Antiferromagnetic Transition in Sr14−x Ca x Cu24O41 Investigated by Specific Heat Measurements

Results for low temperature specific heat for single crystals of ladder/chain materials Sr _14–x Ca _x Cu ₂₄ O ₄₁ (x= 0, 8, 10, 11.5) in magnetic field up to 15 T are reported. A sharp peak of the specific heat of a single crystal Sr _2.5 Ca _11.5 Cu ₂₄ O ₄₁, was found at 2.3 K, clearly indicating the onset of a long range order. In measurements of the specific heat in magnetic fields the transition temperature, T _N , was found to shift to lower temperatures with increasing magnetic field, indicating that the ordering is antiferromagnetic, which has been confirmed by neutron scattering measurements. With decreasing x T _N rapidly shifts to lower temperatures. For x=10 T _N decreases to 1 K and for x= 0 no anomaly indicating the transition was found down to 0.6K     

Magnetic Phase Transition and Electronic Anomalies in La1.96−x Y0.04Sr x CuO4 around x∼1/8

La-NMR, Hall coefficient and sound velocity have been measured in order to investigate the correlation between the magnetic ordering, the prominent change of transport properties, the suppression of T _c and the structural transformation around x1/8 in La _1.96–x Y _0.04 Sr _x CuO ₄. By the substitution of 0.04/2 Y for La-sites, the structural transformation to the low temperature tetragonal phase (LTT) is caused at 57±14 K for samples around x0.115. The. prominent decrease of the Hall coefficient followed by the sign reversal and the magnetic ordering are observed around x0.115 below temperatures T _s and T _N, respectively. T _s vs x shows a bell shaped curve with the maximum value of 65 K at x00115 where most prominent suppression of T _c appears. T _N shows similar x dependence to T _s with the maximum value of 40 K at x0.115. The change of the electronic state below T _s and the suppression of T _c become more prominent and the magnetic ordering is observed more wide range of x under the LTT phase.     

Low-temperature spin fluctuations in La2−xSrxCuO4 studied by Cu nuclear spin-lattice relaxation

Measurements of the temperature (T) dependence of the planar⁶³Cu nuclear spin-lattice relaxation rate (1/T₁)for the⁶³Cu-enriched high-T_c Cu oxides, La_2–xSr_xCuO₄ (x=0.13 and 0.18; T_c34K), are reported. 1/T₁T shows a maximum around 50 K, similar to that for V₂O₃ under a high pressure and for the bilayered high-T_c Cu oxides. The T-dependent anisotropy of 1/T₁ suggests that the T-dependent antiferromagnetic correlation length can explain the peaked T-dependence of 1/T₁T.     

Susceptibility and X-Ray Absorption Measurements on Superconducting (Hg,Mo)Sr2CuO4+δ System

In continuation of our recent work [1] on superconducting 1201-type (Hg,M)Sr₂CuO₄₊, M = Cr system, the samples of (Hg_1–x Mx)Sr₂CuO₄₊, M = Mo (x = 0.15, 0.25, and 0.30), were synthesised in partial vacuum. The x = 0.15 composition shows a maximum T _c ^onset of 66 K, while x = 0.25 exhibits T _c ^onset of 50 K. For x = 0.30 sample, the observed T _c ^onset of 36 K is discretely different from the reported values of 20 and 58 K. X-ray absorption measurements on (Hg,Mo)-1201 samples show Hg in divalent state and Cu predominantly in Cu²⁺ state. A weak but distinct Cu¹⁺ signature is also observed in the SC (Hg,Mo)-1201 samples, which is absent in the impurity SrCuO₂ and Sr₂CuO₃ phases. On the basis of available reports, it is viewed that Cu¹⁺ can arise because of a small fraction of Cu occupying the Hg site in (Hg,Mo)-1201 system.     

Non fermi liquid behavior in strongly correlated f-electron materials

Evidence for non Fermi liquid (NFL) behavior in y_1–x U_xPd₃ and related systems is reviewed and discussed within the context of possible microscopic mechanisms. Low temperature electrical resistivity, specific heat, and magnetic susceptibility measurements on the Th_1–xU_xPd₂Al₃ system reveal unconventional Kondo behavior with NFL low temperature characteristics. Magnetic susceptibility measurements on UCu_3.5Pd_1.5, which has previously been shown to exhibit NFL behavior, are presented. Some systematics of the NFL low temperature behavior observed in several f-electron materials include a linear temperature dependence of the electrical resistivity 1–aT with either positive or negative coefficient a, a logarithmically diverging specific heat C/T –lnT, and T^1/2 asymptotic behavior of the magnetic susceptibility 1 – T^1/2.     

