Observation of bulk superconductivity in NaxCoO2.yH(2)O and NaxCoO2.yD(2)O powder and single crystals

Poly- and single-crystalline NaxCoO2 has been successfully intercalated with H2O and D2O as confirmed by x-ray diffraction and thermogravimetric analysis. Resistivity, magnetic susceptibility, and specific heat measurements show bulk superconductivity with T(c) close to 5 K in both cases. The substitution of deuterium for hydrogen has an effect on T(c) of less than 0.2 K. Investigation of the resistivity anisotropy of NaxCoO2.yH(2)O single crystals shows (a). almost zero resistivity below T(c), and (b). an abrupt upturn at T(*) approximately 52 K in both the ab plane and the c direction. The implications of our results on the possible superconducting mechanism will be discussed.     

Optical Tomograms of Multiple-Photon-Added Gaussian States via the Intermediate State Representation Theory

The oxygen isotope effect in PrBaMn₂^16-18 O_5.97 manganite with an ordered cation arrangement is studied. The field dependences of magnetic susceptibility and magnetization are measured in the temperature range 100–270 K and magnetic fields up to 32 T. A significant increase in the temperature of the spin-reorientation antiferromagnet–ferromagnet phase transition is detected in samples enriched in heavy oxygen ¹⁸O (negative isotope effect). The transition temperature and the isotope effect depend strongly on the magnetic field. An H–T phase diagram is plotted for samples with various isotope compositions. An analysis of the experimental results demonstrates that the detected negative isotope effect and the giant positive isotope effect revealed earlier in doped manganites have the same nature. The mechanisms of appearance of isotope effects are discussed in terms of the double exchange model under a polaron narrowing of the free carrier band.     

Scanning tunneling spectroscopy of Bi(2)Sr(2)CuO(6+delta): new evidence for the common origin of the pseudogap and superconductivity

Using scanning tunneling spectroscopy, we investigated the temperature dependence of the quasiparticle density of states of overdoped Bi(2)Sr(2)CuO(6+delta) between 275 mK and 82 K. Below T(c) = 10 K, the spectra show a gap with well-defined coherence peaks at +/-Delta(p) approximately 12 meV, which disappear at T(c). Above T(c), the spectra display a clear pseudogap of the same magnitude, gradually filling up and vanishing at T(*) approximately 68 K. The comparison with Bi(2)Sr(2)CaCu(2)O(8+delta) demonstrates that the pseudogap and the superconducting gap scale with each other, providing strong evidence that they have a common origin.     

Oxygen isotope effect in the ab-plane reflectance of underdoped YBa(2)Cu(3)O(7-delta)

We have measured the effect of oxygen isotope substitution on the ab-plane reflectance of underdoped YBa(2)Cu(3)O(7-delta). The frequency shift of the transverse optic phonons due to the substitution of 16O by 18O yields an isotope effect of the expected magnitude for copper-oxygen stretching modes with alpha=0.5+/-0.1. The reflectance shoulder at 400-500 cm(-1) shows a much smaller exponent of alpha=0.1+/-0.1 in the normal state and alpha=0.23+/-0.1 in the superconducting state. These observations suggest that the shoulder is of electronic origin and not due to a phonon mode as has been suggested recently.     

Pressure-tuned collapse of the Mott-like state in Ca(n+1)Ru(n)O(3n+1) (n=1,2): Raman spectroscopic studies

We report a Raman scattering study of the pressure-induced collapse of the Mott-like phases of Ca3Ru2O7 (T(N)=56 K) and Ca2RuO4 (T(N)=110 K). The pressure dependence of the phonon and two-magnon excitations in these materials indicate (i) a T approximately 0 pressure-induced collapse of the antiferromagnetic (AF) insulating phase above P(*) approximately 55 kbar in Ca3Ru2O7 and P(*) approximately 5-10 kbar in Ca2RuO4, (ii) a remarkable insensitivity of the exchange interaction to pressure in both systems, and (iii) evidence for persistent AF correlations above the critical pressure of Ca2RuO4, suggestive of phase separation involving AF insulator and ferromagnetic metal phases.     

