arXiv subjects

M. Tokumoto

Publications and source records attributed to M. Tokumoto.

13 recordsLinked to original sources

arXiv2010

A bulk 2D Pauli Limited Superconductor

We present a nearly perfect Pauli-limited critical field phase diagram for the anisotropic organic superconductor \$α$-(ET)$_2$NH$_4$(SCN)$_4$ when the applied magnetic field is oriented parallel to the conducting layers. The critical fields (${H_{c_2}$) were found by use of penetration depth measurements. Because ${H_{c_2}$ is Pauli-limited, the size of the superconducting energy gap can be calculated. The role of spin-orbit scattering and many-body effects play a role in explaining our measurements.

cond-mat.supr-con↗

arXiv2003

Critical field and Shubnikov-de Haas oscillations of k-(ET)2Cu(NCS)2 under pressure

A tuned tank circuit in combination with a nonmetallic diamond anvil cell has been successfully used to measure the change in critical field with angle in k-(ET)2Cu(NCS)2 at pressures up to 1.75 kbar and at temperatures down to 70 mK. The critical field has been found to decrease by more than 90% within less than 2 kbar and at a much higher rate for the field applied parallel to the conducting planes. For this orientation, at 1.75 kbar, we have seen a clear change from the ambient pressure behavior of the critical field with temperature at low temperatures. Up to P = 1.75 kbar, the H_c2($θ$) phase diagram is in good agreement with the theoretical prediction for weakly coupled layered superconductors. We have also succeeded in measuring oscillations in the resistivity of the normal state at higher magnetic field. The $α$-orbit Shubnikov-de Haas frequency was found to increase at a rate of 44 T/kbar. Our experiment opens the possibility for further investigations of the effective mass with pressure, especially because the setup is suitable for pulse fields as well.

cond-mat.supr-con↗

arXiv2002

Fermiology and superconductivity studies on the non-tetrachalcogenafulvalene structured organic superconductor beta-(BDA-TTP)_2SbF_6

The quantum oscillatory effect and superconductivity in a non-tetrachalcogenafulvalene (TCF) structure based organic superconductor beta-(BDA-TTP)_2SbF_6 are studied. Here the Shubnikov-de Haas effect (SdH) and angular dependent magnetoresistance oscillations (AMRO) are observed. The oscillation frequency associated with a cylindrical Fermi surface is found to be about 4050 tesla, which is also verified by the tunnel diode oscillator (TDO) measurement. The upper critical field Hc2 measurement in a tilted magnetic field and the TDO measurement in the mixed state reveal a highly anisotropic superconducting nature in this material. We compared physical properties of beta-(BDA-TTP)_2SbF_6 with typical TCF structure based quasi two-dimensional organic conductors. A notable feature of beta-(BDA-TTP)_2SbF_6 superconductor is a large value of effective cyclotron mass m_c^*=12.4+/1.1 m_e, which is the largest yet found in an organic superconductor. A possible origin of the enhanced effective mass and its relation to the superconductivity are briefly discussed.

cond-mat.str-el↗

arXiv2001

Novel Features of the Newly Discovered Field-Induced Superconducting Phase of lambda-BETS2FeCl4

We examine magnetic field dependent properties associated with the newly discovered field-induced superconducting state (FISC) in lambda-BETS2FeCl4. These include the metal-to-antiferromagnetic insulator transition, the critical field of the FISC state in tilted magnetic fields, and the low-pressure, magnetic field dependence of the insulating and superconducting phases.

cond-mat.supr-con↗

arXiv2001

Experimental observation of Frohlich superconductivity in high magnetic fields

Resistivity and irreversible magnetisation data taken within the high-magnetic-field CDWx phase of the quasi-two-dimensional organic metal alpha-(BEDT-TTF)2KHg(SCN)4 are shown to be consistent with a field-induced inhomogeneous superconducting phase. In-plane skin-depth measurements show that the resistive transition on entering the CDWx phase is both isotropic and representative of the bulk.

cond-mat.supr-con↗

arXiv2001

Superconductivity in an organic insulator at very high magnetic fields

We investigate by electrical transport the field-induced superconducting state (FISC) in the organic conductor $λ$-(BETS)$_2$FeCl$_4$. Below 4 K, antiferromagnetic-insulator, metallic, and eventually superconducting (FISC) ground states are observed with increasing in-plane magnetic field. The FISC state survives between 18 and 41 T, and can be interpreted in terms of the Jaccarino-Peter effect, where the external magnetic field {\em compensates} the exchange field of aligned Fe$^{3+}$ ions. We further argue that the Fe$^{3+}$ moments are essential to stabilize the resulting singlet, two-dimensional superconducting state

cond-mat.str-el↗

arXiv2000

Electronic Structure of Novel Cation-Radical Salts in High Magnetic Fields

Two organic conducting materials, where unusual aspects of their composition play important roles, are explored: beta''-(BEDT-TTF)2SF5XSO3 which exhibits superconductivity, or a metal-insulator transition (for X=CH2CF2 or CHF respectively), and tau-(P-S, S -DMEDT-TTF)2 (AuBr2) (AuBr2)y which exhibits a large, hysteretic, negative magnetoresistance. Detailed angular dependent magnetoresistance studies that allow a tomographic view of the electronic structure of these materials with increasing magnetic fields are presented.

