arXiv subjects

M. Fujisawa

Publications and source records attributed to M. Fujisawa.

9 recordsLinked to original sources

arXiv2023

Time-, spin-, and angle-resolved photoemission spectroscopy with a 1-MHz 10.7-eV pulse laser

We describe a setup of time-, spin-, and angle-resolved photoemission spectroscopy (tr-SARPES) employing a 10.7-eV ($λ$=115.6 nm) pulse laser at 1-MHz repetition rate as a probe photon source. This equipment effectively combines technologies of a high-power Yb:fiber laser, ultraviolet-driven harmonic generation in Xe gas, and a SARPES apparatus equipped with very-low-energy-electron-diffraction (VLEED) spin detectors. A high repetition rate (1 MHz) of the probe laser allows experiments with the photoemission space-charge effects significantly reduced, despite a high flux of 10$^{13}$ photons/s on the sample. The relatively high photon energy (10.7 eV) also brings the capability of observing a wide momentum range that covers the entire Brillouin zone of many materials while ensuring high momentum resolution. The experimental setup overcomes a low efficiency of spin-resolved measurements, which gets even more severe for the pump-probed unoccupied states, and affords for investigating ultrafast electron and spin dynamics of modern quantum materials with energy and time resolutions of 25 meV and 360 fs, respectively.

cond-mat.mtrl-sci↗

arXiv2019

Ultrafast unbalanced electron distributions in quasicrystalline 30° twisted bilayer graphene

Layers of twisted bilayer graphene exhibit varieties of exotic quantum phenomena1-5. Today, the twist angle Θ has become an important degree of freedom for exploring novel states of matters, i.e. two-dimensional superconductivity ( Θ = 1.1°)6, 7 and a two-dimensional quasicrystal (Θ = 30°)8, 9. We report herein experimental observation on the photo-induced ultrafast dynamics of Dirac fermions in the quasicrystalline 30° twisted bilayer graphene (QCTBG). We discover that hot carriers are asymmetrically distributed between the two graphene layers, followed by the opposing femtosecond relaxations, by using time- and angle-resolved photoemission spectroscopy. The key mechanism involves the differing carrier transport between layers and the transient doping from the substrate interface. The ultrafast dynamics scheme continues after the Umklapp scattering, which is induced by the incommensurate interlayer stacking of the quasi-crystallinity. The dynamics in the atomic layer opens the possibility of new applications and creates interdisciplinary links in the optoelectronics of van der Waals crystals.

cond-mat.mes-hall↗

arXiv2016

Ultrafast Melting of Spin Density Wave Order in BaFe$_{2}$As$_{2}$ Observed by Time- and Angle-Resolved Photoemission Spectroscopy with Extreme-Ultraviolet Higher Harmonic Generation

Transient single-particle spectral function of BaFe$_{2}$As$_{2}$, a parent compound of iron-based superconductors, has been studied by time- and angle-resolved photoemission spectroscopy with an extreme-ultraviolet laser generated by higher harmonics from Ar gas, which enables us to investigate the dynamics in the entire Brillouin zone. We observed electronic modifications from the spin-density-wave (SDW) ordered state within $\sim$ 1 ps after the arrival of a 1.5 eV pump pulse. We observed optically excited electrons at the zone center above $E_{F}$ at 0.12 ps, and their rapid decay. After the fast decay of the optically excited electrons, a thermalized state appears and survives for a relatively long time. From the comparison with the density-functional theory band structure for the paramagnetic and SDW states, we interpret the experimental observations as the melting of the SDW. Exponential decay constants for the thermalized state to recover back to the SDW ground state are $\sim$ 0.60 ps both around the zone center and the zone corner.

cond-mat.supr-con↗

arXiv2011

Magnetic and electric properties in the distorted tetrahedral spin chain system Cu3Mo2O9

We study the multiferroic properties in the distorted tetrahedral quasi-one dimensional spin system Cu$_3$Mo$_2$O$_9$, in which the effects of the low dimensionality and the magnetic frustration are expected to appear simultaneously. We clarify that the antiferromagnetic order is formed together with ferroelectric properties at $T_{\rm N}=7.9$ K under zero magnetic field and obtain the magnetic-field-temperature phase diagram by measuring dielectric constant and spontaneous electric polarization. It is found that the antiferromagnetic phase possesses a spontaneous electric polarization parallel to the c axis when the magnetic field $H$ is applied parallel to the a axis. On the other hand, there are three different ferroelectric phases in the antiferromagnetic phase for $H$ parallel to the c axis.

cond-mat.str-el↗

arXiv2009

$^{63/65}$Cu- and $^{35/37}$Cl-NMR Studies of Triplet Localization in the Quantum Spin System NH$_4$CuCl$_3$

