Diamagnetic Signal Research Articles

Superconductivity at 31 K in Alkaline Metal-Doped Cobalt Oxides

By using a simple solid reaction method, we have fabricated alkalinemetal doped cobalt oxides AnxCoO2 (An = Na, K). Themagnetic measurement shows a superconducting-like diamagnetic signal at31 K based on a strong superparamagnetic signal. Below 31 K, themagnetization hysteresis loops contain a strong rough linearsuperparamagnetic background and a superconducting hysteresis. Thetypical magnetization hysteresis loops for a type-II superconductor arefound. Preliminary resistive data also show a fast dropping ofresistance below Tc. These give indication ofsuperconductivity below 31 K in AnxCoO2 (An = Na, K).

Competition of superconductivity and ferromagnetism in CexC60 compounds

High temperature-treated compounds of mixed powders of cerium metal and C 60 solid show fairly strong diamagnetic signal in the zero-field-cooling process and distinct ferromagnetic behavior in the field-cooling path below 13.5K and 15K, respectively. In the resistivity measurement of a compacted and long-term heated pellet of the powder a distinct drop of the resistivity with decreasing temperature is recognized around 20K, which is followed by a steep rise of the resistivity below 14K. These findings indicate an existing competition of superconductivity and ferromagnetism in this compound.

Synthesis of new Ru-based cuprate with 1232 structure, RuSr2+xSmCe2−xCu2Oz

New phase of the Ru-based cuprate with 1232 structure has been synthesized in the RuSr2+xSmCe2−xCu2Oz system. The powder X-ray diffraction analysis shows that nearly the single phase can be formed in the x composition range from 0.0 to 0.2. The new phase has a tetragonal unit cell with the lattice parameters of about a=0.384 nm and c=1.70 nm. A bulk sample with x=0.2 shows two anomalies in resistivity at about 12 and 40 K. The former anomaly is considered to originate from the Ru-1232 phase. In temperature dependence of magnetization, the sample shows a diamagnetic signal at about 20 K and ferromagnetic order at about 80 K.

Coexistence of antiferromagnetism and superconductivity near the quantum criticality of the heavy-fermion compound CeRhIn5.

We report a study on the interplay between antiferromagnetism (AFM) and superconductivity (SC) in a heavy-fermion compound CeRhIn5 under pressure P=1.75 GPa. The onset of the magnetic order is evidenced from a clear split of 115In nuclear quadrupole resonance spectrum due to the spontaneous internal field below the Néel temperature T(N)=2.5 K. Simultaneously, bulk SC below T(c)=2.0 K is demonstrated by the observation of the Meissner diamagnetism signal whose size is the same as in the exclusively superconducting phase. These results indicate that the AFM coexists homogeneously with the SC at a microscopic level.

A muoniated radical in selenium

We report new 0.3 T transverse-field μSR experiments in crystalline Se which show only a small, slowly relaxing muon signal at 300 K , accounting for about 30% of the incoming muon polarization. However, at 90 K signals are observed around 74 and 157 MHz , characteristic of a radical with a hyperfine coupling of 231 MHz . Very fast relaxation which increases with temperature makes these signals impossible to follow beyond 200 K . Above 400 K a quickly relaxing diamagnetic signal becomes visible, its relaxation falling with increasing temperature. In the melt (>490 K) just a single non-relaxing diamagnetic signal is seen. These observations may be explained by electron spin-exchange with a muoniated radical.

Review of Experiments on the Gamma 10 Tandem Mirror

After the attainment of the density doubling due to the potential confinement in 1998, GAMMA 10 experiments have been directed to realization of a high density plasma with potential confinement and also to study dependencies between plasma parameters. These studies are important for understanding of the physics of potential formation in tandem mirrors and also for the development of a tandem mirror reactor. GAMMA 10 experiments have advanced after the last OS2000 Conference where we reported high density plasma production by using an ion cyclotron range of frequency (ICRF) heating at high harmonic frequency and a neutral beam injection in the anchor cell. However, the diamagnetic signal of the high density plasma decreased with application of ECRH. Recently a high density plasma was attained without degradation of the diamagnetic signal, which was attained by adjusting the spacing of the conducting plates installed in the anchor transition regions. Dependencies of particle and energy confinement times and plasma confining potential on the density up to a density of 4×1012 cm−3 are studied for the first time in the high density region.

