arXiv subjects

Nobuyuki Kurita

Publications and source records attributed to Nobuyuki Kurita.

43 records · Page 3Linked to original sources

arXiv2011

Upper Critical Field of Pressure-Induced Superconductor EuFe$_2$As$_2$

We have carried out high-field resistivity measurements up to 27\,T in EuFe$_2$As$_2$ at $P$\,=\,2.5\,GPa, a virtually optimal pressure for the $P$-induced superconductivity, where $T_\mathrm{c}$\,=\,30\,K. The $B_\mathrm{c2}-T_\mathrm{c}$ phase diagram has been constructed in a wide temperature range with a minimum temperature of 1.6 K ($\approx 0.05 \times T_\mathrm{c}$), for both $B \parallel ab$ ($B_\mathrm{c2}^\mathrm{ab}$) and $B \parallel c$ ($B_\mathrm{c2}^\mathrm{c}$). The upper critical fields $B_\mathrm{c2}^\mathrm{ab}$(0) and $B_\mathrm{c2}^\mathrm{c}$(0), determined by the onset of resistive transitions, are 25 T and 22 T, respectively, which are significantly smaller than those of other Fe-based superconductors with similar values of $T_\mathrm{c}$. The small $B_\mathrm{c2}(0)$ values and the $B_\mathrm{c2}(T)$ curves with positive curvature around 20 K can be explained by a multiple pair-breaking model that includes the exchange field due to the magnetic Eu$^{2+}$ moments. The anisotropy parameter, $Γ=B_\mathrm{c2}^{ab}/B_\mathrm{c2}^{c}$, in EuFe$_2$As$_2$ at low temperatures is comparable to that of other "122" Fe-based systems.

cond-mat.supr-con↗

arXiv2010

Thermal and magnetic properties of a low-temperature antiferromagnet Ce$_4$Pt$_{12}$Sn$_{25}$

We report specific heat ($C$) and magnetization ($M$) of single crystalline Ce$_4$Pt$_{12}$Sn$_{25}$ at temperature down to $\sim$50mK and in fields up to 3T. $C/T$ exhibits a sharp anomaly at 180mK, with a large $ΔC/T\sim$30J/molK$^2$-Ce, which, together with the corresponding cusp-like magnetization anomaly, indicates an antiferromagnetic (AFM) ground state with a Néel temperature $T_N$=180mK. Numerical calculations based on a Heisenberg model reproduce both zero-field $C$ and $M$ data, thus placing Ce$_4$Pt$_{12}$Sn$_{25}$ in the weak exchange coupling $J<J_c$ limit of the Doniach diagram, with a very small Kondo scale $T_K\ll T_N$. Magnetic field suppresses the AFM state at $H^*\approx$0.7T, much more effectively than expected from the Heisenberg model, indicating additional effects possibly due to frustration or residual Kondo screening.

cond-mat.str-el↗

arXiv2010

Magnetotransport studies of EuFe$_2$As$_2$: the influence of the Eu$^{2+}$ magnetic moments

We report resistivity $ρ$ and Hall effect measurements on EuFe$_2$As$_2$ at ambient pressure and 28 kbar and magnetization measurements at ambient pressure. We analyze the temperature and magnetic-field dependence of $ρ$ and the Hall effect using a molecular-field theory for magnetoresistance and an empirical formula for the anomalous Hall effect and find that electron scattering due to the Eu$^{2+}$ local moments plays only a minor role in determining electronic transport properties of EuFe$_2$As$_2$.

cond-mat.supr-con↗

arXiv2010

Upper critical field and de Haas-van Alphen oscillations in KOs$_2$O$_6$ measured in a hybrid magnet

Magnetic torque measurements have been performed on a KOs$_2$O$_6$ single crystal in magnetic fields up to 35.3 T and at temperatures down to 0.6 K. The upper critical field is determined to be $\sim$30 T. De Haas-van Alphen oscillations are observed. A large mass enhancement of (1+$λ$) = $m^* / m_{band}$ = 7.6 is found. It is suggested that, for the large upper critical field to be reconciled with Pauli paramagnetic limiting, the observed mass enhancement must be of electron-phonon origin for the most part.

cond-mat.supr-con↗

arXiv2010

Comment on "Quantum Criticality and Nodal Superconductivity in the FeAs-Based Superconductor KFe$_2$As$_2$"

In a recent Letter [J. K. Dong et al., Phys. Rev. Lett. 104, 087005 (2010)], Dong \textit{et al}. have observed a $T^{1.5}$ dependence of resistivity $ρ$ in KFe$_2$As$_2$ at the upper critical field $B_{c2}$ = 5 T parallel to the c axis and have suggested the existence of a field-induced quantum critical point (QCP) at $B_{c2}$. In this comment, we argue that observation of a $T^{1.5}$ dependence of $ρ$ in a sample showing broad resistive transitions does not constitute evidence for a QCP and that recent dHvA results do not support the proposed QCP.

cond-mat.supr-con↗

arXiv2010

Fermi surface and mass enhancement in KFe$_2$As$_2$ from de Haas-van Alphen effect measurements

We report on a band structure calculation and de Haas-van Alphen measurements of KFe$_2$As$_2$. Three cylindrical Fermi surfaces are found. Effective masses of electrons range from 6 to 18$m_e$, $m_e$ being the free electron mass. Remarkable discrepancies between the calculated and observed Fermi surface areas and the large mass enhancement ($\gtrsim 3$) highlight the importance of electronic correlations in determining the electronic structures of iron pnicitide superconductors.

cond-mat.supr-con↗

arXiv2005

On the Mechanism of the Spin State Transition of (Pr1-ySmy)1-xCaxCoO3

Transport, thermal and magnetic measurements have been carried out on (Pr1-ySmy)1-xCaxCoO3. The system exhibits a structural phase transition accompanied by the spin state change from the intermediate spin (IS) state to the low spin (LS) state with decreasing temperature T. We have constructed a T-y phase diagram for x=0.3 and T-x ones for y=0.2 and 0.3. By analyzing their magnetic susceptibilities, the number of Co ions excited to IS state (or the electron number in the eg orbitals), nIS, are roughly estimated. With increasing y or with decreasing x, nIS decreases, and the phase transition changes gradually to the (IS-LS) crossover-like one. We discuss on the possible role of the Pr atoms in realizing the transition.

cond-mat.str-el↗