Temperature dependence of the magnetic penetration depth in the vortex state of the pyrochlore superconductor,Cd2Re2O7

We report transverse-field and zero-field muon spin rotation and relaxation studies of the superconducting rhenium oxide pyrochlore, Cd2Re2O7. Transverse-field measurements (H=0.007 T) show line broadening below T(c), which is characteristic of a vortex state, demonstrating conclusively the type-II nature of this superconductor. The penetration depth is seen to level off below about 400 mK (T/T(c) approximately 0.4), with a rather large value of lambda(T=0) approximately 7500 A. The temperature independent behavior below approximately 400 mK is consistent with a nodeless superconducting energy gap. Zero-field measurements indicate no static magnetic fields developing below the transition temperature.     

Temperature evolution of spin-polaron in-gap states in undoped antiferromagnetic cuprates

The temperature evolution of in-gap states created by the spin polaron effect and located within the gap with charge transfer between the valence and conduction bands is studied for the case of strong electron correlations using the t-t′-t″-J model of antiferromagnetic undoped cuprates. The effect of temperature is taken into account by temperature renormalization of the magnon concentration, which is calculated using the Heisenberg model with inclusion of weak interlayer exchange and weak in-plane spin anisotropy, and by introducing a Lorentzian with a temperature-dependent half-width in the form corresponding to the marginal Fermi liquid model. With increasing temperature, the spectral weight of the in-gap state, which is proportional to the magnon concentration, grows leading to an increased intensity of the corresponding peak of the spectral function in all points of the Brillouin zone. At points (π/2, π/2) and (π, 0), the main peak is approached by the satellite peak related to the in-gap band and, at points (0, 0) and (π, π), the peaks move away from each other.     

Coherent magnetic oscillation in the spin ladder system alpha(')- NaV2O5

We report the first observation of coherent magnetic excitations in a spin ladder system NaV2O5 by using femtosecond time-domain spectroscopy. A pronounced coherent oscillation is observed at 127 cm(-1) (nearly twice the spin gap energy) and assigned to a two-magnon bound state, based on the temperature dependence of the intensity below the charge ordering phase transition at T(C) = 34 K. This mode can be observable only when circularly polarized light is used as a pump or a probe beam, suggesting that it corresponds to a spin-flip excitation from the singlet ground state. A phonon mode strongly coupled to the spin state is also found at 303 cm(-1).     

Enhancement of superconducting transition temperature due to the strong antiferromagnetic spin fluctuations in the noncentrosymmetric heavy-fermion superconductor CeIrSi3: A 29Si NMR study under pressure

We report a (29)Si NMR study on the pressure-induced superconductivity (SC) in an antiferromagnetic (AFM) heavy-fermion compound CeIrSi(3) without inversion symmetry. In the SC state at P = 2.7-2.8 GPa, the temperature (T) dependence of the nuclear-spin lattice relaxation rate 1/T(1) below T(c) exhibits a T(3) behavior without any coherence peak just below T(c), revealing the presence of line nodes in the SC gap. In the normal state, 1/T(1) follows a square root T-like behavior, suggesting that the SC emerges under the non-Fermi-liquid state dominated by AFM spin fluctuations enhanced around a quantum critical point. The reason why the maximum T(c) in CeIrSi(3) is relatively high among the Ce-based heavy-fermion superconductors may be the existence of the strong AFM spin fluctuations. We discuss the comparison with the other Ce-based heavy-fermion superconductors.     

Commensurate spin dynamics in the superconducting state of an electron-doped cuprate superconductor

We report neutron scattering studies on two single crystal samples of the electron-doped (n-type) superconducting (SC) cuprate Nd2-xCexCuO4 (x=0.15) with T(c)=18 and 25 K. Unlike the hole-doped (p-type) SC cuprates, where incommensurate magnetic fluctuations commonly exist, the n-type cuprate shows commensurate magnetic fluctuations at the tetragonal (1/2 1/2 0) reciprocal points both in the SC and in the normal state. A spin gap opens up when the n-type cuprate becomes SC, as in the optimally doped p-type La2-xSrxCuO4. The gap energy, however, increases gradually up to about 4 meV as T decreases from T(c) to 2 K, which contrasts with the spin pseudogap behavior with a T-independent gap energy in the SC state of p-type cuprates.     