New Ru-Based Layered Cuprates with 1232 Structure in the RuSr2(LnCe2−xSrx)Cu2Oz (Ln = Sm, Eu, and Gd) System

New Ru-based layered cuprates with 1232 structure have been synthesized in the RuSr2(LnCe2−x Sr x )Cu2O z (Ln = Sm, Eu, and Gd) system. Nearly single-phase samples are obtained in the composition range from x = 0.0 to 0.2 for Ln = Sm and Eu, and at the composition of x = 0.2 for Ln = Gd. Each of the samples of Ln = Sm, Eu, and Gd with x = 0.2 crystallizes in a tetragonal lattice and lattice parameters a and c are 0.3845 and 1.700 nm, 0.3842 and 1.699 nm, and then 0.3839 and 1.698 nm, respectively. Each of them shows some anomalies at a low temperature in addition to a semiconductor-like temperature dependence of the resistivity. Especially, the sample with Ln = Gd shows two resistivity dropping phenomena at about 16 and 22 K. Then, all of nearly single-phase samples show diamagnetic signals originating from superconductivity, and also rapid increase of magnetization due to a transition from paramagnetic state to ferromagnetic one below 80 K.

Origin of superconductivity transition broadening in MgB 2

We report resistivity and magnetization of single crystal MgB 2, focusing on the broadening of superconducting (SC) transition in magnetic fields. In-plane and out-of-plane resistivity indicate that the broadening of SC transition is independent of Lorentz force and that it is merely dependent on the magnetic field direction. In magnetization, diamagnetic signal begins to appear at almost the same temperature as the onset temperature of resistivity transition. These results suggest that the broadening is attributed not to the surface superconductivity but to the SC fluctuation or the vortex-liquid picture, owing to the short coherence length and the high transition temperature of MgB 2.

Synthesis and search for superconductivity in LiBC

Following the recent theoretical prediction of superconductivity in hole doped LiBC by Rosner et al. [Phys. Rev. Lett. 88 (2002) 127001], we have attempted to synthesize Li deficient Li x BC ( x=1, 0.8, 0.6 and 0.4) and looked for superconductivity in this system. Our synthesis procedure, following the recipe for MgB 2, involves reaction of elemental components in a Ta crucible at 900 °C under 50 bar of argon pressure. X-ray diffraction measurements indicate the formation of P6 3/ mmc structure up to x=0.6. However, no diamagnetic signal or zero resistance, corresponding to the superconducting transition, were observed in the temperature range of 4–300 K. This is possibly related to the presence of disorder in the B–C stacking; evidence for which is suggested from a study of the vibrational modes of Li x BC through infrared spectroscopy.

Novel Conductive Radical Cation Salts Based on Methylenediselenotetraselenafulvalene (MDSe-TSF): A Sign of Superconductivity in κ-(MDSe-TSF) 2Br Below 4 K

Seven conductive radical cation salts based on MDSe-TSF (methylenediselenotetraselenafulvalene) have been synthesized by electrocrystallization in the presence of Cl −, Br −, I 3 −, I 2Br −, PF 6 −, ClO 4 −, and Cu(NCS) 2 − counter anions. The crystal appearances of these salts fairly depend on the anions employed. X-ray crystallographic analyses have revealed that the PF 6 and ClO 4 salts in the shape of brown thin plates adopt the θ-type structures characterized by the herringbone arrangement of donor stacks, whereas the Cl and Br salts in the shape of black thick plates favor the κ-type structures with the orthogonal arrangement of donor dimers. Regardless of different crystal appearances or crystal packing patterns, all these salts show high conductivity (>10 2 S cm −1) at room temperature and retain metallic properties down to 4.2 K. Of them, the Br salt shows a weak but distinct diamagnetic shielding signal below 4 K in the dc magnetization measurement under zero-field-cooled (ZFC) condition, suggesting a sign of superconductivity. The band calculations of both PF 6 and Br salts demonstrate closed Fermi surfaces indicative of two-dimensional molecular conductors.

