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Register a new user$^{77}$Se NMR evidence for the Jaccarino-Peter mechanism in the field induced superconductor, $lambda$(BETS)$_2$FeCl$_4$}
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K. Hiraki
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We have performed $^{77}$Se NMR on a single crystal sample of the field induced superconductor $lambda$-(BETS)$_{2}$FeCl$_{4}$. Our results obtained in the paramagnetic state provide a microscopic insight on the exchange interaction $J$ between the spins textbf{s} of the BETS $pi$ conduction electrons and the Fe localized $d$ spins textbf{S}. The absolute value of the Knight shift textbf{K} decreases when the polarization of the Fe spins increases. This reflects the ``negative spin polarization of the $pi$ electrons through the exchange interaction $J$. The value of $J$ has been estimated from the temperature and the magnetic field dependence of textbf{K} and found in good agreement with that deduced from transport measurements (L. Balicas textit{et al}. Phys. Rev. Lett. textbf{87}, 067002 (2001)). This provides a direct microscopic evidence that the field induced superconductivity is due to the compensation effect predicted by Jaccarino and Peter (Phys. Rev. Lett. textbf{9}, 290 (1962)). Furthermore, an anomalous broadening of the NMR line has been observed at low temperature, which suggests the existence of charge disproportionation in the metallic state neighboring the superconducting phase.
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Complementary $^{77}$Se nuclear magnetic resonance (NMR) and electrical transport have been used to correlate the spin density dynamics with the subphases of the field-induced spin density wave (FISDW) ground state in tmt. We find that the peaks in the spin-lattice relaxation rate 1/T$_1$ appear within the metal-FISDW phase boundary and/or at first-order subphase transitions. In the quantum limit above 25 T, the NMR data gives an insight into the FISDW electronic structure.
$^{1}$H-NMR spin-echo measurements of the spin-echo decay $M(2tau)$ with a decay rate 1/$T_{2}$ and the frequency shift $Delta u/ u_{0}$ under applied magnetic field $mathbf{B}$$_{0}$ = 9 T along the a-axis over a temperature range 2.0$-$180 K are reported for a single crystal of the organic conductor $lambda$-(BETS)$_{2}$FeCl$_{4}$. It provides the spin dynamic and static properties in the paramagnetic metal (PM) and antiferromagnetic insulator (AFI) states as well as across the PM$-$AFI phase transition. A large slow beat structure in the spin-echo decay is observed with a typical beat frequency of $f$ $sim$ 7 kHz and it varies across the spectrum. Its origin is attributed to the $^{1}$H$-$$^{1}$H dipole interactions rather than to the much larger dipolar field contribution from the Fe$^{3+}$ electrons (spin $S$ = 5/2). A simple phenomenological model provides an excellent fit to the data. The dominant $^{1}$H-NMR frequency shift comes from the dipolar field from the 3d Fe$^{3+}$ ions, and the Fe$^{3+}$ $-$ Fe$^{3+}$ exchange interactions ($J_{0}$) ($J_{0} $ includes the d$-$d exchange interactions through the $pi-$electrons) have a substantial effect to the local field at the proton sites expecially at low temperatures. A good fit is obtained with $J_{0}$ = - 1.7 K. The data of the spin-echo decay rate 1/$T_{2}$ indicates that there is a significant change in the slow fluctuations of the local magnetic field at the $^{1}$H-sites on traversing the PM to AFI phase. This evidence supports earlier reports that the PM$-$AFI phase transition in $lambda$-(BETS)$_{2}% $FeCl$_{4} $ is driven magnetically and first order.
We report measurements of low-temperature specific heat on the 4f^2-based heavy-fermion superconductor PrOs4Sb12. In magnetic fields above 4.5 T in the normal state, distinct anomalies are found which demonstrate the existence of a field-induced ordered phase (FIOP). The Pr nuclear specific heat indicates an enhancement of the 4f magnetic moment in the FIOP. Utilizing a Maxwell relation, we conclude that anomalous entropy, which is expected for a single-site quadrupole Kondo model, is not concealed below 0.16 K in zero field. We also discuss two possible interpretations of the Schottky-like anomaly at ~3 K, i.e., a crystalline-field excitation or a hybridization gap formation.
In this work, the thermodynamic properties of the organic superconductor $lambda$-(BETS)$_2$GaCl$_4$ are investigated to study a high-field superconducting state known as the putative Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase. We observed a small thermodynamic anomaly in the field $H_{rm FFLO}$ $sim$ 10~T, which corresponds to the Pauli limiting field $H_{rm P}$. This anomaly probably originates from a transition from a uniform superconducting state to the FFLO state. $H_{rm FFLO}$ does not show a strong field-angular dependence due to a quasi-isotropic paramagnetic effect in $lambda$-(BETS)$_2$GaCl$_4$. The thermodynamic anomaly at $H_{rm FFLO}$ is smeared out and low-temperature upper critical field $H_{rm c2}$ changes significantly if fields are not parallel to the conducting plane even for a deviation of $sim$0.5$^{circ}$. This behavior indicates that the high-field state is very unstable, as it is influenced by the strongly anisotropic orbital effect. Our results are consistent with the theoretical predictions on the FFLO state, and show that the high-field superconductivity is probably an FFLO state in $lambda$-(BETS)$_2$GaCl$_4$ from a thermodynamic point of view.
We present magnetoresistivity measurements on the heavy-fermion superconductor UTe$_{2}$ in pulsed magnetic fields $mu_0H$ up to 68~T and temperatures $T$ from 1.4 to 80~K. Magnetic fields applied along the three crystallographic directions $mathbf{a}$ (easy magnetic axis), $mathbf{b}$, and $mathbf{c}$ (hard magnetic axes), are found to induce different phenomena - depending on the field direction - beyond the low-field suppression of the superconducting state. For $mathbf{H}parallelmathbf{a}$, a broad anomaly in the resistivity is observed at $mu_0H^*simeq10$~T and $T = 1.4$~K. For $mathbf{H}parallelmathbf{c}$, no magnetic transition nor crossover are observed. For $mathbf{H}parallelmathbf{b}$, a sharp first-order-like step in the resistivity indicates a metamagnetic transition at the field $mu_0H_m simeq 35$~T. When the temperature is raised signature of first-order metamagnetism is observed up to a critical endpoint at $T_{CEP}simeq7$~K. At higher temperatures a crossover persists up to 28~K, i.e., below the temperature $T_chi^{max} = 35$~K where the magnetic susceptibility is maximal. A sharp maximum in the Fermi-liquid quadratic coefficient $A$ of the low-temperature resistivity is found at $H_m$. It indicates an enhanced effective mass associated with critical magnetic fluctuations, possibly coupled with a Fermi surface instability. Similarly to the URhGe case, we show that UTe$_{2}$ is a candidate for field-induced reentrant superconductivity in the proximity of $H_m$.
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