Non fermi liquid ground states in strongly correlated f-electron materials

Experimental efforts to characterize and develop an understanding of non Fermi liquid (NFL) behavior at low temperature in f-electron materials are reviewed for three f-electron systems: M_1–xU_xPd₃ (M = Sc, Y), U_1–xTh_xPd₂Al₃, and UCu_5–xPd_x. The emerging systematics of NFL behavior in f-electron systems, based on the present sample of nearly ten f-electron systems, is updated. Many of the f-electron systems exhibit the following temperature dependences of the electrical resistivity p, specific heat C, and magnetic susceptibility for T T₀, where T_o is a characteristic temperature: P(T) 1 –aT/T ₀, where a < 0 or > 0, C(T)/T (-1/T _o) In (T/bT ₀), and (T) 1 –c(T/T_o)^1/2. In several of the f-electron systems, the characteristic temperature T_o can be identified with the Kondo temperature T_k.     

Effects of Substituting Ni and Zn for Cu in the Electron Doped Sr0.9La0.1CuO2 Superconductor

We used X-ray diffraction, energy dispersive spectroscopy and low field magnetization to characterize Sr_0.9La_0.1Cu_1–x R _x O₂ samples for R=Zn and Ni. We found that both Ni and Zn are substituted at the Cu-site. The value of T _c was almost independent of composition until x0.03 for R=Zn while it was reduced to 30 K at x0.02 for R=Ni. This trend of T _c reduction is very similar to the pair breaking-effect in conventional superconductors and electron-doped (Nd, Ce)₂CuO_4– but the rate of reduction of T _c for R=Ni is larger in Sr_0.9La_0.1CuO₂ than in (Nd, Ce)₂CuO_4–. These results are discussed along with the recent experimental observation of gap symmetry in electron-doped superconductors.     

Pressure effect on anomalous suppression ofT c around x=1/8 in La2−xBaxCuO4 and La1.8−xx Nd0.2BaxCuO4

In La_2–xBa_xCuO₄ (LBCO) and La_1.8–xNd_0.2Ba_xCuO₄ (LNBCO), the low-temperature structural change at T₁ (=60–110K), resulting from tilt of the CuO₆ octahedra, is easily suppressed at high pressures. However, the structural change of LNBCO (x=0.125) remains up to 2GPa though it jumps from T₁(110K) to T* (60K), where the electric resistivity begins to upturn, at 0-1.5GPa. The upturn of is also suppressed at high pressures, and the superconducting critical temperature T_c is fully restored except in a very narrow x-range around x=0.125. The T_c for x 0.125 is appreciably lower at 2GPa than those for other Ba-contents.     

Magnetization and Re LIII-Edge Studies of (Hg,Re)Sr2CuO4 + δ System

Chemically stable nominal compositions of (Hg_1-x Re _x )Sr₂CuO_{4 +} (x = 0.05, 0.10, and 0.20), designated as (Hg,Re)-1201 phase, were synthesized at 920°C in partial vacuum. The samples with x = 0.05 and 0.10 are superconducting (SC) with T _c ^onset 40 and 54 K, respectively. On the other hand, for the composition with x = 0.20, the sample is nonsuperconducting down to 5 K. The sample with composition (Hg_0.9Re_0.1)Sr₂CuO_{4 +} on cooling, below 10 K in dc-field of 4 kOe, shows a dramatic decrease (reversal) in diamagnetic signal. Such a reduction in diamagnetic response is also reflected by the sample's supercurrent induced magnetic hysteresis curve generated at 9 K. In the light of the available reports and our Re L_III-edge measurements, we attribute this effect to a field enhanced paramagnetic contribution probably arising mainly from mixed valent (6⁺, 7⁺ dominant) Re ions. The critical current density (J _c), calculated using Bean's model, for (Hg_0.9Re_0.1)Sr₂CuO_{4 +} is 6 × 10³ A/cm² (4.5 K, 5 kOe).     