Search for superconductivity of magnetic metals

A search for superconductivity of magnetic elemental metals is performed.A successful discovery of the onset of superconductivity is reported inthe case of iron under pressure. By electrical resistance measurement,a maximum value of the superconducting transition temperatureTc of 2 K and the uppercritical magnetic field Hcof 0.2 T are observed under pressure of 20 GPa where iron is in the crystallographichcp phase and non-magnetic. Further confirmation of the superconductingtransition of hcp iron was obtained by the detection of the diamagnetic signal dueto the Meissner effect in accordance with the results of the electrical resistancemeasurements.

Diamagnetism and structural transition in the Ba2Cu3O4Cl2 compound

This work presents a study of structural and magnetic properties of the Ba2Cu3O4Cl2 (Ba2342) compound. Doped samples with K or Sr in the Ba site and Zn or Mg in the Cu site of the Ba2342 were prepared using a solid state diffusion method. They were characterized by X-ray powder diffractometry, scanning electron microscopy and dc-magnetization. Samples with Ba2342 composition presented tetragonal structure of the I4/mmm space group when heat-treated at 750ºC. A structural transition was observed when the samples were doped with Zn or Mg or heat-treated at 700ºC. dc-Magnetization measurements performed in SQUID showed the existence of diamagnetic signal. A difference of the magnetic moments measured at zero field cooled (ZFC) and field-cooled (FC) procedures was observed suggesting the existence of localized superconductivity in this oxychloride system.

Magnetic properties of YbFe 4Al 8 intermetallic compound. New antiferromagnetic superconductor

The magnetic properties of the YbFe 4Al 8 antiferromagnetic ( T N=102 K) compound are reported in wide temperature and magnetic field ranges. The pronounced diamagnetic signal of the dc susceptibility has been discovered for this compound below T<50 K when the sample was cooled and measured in an applied magnetic field lower than H<500 Oe. The results obtained indicate that this magnetically active compound probably belongs to a new class of superconductors of the RFe 4Al 8 type structure. The estimated volume fraction of the superconducting phase was not higher than 15% in the studied sample.

High pressure synthesis and superconductivity of Ba 1− xK xBiO 3 (0.35< x

The whole region of superconductivity in the Ba 1− x K x BiO 3 (BKBO) system (0.35< x<1) has been investigated by synthesizing the samples with the high pressure and high temperature technique. As the K doping ratio ( x) increases, the superconducting transition temperature ( T c) of the BKBO system decreases from 30.4 K for x=0.37 to very low diamagnetic signal below 3 K for x=0.74. The synthesis of the samples without including any bariums in the starting composition ( x=1) resulted in different T c and lattice parameters from sample to sample with partial substitutions of Bi at the K site in final products. With detailed synthesis information of the BKBO samples which varies with x, the evolution of T c by doping in the cubic BKBO system is compared with other superconducting bismuthates and discussed on a basis of its electronic band structure.

X-ray absorption near-edge structure spectra study of the Ba 1− xK xBiO 3 (0.37⩽ x⩽1.0) superconductor

X-ray absorption near-edge structure spectra (XANES) of the Ba 1− x K x BiO 3 (BKBO) (0.37⩽ x⩽1.0) superconductor have been measured at the Bi L III edge. To estimate changes of the Bi valence state, and reveal its influence on the superconducting properties of this solid solution, quantitative spectral characteristics of the 2p 3/2→6s transition in the series comprising different Ba/K ratios were determined. The superconducting transition T c of the BKBO samples, determined from dc magnetic susceptibility measurements, monotonously decreases from Ba 0.63K 0.37 BiO 3 ( T c=30.4 K) to Ba 0.26K 0.74BiO 3 (a very low diamagnetic signal was detected at T∼3 K). This corresponds to the increase of Bi valence determined from XANES data in the range +4.36⩽ ν⩽+4.74. Further increase of the K content to x>0.8 results in reappearance of superconductivity with T c=9 K. The maximum value of formal Bi valence versus K content corresponds with the minimum in T c.