Nitrogen-15 fractionation in the thermal decomposition of nitrous oxide of natural isotopic composition

The 15N fractionation in the thermal decomposition of nitrous oxide (N2O) of natural isotopic composition has been investigated in quartz reaction vessel in the temperature interval 888-1073 K. The formulas relating the observed experimentally 15N fractionations with the primary 15N kinetic isotope effect, (k14/k15)p for 14N15N16O, and secondary 15N kinetic isotope effect, (k14/k15)s for 15N14N16O, have been derived. The experimentally estimated 15N kinetic isotope effects have been compared with the primary and secondary 15N kinetic isotope effects calculated with the absolute rate theory formulations applied to linear three atom molecules. A good agreement was found for the primary 15N kinetic isotope effect, (k14/k15)p, in the temperature interval 888-1007 K. But at 1073 K the decompositions of N2O, accompanied by NO (nitric oxide) formation proceed with a twice times smaller primary kinetic isotope effect, (k14/k15)p of 1.0251 +/- 0.0009, only, suggesting the nonlinear transition state structures with participation of the fourth external atom at high temperature decompositions of nitrous oxide. The nitrogen isotope effects determined in this study correlate well with nitrogen isotope fractionations observed in the natural biological, earth and atmospheric processes.     

Structural and magnetic properties of ion-beam deposited NiFe/Co-oxide bilayers

The structural and magnetic properties of NiFe/Co-oxide bilayers were studied. XRD investigation indicates the NiFe (Permalloy) layers (a= 3.53 Å) grow with a (111) preferred orientation. The Co-oxide layers were fabricated with oxygen content in the deposition assist beam ranging from 8% (rock-salt CoO, a= 4.27 Å) to 34% (spinel Co3O4, a= 8.21 Å). Both the coercivity Hc and exchange bias field Hex closely follow an inverse NiFe thickness relationship. A strong temperature dependence of Hc and Hex is found in these NiFe/Co-oxide bilayers. At T= 289 K, the NiFe/CoO film exhibits an enhanced Hc relative to pure NiFe/Co or NiFe/Co3O4 bilayers, an indication of exchange coupling between the NiFe and CoO phases. At T= 10 K, the NiFe/Co3O4 film exhibits an exchange-bias loop shift of Hex∼- 200 Oe that persists to temperatures greater than 30 K (the Néel temperature of bulk Co3O4). The transition temperature of the Co3O4 film component has increased above the bulk value via exchange coupling with the permalloy. Results indicate that the exchange coupling interaction between FM and AFM layers are responsible for both enhanced coercivity and cross-tie domains.     

Isotope effect in hydrogen peroxide formation during H2O and D2O sonication

The kinetics of hydrogen peroxide formation have been studied during H2O and D2O sonication in the presence of argon and oxygen (f = 22 kHz, I = 3.0 W cm-2, Pac = 0.52 W ml-1, V = 20 ml, T = 20 degrees C). It was found that the sonochemical reaction rate W has a zero order with respect to hydrogen peroxide (H2O, D2O or DHO2) concentration. In argon atmosphere the kinetic isotope effect was found to be equal to alpha = WH2O/WD2O = 2.2 +/- 0.3. The alpha value decreases in H2O-D2O mixtures with increasing H2O concentration. In oxygen atmosphere the isotope effect is not observed (alpha = 1.05 +/- 0.10). It is assumed that the revealed isotope effect is related to the mechanism of water sonolysis including the H2O-Ar* and D2O-Ar* energy transition, where Ar* is an argon atom in an excited state, in nonequilibrium plasma generated by the shock-wave.     

Magneto-optical evidence for a gapped fermi surface in underdoped YBa2Cu3O6+x

The infrared (900-1100 cm(-1)) Faraday rotation and circular dichroism are measured in the normal state of underdoped High T(c) superconductors and used to study the magnetotransport. YBa2Cu3O6+x thin films are investigated in the temperature range 10-300 K in magnetic fields up to 8 T and as a function of oxygen concentration. A dramatic increase of the Hall frequency is observed for underdoped samples, which is not consistent with the approach to a Mott transition but is consistent with a partial gapping of the Fermi surface as predicted in density wave models.     

Boron isotope effect in superconducting MgB2

We report the preparation method of and boron isotope effect for MgB2, a new binary intermetallic superconductor with a remarkably high superconducting transition temperature T(c)(10B) = 40.2 K. Measurements of both temperature dependent magnetization and specific heat reveal a 1.0 K shift in T(c) between Mg11B2 and Mg10B2. Whereas such a high transition temperature might imply exotic coupling mechanisms, the boron isotope effect in MgB2 is consistent with the material being a phonon-mediated BCS superconductor.     