cond-mat.str-el↗

arXiv2000

On the absence of spin-splitting in alpha-(BEDT-TTF)2KHg(SCN)4

We report the results of a detailed study of the field orientation-dependence of the de Haas-van Alphen waveform in $α$-(BEDT-TTF)$_2$KHg(SCN)$_4$. By considering the field orientation-dependence of the sign and phase of the fundamental $α$ frequency, at fields both well above and below the kink transition field, it is found that a single value for the product of the effective mass with the electron {\it g}-factor can explain the experimental data deep within both the high magnetic field and low magnetic field phases. This implies that spin-splitting does not occur within the low magnetic field phase until the angle between the magnetic field and the normal to the conducting planes is $\sim$~42$^\circ$. This finding contrasts greatly with that recently published by Sasaki and Fukase, implying that electron-electron interactions do not play a significant role in the formation of the charge-density wave ground state. The manner in which the amplitude of the waveform of the oscillations is damped within the low magnetic field phase is indicative of a non harmonically indexed reduction of the amplitude, thereby eliminating both magnetic breakdown and impurity scattering as dominant damping mechanisms within this phase. Meanwhile, the presence of a large amplitude second harmonic within the low magnetic field phase that has a negative sign over a broad range of angles can only be explained by the frequency doubling effect.

cond-mat.str-el↗

arXiv2000

Shubnikov-de Haas effect and Yamaji oscillations in the antiferromagnetically ordered organic superconductor k-(BETS)2FeBr4: A Fermiology study

Shubnikov-De Haas effect (SdH) effect and angular dependent magnetoresistance oscillations (AMRO) were observed in the organic superconductor: $κ$-(BETS)$_2$FeBr$_4$. In contrast to its isostructural compound $κ-(BETS)$_2$FeCl$_4$, SdH oscillations, for fields perpendicular to the conducting planes, reveal three Fermi Surface (FS) closed orbits $α$, $β$, and $γ$ whose cross sectional areas are 19.8 %, 99.9 %, and 2.4 % of the first Brillouin zone, respectively. The conduction electron effective masses were found to be: $μ_α = (4.7 \pm 0.2)$ m$_e$, $μ_β = (8.0 \pm 1.0)$ m$_e$, and $μ_γ = (2.0 \pm 0.2)$ m$_e$. The observation of a $γ$ orbit is not expected from band structure calculations, $μ_β$ is among the heaviest masses ever reported for an organic conductor. The observed Yamaji-like AMRO indicates a 2-D closed FS, warped along the $k_z$ direction.

cond-mat.str-el↗

arXiv2000

Competition between Pauli and orbital effects in a charge-density wave system

We present angular dependent magneto-transport and magnetization measurements on alpha-(ET)2MHg(SCN)4 compounds at high magnetic fields and low temperatures. We find that the low temperature ground state undergoes two subsequent field-induced density-wave type phase transitions above a critical angle of the magnetic field with respect to the crystallographic axes. This new phase diagram may be qualitatively described assuming a charge density wave ground state which undergoes field-induced transitions due to the interplay of Pauli and orbital effects.

cond-mat.mtrl-sci↗

arXiv2000

Critical State Behaviour in a Low Dimensional Metal Induced by Strong Magnetic Fields

We present the results of magnetotransport and magnetic torque measurements on the alpha-(BEDT-TTF)2KHg(SCN)4 charge-transfer salt within the high magnetic field phase, in magnetic fields extending to 33 T and temperatures as low as 27 mK. While the high magnetic field phase (at fields greater than ~ 23 T) is expected, on theoretical grounds, to be either a modulated charge-density wave phase or a charge/spin-density wave hybrid, the resistivity undergoes a dramatic drop below ~ 3 K within the high magnetic field phase, falling in an approximately exponential fashion at low temperatures, while the magnetic torque exhibits pronounced hysteresis effects. This hysteresis, which occurs over a broad range of fields, is both strongly temperature-dependent and has several of the behavioural characteristics predicted by critical-state models used to describe the pinning of vortices in type II superconductors in strong magnetic fields. Thus, rather than exhibiting the usual behaviour expected for a density wave ground state, both the transport and the magnetic properties of alpha-(BEDT-TTF)2KHg(SCN)4, at high magnetic fields, closely resembles those of a type II superconductor.

cond-mat.str-el↗

arXiv1996

Magnetoresistance and magnetic breakdown in the quasi-two-dimensional conductors (BEDT-TTF)$_2$MHg(SCN)$_4$[M=K,Rb,Tl]

The magnetic field dependence of the resistance of (BEDT-TTF)$_2$MHg(SCN)$_4$[M=K,Rb,Tl] in the density-wave phase is explained in terms of a simple model involving magnetic breakdown and a reconstructed Fermi surface. The theory is compared to measurements in pulsed magnetic fields up to 51 T. The value implied for the scattering time is consistent with independent determinations. The energy gap associated with the density-wave phase is deduced from the magnetic breakdown field. Our results have important implications for the phase diagram.

cond-mat↗

arXiv1996

Quantum Hall Transitions in (TMTSF)$_2$PF$_6$

We have studied the temperature dependence of the integer quantum Hall transitions in the molecular crystal (TMTSF)$_2$PF$_6$. We find that the transition width between the quantum Hall plateaus does not exhibit the universal power-law scaling behavior of the integer quantum Hall effect observed in semiconducting devices. Instead, the slope of the $ρ_{xy}$ risers, $dρ_{xy}/dB$, and the (inverse) width of the $ρ_{xx}$ peaks, $(ΔB)^{-1}$, show a BCS-like energy gap temperature dependence. We discuss these results in terms of the field-induced spin-density wave gap and order parameter of the system.

cond-mat↗