(t_w)$ in the $B$-$T$ phase diagram and in different types of samples. The results suggest that the relaxation by thermal fluctuation is strongly affected by the pinning strength, the vortex bundle size and the system size. Based on these results, we found crucial conditions to determine the validity of the Amontons-Coulomb's law, and proposed a criterion.

cond-mat.supr-con↗