Evidence for unconventional strong-coupling superconductivity in PrOs4Sb12: an Sb nuclear quadrupole resonance study

We report Sb-NQR results which evidence a heavy-fermion (HF) behavior and an unconventional superconducting (SC) property in Pr(Os4Sb12 with T(c)=1.85 K. The temperature (T) dependence of nuclear-spin-lattice-relaxation rate, 1/T(1), and NQR frequency unravel a low-lying crystal-electric-field splitting below T0 approximately 10 K, associated with Pr3+(4f(2))-derived ground state. In the SC state, 1/T(1) shows neither a coherence peak just below T(c) K nor a T3-like power-law behavior observed for anisotropic HF superconductors with the line-node gap. The isotropic energy gap with its size Delta/k(B)=4.8 K seems to open up across T(c) below T(*) approximately 2.3 K. It is surprising that Pr(Os4Sb12 looks like an isotropic HF superconductor-it may indeed argue for Cooper pairing via quadrupolar fluctuations.     

Search for the rare decays B-->Kl(+)l(-) and B-->K(*)l(+)l(-)

We present results from a search for the flavor-changing neutral current decays B-->Kl(+)l(-) and B-->K(*)l(+)l(-), where l(+)l(-) is either an e(+)e(-) or mu(+)mu(-) pair. The data sample comprises 22.7 x 10(6) Upsilon(4S)-->B(-)B decays collected with the BABAR detector at the PEP-II B Factory. We obtain the 90% C.L. upper limits B(B-->Kl(+)l(-))<0.51 x 10(-6) and B(B-->K(*)l(+)l(-))<3.1 x 10(-6), close to standard model predictions for these branching fractions. We have also obtained limits on the lepton-family-violating decays B-->Ke+/-mu(-/+) and B-->K(*)e(+/-)mu(-/+).     

压力对HgBa2Ca2Cu3O8+y超导转变温度的影响

 利用低温超高压装置，测量了Hg系样品HgBa₂Ca₂Cu₃O_8+y（Hg-1223）超导转变温度T_c在压力作用下的增强效应。压力最高达7.8 GPa，超导起始转变温度常压下为130 K，加压到5.4 GPa时获得最高温度为140 K。在5.4 GPa以下获得dT_c/dp为1.85 K/GPa。用压力作用下氧原子位置的改变使载流子浓度提高和CuO₂面间的耦合作用来解释高温超导的压力效应。     

Absence of a loss of in-plane infrared spectral weight in the pseudogap regime of Bi(2)Sr(2)CaCu(2)O(8+delta)

The ab-plane reflectance of Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi-2212) thin films was measured in the 30-25 000 cm(-1) range for one underdoped ( T(c) = 70 K), and one overdoped sample ( T(c) = 63 K) down to 10 K. We find similar behaviors in the temperature dependence of the normal-state infrared response of both samples. Above T(c), the effective spectral weight, obtained from the integrated conductivity, does not decrease when T decreases, so that no opening of an optical pseudogap is seen. We suggest that these are consequences of the pseudogap opening in the k = (0,pi) direction and of the in-plane infrared conductivity being mostly sensitive to the k = (pi,pi) direction.     

Effect of the 16O → 18O isotopic substitution and a magnetic field on the spin dynamics in the (La0.25Pr0.75)0.7Ca0.3MnO3 manganite according to the 139La NMR data

¹³⁹La nuclear magnetic resonance spectra have been recorded and the spin-spin relaxation times in the paramagnetic region in external magnetic fields of 5 and 9.4 T have been measured for two types of manganites: initial (La_0.25Pr_0.75)_0.7Ca_0.3Mn¹⁶O₃ and enriched with the ¹⁸O isotope ((La_0.25Pr_0.75)_0.7Ca_0.3Mn¹⁸O₃). The manganite enriched with the heavier oxygen isotope exhibits disappearance of a signal in the charge-ordering region (T < T _co) in an external field of 5 T. This effect is related to the anomalous increase in the spin-spin relaxation rate. With an increase in the external magnetic field to 9.4 T, the difference in the behavior of the relaxation rate for the two samples in the charge-ordering region remains pronounced, although the magnitude of this effect becomes much smaller. The observed giant isotope effect is a bright evidence of the important role of oxygen motion in the formation of long-range magnetic order in the manganites under consideration. Original Russian Text ? K.N. Mikhalev, I.E. Litvinov, Z.N. Nigamat’yanova, A.Yu. Yakubovskii, A.R. Kaul’, O.Yu. Gorbenko, K. Kumagai, Y. Furukawa, 2007, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2007, Vol. 71, No. 5, pp. 726–729.     

Oxygen isotope effect in the Ruddlesden-Popper phases

The oxygen isotope effect in ceramic samples of bilayer manganites (La_1−z Pr _z )_1.2Sr_1.8Mn₂ ^16–18O₇(z = 0, 0.3) has been investigated. The real and imaginary parts of the magnetic susceptibility have been measured and a significant (more than 20 K) decrease in the ferromagnetic transition temperature upon oxygen isotope substitution ¹⁶O—¹⁸O is found. At temperatures T > 170 K, a number of additional ferromagnetic transitions are observed, whose critical temperatures also shift as a result of the isotope substitution. The obtained results are compared from the data for other manganite systems where giant isotope effect is observed. Original Russian Text ? A.N. Taldenkov, N.A. Babushkina, A.V. Inyushkin, R. Suryanarayanan, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 1, pp. 124–127.     