Magnetic ground state of an experimental S=1/2 kagome antiferromagnet

We present a detailed analysis of the heat capacity of a near-perfect S=1/2 kagome antiferromagnet, zinc paratacamite Zn(x)Cu(4-x)(OH)(6)Cl(2), as a function of stoichiometry x-->1 and for fields of up to 9 T. We obtain the heat capacity intrinsic to the kagome layers by accounting for the weak Cu2+/Zn2+ exchange between the Cu and the Zn sites, which was measured independently for x=1 using neutron diffraction. The evolution of the heat capacity for x=0.8...1 is then related to the hysteresis in the magnetic susceptibility. We conclude that for x>0.8 zinc paratacamite is a spin liquid without a spin gap, in which unpaired spins give rise to a macroscopically degenerate ground state manifold with increasingly glassy dynamics as x is lowered.     

NMR evidence for gapped spin excitations in metallic carbon nanotubes

We report on the spin dynamics of 13C isotope enriched inner walls in double-wall carbon nanotubes using 13C nuclear magnetic resonance. Contrary to expectations, we find that our data set implies that the spin-lattice relaxation time (T1) has the same temperature (T) and magnetic field (H) dependence for most of the inner-wall nanotubes detected by NMR. In the high-temperature regime (T approximately > or = 150 K), we find that the T and H dependence of 1/T1T is consistent with a 1D metallic chain. For T approximately < or = 150 K we find a significant increase in 1/T1T with decreasing T, followed by a sharp drop below approximately = 20 K. The data clearly indicate the formation of a gap in the spin excitation spectrum, where the gap value 2delta approximately = 40 K (congruent to 3.7 meV) is H independent.     

Spin excitations and spin wave gap in the ferromagnetic Weyl semimetal Co_3Sn_2S_2

We report a comprehensive neutron scattering study on the spin excitations in the magnetic Weyl semimetal Co3Sn2S2 with a quasi-two-dimensional structure.Both in-plane and out-of-plane dispersions of the spin waves were revealed in the ferromagnetic state.Similarly,dispersive but damped spin excitations were found in the paramagnetic state.The effective exchange interactions were estimated using a semi-classical Heisenberg model to consistently reproduce the experimental TCand spin stiffness.However,a full spin wave gap below Eg=2.3 meV was observed at T=4 K.This value was considerably larger than the estimated magnetic anisotropy energy(~0.6 meV),and its temperature dependence indicated a significant contribution from the Weyl fermions.These results suggest that Co3Sn2S2 is a three-dimensional correlated system with a large spin stiffness,and the low-energy spin dynamics can interplay with the topological electron states.     

Magnetic field effect on the superconducting magnetic gap of Nd1.85Ce0.15CuO4

Inelastic neutron-scattering measurements on the archetypical electron-doped material Nd1.85Ce0.15CuO4 up to a high relative magnetic-field strength, H/H(c2) approximately 50%, reveal a simple linear magnetic-field effect on the superconducting magnetic gap and the absence of field-induced in-gap states. The extrapolated gap-closing field value is consistent with the upper critical field H(c2), and the high-field response resembles that of the paramagnetic normal state.     

Antiferromagnetic spin fluctuations above the dome-shaped and full-gap superconducting states of LaFeAsO1-xFx revealed by (75)As-nuclear quadrupole resonance

We report a systematic study by (75)As nuclear-quadrupole resonance in LaFeAsO(1-x)F(x). The antiferromagnetic spin fluctuation found above the magnetic ordering temperature T(N) = 58 K for x = 0.03 persists in the regime 0.04 ≤ x ≤ 0.08, where superconductivity sets in. A dome-shaped x dependence of the superconducting transition temperature T(c) is found, with the highest T(c) = 27 K at x = 0.06, which is realized under significant antiferromagnetic spin fluctuation. With increasing x further, the antiferromagnetic spin fluctuation decreases, and so does T(c). These features resemble closely the cuprates La(2-x)Sr(x)CuO(4). In x = 0.06, the spin-lattice relaxation rate (1/T(1)) below T(c) decreases exponentially down to 0.13T(c), which unambiguously indicates that the energy gaps are fully opened. The temperature variation of 1/T(1) below T(c) is rendered nonexponential for other x by impurity scattering.     