Annealing Effects in (Hg,Cr)Sr2CuO4+δ: Transport and X-Ray Absorption Studies

Superconducting (Hg_1–x Cr _x )Sr₂CuO₄₊, x 0.36, samples of 1201-type, synthesized in partial vacuum, show T _c ^onset of 58 K, T _c (R = 0) 52 K. It is found that T _c is not affected by the subsequent Ar or O₂ annealing treatment. Remarkably, the annealed samples show significant improvement in the diamagnetic signal. Hg L₃-edge measurements on 1201 samples show divalent state of mercury. The Cu K-edge spectra for the samples, after taking due account of the impurity phases, show noticeable modifications in the split main peak features on Ar or O₂ annealing, suggesting changes in the crystal field asymmetry. In the superconducting (SC) samples, a weak but distinct signature of Cu¹⁺ is seen. The Cu¹⁺ feature is absent in all the non-SC 1201 samples as well as in the impurity phase SrCuO₂ and Sr₂CuO₃ samples. An attempt is made to explain the observed Cu¹⁺ feature in SC (Hg,Cr)-1201 in the light of the available reports.     

Ferromagnetic Correlations in Ca-Doped Sr2RuO4: 87Sr NMR Study

Magnetic properties on Sr_2–x Ca _x RuO₄ have been investigated by a microscopic probe of ⁸⁷Sr-NMR in order to understand the magnetic character on spin-triplet superconductor of Sr₂RuO₄, which has multibands on the Fermi surface. With substituting Ca for Sr which gives rise to crystal distortion, the Knight shift (K) and the nuclear spin-lattice relaxation rate divided by temperature (1/T _{1 T}) increases progressively up to x=1.5. The Korringa relation from K and 1/T ₁ T becomes smaller, indicative of development of ferromagnetic fluctuations with increasing Ca content. This suggests that the q-independent spin fluctuations originating from the 2-dimensional band are changed to the ferromagnetic ones by the Ca doping.     

Transport anomalies in the Zn-substituted La2−x Sr x Cu1−y Zn y O4 with x ∼ 0.115; Possibility of the Pinning of CDW and SDW by Zn

Anomalies in the thermoelectric power and the Hall coefficient, which are analogous to those observed in La _2–x Ba _x CuO ₄ with x 1/8, have been observed in the Zn-substituted La _2–x Sr _x Cu _1–y Zn _y O ₄ with x = 0.115 and y = 0 – 0.025 and with x = 0.15 and y 0.02. Based on the so-called stripe model, these are explained as being due to the formation of the static order of the stripe-patterned CDW and SDW on account of the pinning by Zn. Accordingly, the long-standing problem on the conspicuous suppression of superconductivity in the Zn-substituted La _2–x Sr _x Cu _1–y Zn _y O ₄ with x 0.115 is likely to be attributed to this static order.     

A simple theory for the cuprates: the Antiferro-magnetic van Hove scenario

A model of weakly interacting hole quasiparticles is proposed to describe the normal state of the high temperature superconductors. The effect of strong correlations is contained in the dispersion of the holes. Many-body effects induce anomalous quasiparticleflat bands similar to those observed in recent angle-resolved photoemission experiments. A model of weakly interacting hole quasiparticles is proposed to describe the physics of carriers in the cuprates. The model predicts superconductivity in the d_x2-_y2 channel, with a typical T_c 100K. The concept of optimal doping appears naturally in this model, as well as a large ratio 2/T _c 5.     

Superconducting Properties of the 3-K Phase of Sr2RuO4 near Hc2

The superconducting transition temperature, T _c , of the impurity-free, intrinsic Sr₂RuO₄ is as high as 1.50 K. However, we recently showed that T _c is remarkably increased up to 3 K in the Sr₂RuO₄–Ru eutectic system, in which plate-like microdomains of Ru metal are embedded in the primary-phase Sr₂RuO₄. The phase diagram of the anisotropic upper critical field of the 3-K phase indicates that H _c2 for the field parallel to the RuO₂ plane is strongly suppressed at low temperatures. We argue that the reorientation of the Cooper-pair spin direction near the Sr₂RuO₄–Ru interface may be responsible for this suppression. In addition, we observed unusual hysteresis in the out-of-plane resistivity, _c , at low temperatures and near H _c2, only when the field was applied parallel to the RuO₂ plane.     

Influence of Charge Doping on Thermal Diffusivity of Double Perovskite Sr2FeMoO6 Studied by Mirage Effect

Effects of charge doping on thermal diffusivity have been investigated in double perovskite ferromagnetic Sr_2–x La _x FeMoO₆ (0 x 0.4) by means of the mirage effect at 300 K ( the critical temperature T _c 420 K). Substitution of the La³⁺ ions for the Sr²⁺ ions significantly increases the value of the thermal diffusivity from 0.39 cm² · s^–1 at x = 0 to 0.54 cm² · s^–1 at x = 0.4. The increased thermal diffusivity is ascribed to the extra itinerant electrons on the Mo4d band.     