Carbon solubility and superconductivity in MgB 2

Successful replacement of B by C in the series MgB 2− x C x for values of x upto 0.3 is reported. Resistivity and ac susceptibility measurements have been carried out in the samples. Solubility of carbon, inferred from the observed change in the lattice parameter with carbon content indicates that carbon substitutes upto x=0.30 into the MgB 2 lattice. The superconducting transition temperature, T c measured both by zero resistivity and the onset of the diamagnetic signal shows a systematic decrease with increase in carbon content upto x=0.30, beyond which the volume fraction decreases drastically. The temperature dependence of resistivity in the normal state fits to the Bloch–Gruneisen formula for all the carbon compositions studied. The Debye temperature, θ D, extracted from the fit, is seen to decrease with carbon content from 900 to 525 K, whereas the electron–phonon interaction parameter, λ, obtained from the McMillan equation using the measured T c and θ D, is seen to increase monotonically from 0.8 in MgB 2 to 0.9 in the x=0.50 sample. The ratio of the resistivities between 300 and 40 K versus T c is seen to follow the Testardi correlation for the C substituted samples. The decrease in T c is argued to mainly arise due to large decrease in θ D with C concentration and a decrease in the hole density of states at N( E F).

Magnetization and Re LIII-Edge studies of (Hg, Re)Sr2CuO4+δ system

Chemically stable nominal compositions of (Hg1-xRe x )Sr2CuO4 + δ (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 Tconset ∼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 (Hg0.9Re0.1)Sr2CuO4 + δ 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 LIII-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 (Jc), calculated using Bean's model, for (Hg0.9Re0.1)Sr2CuO4 + δ is ∼6 × 103 A/cm2 (4.5 K, 5 kOe).

Potential Formation and High Density Plasma Production on the Tandem Mirror GAMMA 10

GAMMA 10 experiments have advanced in high density experiments after the last EPS Workshop where we reported high density plasma production by using an ion cyclotron range of frequency heating at a high harmonic frequency and neutral beam injection in the central cell. Recently a high density plasma was obtained with much improved reproducibility than before and without degradation of diamagnetic signal. The high density plasma was attained by adjusting the spacing of the conducting plates installed in the anchor transition regions. Dependencies of particle confinement time, ion energy confinement time and plasma confining potential on plasma density were obtained for the first time in the high density region.

Magnetic Properties of Mn and Fe-Implanted p-GaN

The structural and magnetic properties of p-GaN implanted with high doses of Mn+ or Fe% (0.1–5 at%) and subsequently annealed at 700–1000 °C were examined by transmission electron microscopy, selected-area diffraction patterns, X-ray diffraction and SQUID magnetometry. The implanted samples showed paramagnetic behavior on a large diamagnetic background signal for implantation doses below 3 at% Mn or Fe. At higher doses the samples showed signatures of ferromagnetism with Curie temperatures <250 K for Mn and <150 K for Fe implantation. The structural analysis of the Mn-implanted GaN showed regions consistent with the formation of GaxMn1—xN platelets occupying ∼5% of the implanted volume. An estimate of ∼(5.5 ± 1.9)μB per Mn was obtained, consistent with the expected value (5.0) for a half-filled shell. The formation of secondary phases such as MnxGay or MnxNy was excluded by careful diffraction analysis. The implantation process may have application in forming selected-area contact regions for spin-polarized carrier injection in device structures and in enabling a quick determination of the Curie temperatures in dilute magnetic semiconductor host materials.

Anomalous superconductivity in bismuthates

We report the anomalous superconductivity found in bismuthates with different compositions. In the Sr 1− x K x BiO 3 (SKBO) compound the field-induced superconductivity is observed, which shows a striking suppression of the reentrant resistance at T< T c by applying an external magnetic field ( H) or increasing the electrical transport current ( I). The magnetotransport measurements on several different samples reveal that the reentrant resistance behavior with H and I is related to the normal state transport properties. Disordered junction barriers between superconducting grains seem to be important for the observed anomalous reentrant resistance phenomena. In the Ba 1− x K x BiO 3 system, the whole region of the superconducting cubic phase diagram from x=0.4 up to x=1 was investigated for the first time by using the high pressure and high temperature synthesis technique. The superconducting temperature ( T c) of Ba 1− x K x BiO 3 (BKBO) samples shows a parabolic decrease of T c from the well-known Ba 0.63K 0.37BiO 3 composition with T c=30.4 K to Ba 0.24K 0.76BiO 3 with very low diamagnetic signal at nearly 3 K. However, we found that further increasing x gives an increase of T c up to 9 K for x=1. Possible physical origins of the field-induced superconductivity in SKBO and the anomalous superconducting region in high potassium concentration ( x>0.8) BKBO are discussed.