Low-temperature heat transport of spin-gapped quantum magnets

This article reviews low-temperature heat transport studies of spin-gapped quantum magnets in the last few decades. Quantum magnets with small spins and low dimensionality exhibit a variety of novel phenomena. Among them, some systems are characteristic of having quantum-mechanism spin gap in their magnetic excitation spectra, including spin-Peierls systems, S=1 Haldane chains, S= 1/2 spin ladders, and spin dimmers. In some particular spin-gapped systems, the XY-type antiferromagnetic state induced by magnetic field that closes the spin gap can be described as a magnon Bose-Einstein condensation (BEC). Heat transport is effective in probing the magnetic excitations and magnetic phase transitions, and has been extensively studied for the spin-gapped systems. A large and ballistic spin thermal conductivity was observed in the two-leg Heisenberg S=1/2 ladder compounds. The characteristic of magnetic thermal transport of the Haldane chain systems is quite controversial on both the theoretical and experimental results. For the spin-Peierls system, the spin excitations can also act as heat carriers. In spin-dimer compounds, the magnetic excitations mainly play a role of scattering phonons. The magnetic excitations in the magnon BEC systems displayed dual roles, carrying heat or scattering phonons, in different materials.     

Unique spin dynamics and unconventional superconductivity in the layered heavy fermion compound CeIrIn5: NQR evidence

We report measurements of the 115In nuclear spin-lattice relaxation rate ( 1/T1) between T = 0.09 and 100 K in the new heavy fermion (HF) compound CeIrIn5. At 0.4 < or = T< or = 100 K, 1/T1 is strongly T-dependent, which indicates that CeIrIn5 is much more itinerant than known Ce-based HFs. We find that 1/T1T, subtracting that for LaIrIn5, follows a (1 / T+straight theta)3/4 variation with straight theta = 8 K. We argue that this novel feature points to anisotropic, due to a layered crystal structure, spin fluctuations near a magnetic ordering. The bulk superconductivity sets in at 0.40 K below which the coherence peak is absent and 1/T1 follows a T3 variation, which suggests unconventional superconductivity with line-node gap.     

Statics and dynamics of incommensurate spin order in a geometrically frustrated antiferromagnet CdCr2O4

Using elastic and inelastic neutron scattering we show that a cubic spinel, CdCr2O4, undergoes an elongation along the c axis (c > a = b) at its spin-Peierls-like phase transition at T(N) = 7.8 K. The Néel phase (T < T(N)) has an incommensurate spin structure with a characteristic wave vector Q(M) = (0, delta,1) with delta approximately 0.09 and with spins lying on the ac plane. This is in stark contrast to another well-known Cr-based spinel, ZnCr2O4, that undergoes a c-axis contraction and a commensurate spin order. The magnetic excitation of the incommensurate Néel state has a weak anisotropy gap of 0.6 meV and it consists of at least three bands extending up to 5 meV.     

Tomonaga-Luttinger liquid in a quasi-one-dimensional S = 1 antiferromagnet observed by specific heat measurements

Specific-heat experiments on single crystals of the S = 1 quasi-one-dimensional bond-alternating antiferromagnet Ni(C9H24N4)(NO2)ClO2 (NTENP) have been performed in magnetic fields applied both parallel and perpendicular to the spin chains. We have found for the parallel field configuration that the magnetic specific heat (C(mag)) is proportional to temperature (T) above a critical field H(c), at which the energy gap vanishes, in a temperature region above that of the long-range ordered state. The ratio C(mag)/T increases as the magnetic field approaches H(c) from above. The data are in good quantitative agreement with the prediction of the c= 1 conformal field theory in conjunction with the velocity of the excitations calculated by a numerical diagonalization, providing conclusive evidence for a Tomonaga-Luttinger liquid.     

Magnetic field effect on the static antiferromagnetism of the electron-doped superconductor Pr1-xLaCexCuO4 (x=0.11 and 0.15)

Effects of magnetic fields (applied along the c axis) on static spin correlation were studied for the electron-doped superconductors Pr1-xLaCexCuO4 with x=0.11 (T(c)=25 K) and x=0.15 (T(c)=16 K) by neutron-scattering measurements. In the x=0.11 sample, which is located near the antiferromagnetic (AF) and superconducting phase boundary, a commensurate magnetic order develops below around T(c) at zero field. Upon applying a magnetic field up to 9 T both the magnetic intensity and the onset temperature of the order increase with the maximum field effect at approximately 5 T. In contrast, in the overdoped x=0.15 sample any static AF order is neither observed at zero field nor induced by the field up to 8.5 T. Difference and similarity of the field effect between the hole- and electron-doped high-T(c) cuprates are discussed.     