Cyclotron Resonance of Electrons and Holes in Paramagnetic and Ferromagnetic InMnAs-Based Films and Heterostructures

We have studied the cyclotron resonance of electrons and holes in various types of InMnAs-based structures at ultrahigh magnetic fields. Our observations, in conjunction with an eight-band effective mass model including the s–d and p–d exchange interactions with Mn d-electrons, unambiguously suggest the existence of s-like and p-like delocalized carriers in all samples studied. The samples studied include Paramagnetic n-type In_1–x Mn _x As films (x 0.12) grown on GaAs, ferromagnetic p-type In_1–x Mn _x As films (x 0.025) grown on GaAs with Curie temperatures (T _C) > 5 K, paramagnetic n-type In_1–x Mn _x As/InAs superlattices, ferromagnetic p-type In_1–x Mn _x As/GaSb heterostructures (x 0.09) with T _C = 30-60 K, and ferromagnetic (In_0.53Ga_0.47)_1–x Mn _x As/In_0.53Ga_0.47As heterostructures (x 0.05) grown on InP with T _C up to 120 K.     

Spin susceptibility of La2−xSrxCuO4; Modification of localized character of Cu 3d-Electrons at x≳0.15

The magnetic susceptibility X of superconducting La_2–xSr_xCuO₄ (LSCO) was investigated over a wide x-range. The effective magnetic moment on the Cu-site of LSCO is progressively suppressed at x0.15 owing to deterioration of the localized character of Cu 3d-electrons, and the superconducting transition temperature T_c begins to decrease near x=0.15. The density of states at the Fermi level, estimated from the electronic specific heat or the T-independent spin part of X, is enhanced above x 0.15.     

Substitution effects on the electronic anomaly and superconductivity in La2−xBaxCuO4 and La2−xSrxCuO4 with x ∼ 1/8

The effects of substitution of Y or Bi on the low-temperature structural phase transition, electronic anomaly and superconductivity have been studied in La_2-y-xY_yBa_xCuO La_2-y-xBi_yBa_xCuO₄ and La_1.9-y-xTb_0.1Bi_ySr_xCuO₄ with x = 1/8. Moreover, an electronic anomaly, which is analogous to those observed in La_2–xBa_xCuO₄ and La_1.6-xNd_0.4Sr_xCuO₄ with x 1/8, has been found below about 60 K in the thermoelectric power measurements for the Zn-substituted La_2–xSr_xCu_1–yZn_yO₄ with x 0.115 and y = 0.01 – 0.02. These experimental results support the conclusions by Tranquada et al. that static order of the stripe correlations of holes and spins, owing to pinning by the low-temperature structure or impurities (Zn in this case), is the origin of the electronic anomaly, leading to the suppression of superconductivity.     

Anomalous Reentrant Resistance Phenomena in Superconducting Sr1 ? xKxBiO3: Recovery of Superconductivity with Electric or Magnetic Field

The temperature-dependent resistivity of Sr_{1 – x}K_xBiO₃, with x = 0.4–0.6, has been measured as a function of the magnetic field (or electrical current). Although X-ray diffraction results are more or less identical to single phase for the measured 10 samples, the EDS results indicate that the potassium content, x, varies from 0.4 to 0.6 and the electrical resistance varies quite sensitively from sample to sample. For the samples of resistivity less than 4 m cm at room temperature with (15 K)/(273 K) < 0.9, superconductivity is observed with T_c 12 K. Other samples exhibit a reentrant resistance below the superconducting transition temperature. The reentrant resistance, however, decreases as the external magnetic field (or electrical current) is applied, and some samples show the recovery of superconductivity upon the application of a magnetic field (or electrical current). Disorderness in the junction area between superconducting grains seems to be vital for the observed anomalous reentrant resistance. Further investigations are on to understand this intriguing phenomena.     

17O Knight shift study in the superconducting state of Sr2RuO4

¹⁷O Knight shift measurements in Sr₂RuO₄ were performed over the wide range of magnetic field 3.2-11.4kOe parallel to the basal RuO₂ planes. The spin susceptibility is totally unchanged through its T_c, evidencing that the spin-triplet superconducting state is realized in Sr₂RuO₄. The result indicates that the Cooper pairs consist of the parallel spin pairs | > and | > with their quantization axis perpendicular to the c-axis direction. The in-plane 2D nearly ferromagnetic spin fluctuations may play a role for the stabilization of this state among various representations of spin-triplet order parameter.