$^{63/65}$Cu- and $^{35/37}$Cl-NMR experiments were performed to investigate triplet localization in the $S=1/2$ dimer compound NH$_4$CuCl$_3$, which shows magnetization plateaus at one-quarter and three-quarters of the saturation magnetization. In $^{63/65}$Cu-NMR experiments, signal from only the singlet Cu site was observed, because that from the triplet Cu site was invisible due to the strong spin fluctuation of onsite 3$d$-spins. We found that the temperature dependence of the shift of $^{63/65}$Cu-NMR spectra at the singlet Cu site deviated from that of macroscopic magnetization below T=6 K. This deviation is interpreted as the triplet localization in this system. From the $^{35/37}$Cl-NMR experiments at the 1/4-plateau phase, we found the two different temperature dependences of Cl-shift, namely the temperature dependence of one deviates below T=6 K from that of the macroscopic magnetization as observed in the $^{63/65}$Cu-NMR experiments, whereas the other corresponds well with that of the macroscopic magnetization in the entire experimental temperature region. We interpreted these dependences as reflecting the transferred hyperfine field at the Cl site located at a singlet site and at a triplet site, respectively. This result also indicates that the triplets are localized at low temperatures. $^{63/65}$Cu-NMR experiments performed at high magnetic fields between the one-quarter and three-quarters magnetization plateaus have revealed that the two differently oriented dimers in the unit cell are equally occupied by triplets, the fact of which limits the theoretical model on the periodic structure of the localized triplets.

cond-mat.stat-mech↗

arXiv2006

Magentic-Field Induced Quantum Phase Transition and Critical Behavior in a Gapped Spin System TlCuCl$_3$

Magnetization measurements were performed on TlCuCl$_3$ with gapped ground state. The critical density and the magnetic phase diagram were obtained. The interacting constant was obtained as $U/k_{\rm B} = 313$ K. The experimental phase boundary for $T < 5$ K agrees perfectly with the magnon BEC theory based on the Hartree-Fock approximation with realistic dispersion relations and $U/k_{\rm B} = 320 $ K. The exponent $ϕ$ obtained with all the data points for $T < 5$ K is $ϕ= 1.99$, which is somewhat larger than theoretical exponent $ϕ_{\rm BEC} =3/2$. However, it was found that the exponent converges at $ϕ_{\rm BEC} =3/2$ with decreasing fitting window.

cond-mat.str-el↗

arXiv2006

Drastic Change of Magnetic Phase Diagram in Doped Quantum Antiferromagnet TlCu$_{1-x}$Mg$_x$Cl$_3$

TlCuCl$_3$ is a coupled spin dimer system, which has a singlet ground state with an excitation gap of $Δ/gμ_{\mathrm B}$ = 5.5 T. TlCu$_{1-x}$Mg$_x$Cl$_3$ doped with nonmagnetic Mg$^{2+}$ ions undergoes impurity-induced magnetic ordering. Because triplet excitation with a finite gap still remains, this doped system can also undergo magnetic-field-induced magnetic ordering. By specific heat measurements and neutron scattering experiments under a magnetic field, we investigated the phase diagram in TlCu$_{1-x}$Mg$_x$Cl$_3$ with $x\sim 0.01$, and found that impurity- and field-induced ordered phases are the same. The gapped spin liquid state observed in pure TlCuCl$_3$ is completely wiped out by the small amount of doping.

cond-mat.str-el↗

arXiv2003

Neutron Scattering Study of Magnetic Ordering and Excitations in the Doped Spin Gap System Tl(Cu$_{1-x}$Mg$_x$)Cl$_3$

Neutron elastic and inelastic scattering measurements have been performed in order to investigate the spin structure and the magnetic excitations in the impurity-induced antiferromagnetic ordered phase of the doped spin gap system Tl(Cu$_{1-x}$Mg$_x$)Cl$_3$ with $x=0.03$. The magnetic Bragg reflections indicative of the ordering were observed at ${\pmb Q}=(h, 0, l)$ with integer $h$ and odd $l$ below $T_{\rm N}=3.45$ K. It was found that the spin structure of the impurity-induced antiferromagnetic ordered phase on average in Tl(Cu$_{1-x}$Mg$_x$)Cl$_3$ with $x=0.03$ is the same as that of the field-induced magnetic ordered phase for ${\pmb H} \parallel b$ in the parent compound TlCuCl$_3$. The triplet magnetic excitation was clearly observed in the $a^*$-$c^*$ plane and the dispersion relations of the triplet excitation were determined along four different directions. The lowest triplet excitation corresponding to the spin gap was observed at ${\pmb Q}=(h, 0, l)$ with integer $h$ and odd $l$, as observed in TlCuCl$_3$. It was also found that the spin gap increases steeply below $T_{\rm N}$ upon decreasing temperature. This strongly indicates that the impurity-induced antiferromagnetic ordering coexists with the spin gap state in Tl(Cu$_{1-x}$Mg$_x$)Cl$_3$ with $x=0.03$.

cond-mat.str-el↗

arXiv2003

Neutron Diffraction Study of the Pressure-Induced Magnetic Ordering in the Spin Gap System TlCuCl$_3$

Neutron elastic scattering measurements have been performed under the hydrostatic pressure in order to investigate the spin structure of the pressure-induced magnetic ordering in the spin gap system TlCuCl$_3$. Below the ordering temperature $T_{\rm N}=16.9$ K for the hydrostatic pressure $P=1.48$ GPa, magnetic Bragg reflections were observed at the reciprocal lattice points ${\mib Q}=(h, 0, l)$ with integer $h$ and odd $l$, which are equivalent to those points with the lowest magnetic excitation energy at ambient pressure. This indicates that the spin gap closes due to the applied pressure. The spin structure of the pressure-induced magnetic ordered state for $P=1.48$ GPa was determined.

cond-mat.str-el↗