Field-dependent diamagnetic transition in magnetic superconductor Sm1.85Ce0.15CuO4-y

The magnetic penetration depth of single crystal Sm(1.85)Ce(0.15)CuO(4-y) was measured down to 0.4 K in dc fields up to 7 kOe. For insulating Sm2CuO4, Sm3+ spins order at the Ne el temperature, T(N)=6 K, independent of the applied field. Superconducting Sm(1.85)Ce(0.15)CuO(4-y) (T(c) approximately 23 K) shows a sharp increase in diamagnetic screening below T(*)(H) which varied from 4.0 K (H=0) to 0.5 K (H=7 kOe) for a field along the c axis. If the field was aligned parallel to the conducting planes, T(*) remained unchanged. The unusual field dependence of T(*) indicates a spin-freezing transition that dramatically increases the superfluid density.     

Pressure effects on the transition temperature and the magnetic field penetration depth in the pyrochlore superconductor RbOs2O6

Magnetization measurements under hydrostatic pressure up to 8 kbar in the pyrochlore superconductor RbOs2O6 (T(c) approximately or equal 6.3 K at p=0) were carried out. A positive pressure effect on T(c) with dT(c)/dp=0.090(3) K/kbar was observed, whereas no pressure effect on the magnetic penetration depth lambda was detected. The pressure independent ratio 2 Delta(0)/k(B)T(c)=3.72(2) (Delta(0) is the superconducting gap at zero temperature) was found to be close to the BCS value 3.52. Magnetization and muon-spin rotation measurements of lambda(T) indicate that RbOs2O6 is an adiabatic s-wave BCS-type superconductor. The value of lambda extrapolated to zero temperature and ambient pressure was estimated to be 230(30) nm.     

Changes in the magnetic structure of (La0.25Pr0.75)0.7Ca0.3MnO3 upon the isotopic substitution of 18O for 16O

A giant isotope effect, wherein the low-temperature metallic state is replaced by an insulator state, was recently observed when ¹⁸O was substituted for ¹⁶O in (La_0.25Pr_0.75)_0.7Ca_0.3MnO₃ [N. A. Babushkina, L. M. Belova, O. Yu. Gorbenko et al., Nature (London) 391, 159 (1998)]. In the present work, the temperature evolution of the magnetic structure of two samples of this compound is studied by neutron diffraction. One sample contained a natural mixture of oxygen isotopes, 99.7% ¹⁶O, while the other was enriched to 75% with the isotope ¹⁸O. It is established that the samples are crystallographically identical at room temperature. As the temperature decreases, the sample with ¹⁶O undergoes successive antiferromagnetic (T _AFM=5150 K) and ferromagnetic (T _FM=110 K) transitions, which lead to the establishment of a noncollinear ferromagnetic structure, while a purely antiferromagnetic (T _AFM=150 K) order arises in the sample with ¹⁸O. The temperature dependences of the intensities of the diffraction peaks associated with the charge ordering differ substantially in samples with ¹⁶O and ¹⁸O, and they correlate with the behavior of their electric resistance and magnetic structure. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 46–51 (10 January 1999)     

Effect of strontium substitution on superconductivity in YBa 2 Cu 4 O 8

Superconducting powders with the composition Y(Ba 1 mx Sr x ) 2 Cu 4 O 8 ( x = 0.00-0.50) have been prepared by an aqueous sol-gel method. The effects of strontium substitution on the properties of compounds were studied by resistivity measurements, X-ray powder diffraction, thermogravimetric analysis, electron microscopy and elemental analysis. The data indicate that single-phase Y(Ba 1 mx Sr x ) 2 Cu 4 O 8 superconducting samples were obtained. They also show that doping with strontium has a strong effect on the superconducting properties of the YBa 2 Cu 4 O 8 phase. The critical temperature is enhanced from 78 K (for a non-substituted sample) to 88 K (for Y(Ba 1 mx Sr x ) 2 Cu 4 O 8 ). Elemental analysis data clearly indicate that small amounts of strontium enter the copper sites in the Y(Ba 1 mx Sr x ) 2 Cu 4 O 8 compounds. A point-defect chemistry approach, which explains the enhancement of T c by substituting strontium for barium, is presented.