Antiferromagnetic ordering driven by the molecular orbital order of C(60) in alpha'-tetra-kis-(Dimethylamino)-ethylene-C(60)

We have studied the ground state of a fullerene-based magnet, the alpha;{'}-phase tetra-kis-(dimethylamino)-ethylene-C60 (alpha'-TDAE-C(60)), by electron spin resonance and magnetic torque measurements. Below T(N) = 7 K, nonparamagnetic field dependent resonances with a finite excitation gap (1.7 GHz) are observed along the a axis. Strong enhancement in their intensity as temperature is decreased is inconsistent with excitation from a singlet state, which had been proposed for the alpha'-phase ground state. Below T(N), nonquadratic field dependence of the magnetic torque signal is also observed in contrast to quadratic field dependence in the paramagnetic phase. The angle-dependent torque signals below T(N) indicate the existence of an anisotropy of the bulk magnetization. From both experiments, we propose an antiferromagnetic ground state driven by the cooperative orientational ordering of C(60) in the alpha'-TDAE-C(60).     

Magnetic order in lightly doped La2-xSrxCuO4

We study long wavelength magnetic excitations in lightly doped La2-xSrxCuO4 (x相似文献   

Spin dynamics in iron-based layered superconductor (La0.87Ca0.13)FePO revealed by 31P and 139La NMR studies

We report 31P and 139La NMR studies of (La0.87Ca0.13)FePO, which is a family member of the recently discovered superconductor LaFeAs(O1-xFx). In the normal state, Knight shift and nuclear spin-lattice relaxation rate divided by T (1/T1T) show that a Fermi-liquid state with moderate ferromagnetic fluctuations emerges below 30 K. From 1/T1T of 31P and 139La, a quasi-two- dimensional electronic structure is suggested, in which the FeP layer is more conductive than the LaO layer. In the superconducting (SC) state, although a clear Meissner signal was observed, 1/T1T increases below Tc, in contrast to a decrease of 1/T1T due to the opening of a SC gap, suggesting that novel low-energy spin dynamics develop in the SC state.     

Cu site substitution effect on the Hall coefficient of La2−xSrxCuO4

The Cu site substitution effects on the Hall coefficient R_H and the electrical resistivity have been studied for La_2−xSr_xCuO₄. In a small x region, R_H decreases largely with Zn doping and increases with Ni doping. In the Ni doped samples, shows the characteristic temperature dependence similar to those of the unsubstituted samples where the decrease of the slope of vs. T curve is observed for x<0.1 above 600K but in the Zn doped samples, the change of the slope of becomes smaller,i.e. the temperature dependence of below 600K becomes smaller. These results indicate that the origin of the change of the slope of around 600K for x<0.1 is magnetic and the spin correlation or the electronic state is rather different below and above 600K. The unusual Zn doping dependences of R_H and are naturally explained by considering that the electronic state at high temperatures above 600K which has a small Hall coefficient expected for the large Fermi surface comes down to lower temperatures by the Zn doping.     

Unconventional superconductivity and electron correlations in the cobalt oxyhydrate Na0.35CoO2.yH2O from nuclear quadrupole resonance

We report a careful 59Co nuclear quadrupolar resonance measurement on the recently discovered cobalt oxyhydrate Na0.35CoO2.yH(2)O superconductor from T=40 K down to 0.2 K. We find that in the normal state the spin-lattice relaxation rate 1/T(1) follows a Curie-Weiss type temperature (T) variation, 1/T(1)T=C/(T-theta), with theta=-42 K, suggesting two-dimensional antiferromagnetic spin correlations. Below T(c)=3.9 K, 1/T(1) decreases with no coherence peak and follows a T(n) dependence with n approximately 2.2 down to approximately 2.0 K but crosses over to a 1/T(1) proportional to T variation below T=1.4 K, which suggests non-s-wave superconductivity. The data in the superconducting state are most consistent with the existence of line nodes in the gap function.