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A few atomic layers of helium adsorbed on graphite have been attracting much attention as one of the ideal quantum systems in two dimension. Although previous reports on neutron diffraction have shown fundamental structural information in these systems, there still remain many open questions. Here, we propose surface crystal truncation rod (CTR) scatterings using synchrotron radiation X-rays as a promising method to reveal surface and interface structures of helium films on graphite at temperatures below 2 K, based on the preliminary experimental results on a monolayer of He-4 on a thin graphite. Our estimation on heat generation by X-ray irradiations also suggests that CTR scatterings are applicable to even at system temperatures near 100 mK., 9 pages, 3 figures, submitted to Journal of Low Temperature Physics

We carried out simulations of crystal truncation rod (CTR) scatterings, i.e., one of the surface X-ray diffraction techniques with atomic resolution, from a monolayer He film adsorbed on graphite. Our simulations reveal that the 00L rod scatterings from the He monolayer exhibit notable intensity modifications for those from a graphite surface in the ranges of approximately L = 0.6 - 1.7 and L = 2.2 - 3.5. The height of the He monolayer from the graphite surface largely affects the CTR scattering profiles, indicating that CTR scatterings have enough sensitivities to determine the surface structure of the various phases in the He layer. In particular, in the incommensurate solid phase, our preliminary experimental data show the intensity modulations that are expected from the present simulations., 6 pages, 4 figures, to be published in JPS Conf. Proc

Bis(glycolato)copper(II) [Cu(HOCH2CO2)2] shows a structural phase transition around 220 K under cooling and 270 K under heating, and ferromagnetic ordering at 1.1 K. Deuterium substitution of the hydroxyl groups in it induced a large shift in the structural transition temperature while no shift in the ferromagnetic transition temperature took place.

We have performed parallel measurements of dc-magnetization and ac-magnetic susceptibility for a ferromagnetic superconductor, ${\mathrm{UGe}}_{2}$, in the ferromagnetic-superconducting phase. dc-magnetization measurements revealed that adequate demagnetizing of the sample allows for the preparation of various magnetized states with different zero-field residual magnetization. We observed that these states exhibit varying ac superconducting response at large ac-field amplitudes. The amount of ac flux penetration is less in the demagnetized state involving many domain walls. This result seems to contradict the theory that considers the domain walls as weak links. Moreover, the ferromagnetic domain walls enforce the shielding capability of superconductivity. This observation sheds light on the role of the domain walls on superconductivity, which has been a controversial issue for several decades. Two possible scenarios are presented to explain the enhancement of the shielding capability by the domain walls.

A novel star-shaped high-spin Fe(III) tetranuclear cluster was unexpectedly obtained by the reaction of an [FeIII(phsal)2]− anion with an Fe(II) coordination cation (H2phsal: N-(2-hydroxyphenyl)sal...

Experimental exploration of highly spin-polarized states of liquid 3He by applying external magnetic field is limited by the availability of static magnetic field. In the “ferromagnetic” superfluid A1 phase of liquid 3He there is an alternate method for boosting spin-polarization by the process of spin pumping without requiring such high magnetic field. The spin pumping in the A1 phase takes advantage of a superleak (SL) acting simultaneously as a filter for both entropy and spin. The spin pump technique that uses the SL-spin filter and a mechanical actuator enables us to directly boost polarization of 3He. The amount of enhancement of spin polarization has been limited so far. We are now developing a new type of SL filter made of packed aluminum oxide powder (referred as PAP-SL), in order to achieve greater enhancement of spin polarization. Several kinds of the PAP-SL filter were constructed by pressing aluminum oxide powders into a cylinder holder. The packed structures were carefully characterized by a flow-rate-measurement, X-ray tomography, and mercury intrusion porosimetry. The preliminary result shows that the PAP-SL works as SL filter for the superfluid 3He.

The Josephson effect between a single-crystal ${\mathrm{UPt}}_{3}$ and a conventional superconductor Al has been investigated under pressure for the junction on the ${\mathrm{UPt}}_{3}$ surface perpendicular to the hexagonal $c[0001]$ axis. Simultaneously measured magnetization has revealed that the Meissner fraction approaches a minimum value of $\ensuremath{\sim}1%$ at the critical pressure of ${P}_{\mathrm{c}}\ensuremath{\sim}0.4\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. The critical temperature ${T}_{\mathrm{c}}$ and the temperature ${T}_{\mathrm{J}}$ where a measurable Josephson critical current ${I}_{\mathrm{c}}$ first appears decrease with increasing pressure; the decreasing rate of $|d{T}_{\mathrm{c}}/dP|$ is almost constant, whereas $|d{T}_{\mathrm{J}}/dP|$ shows an abrupt increase above ${P}_{\mathrm{c}}$, indicating that the Josephson effect is suppressed above ${P}_{\mathrm{c}}$. These results, together with the decrease in penetration depth $\ensuremath{\lambda}$ derived from the magnetic-field dependence of ${I}_{\mathrm{c}}$, suggest that the superconducting phase is modified above ${P}_{\mathrm{c}}$ by the disappearance of the symmetry-breaking antiferromagnetic order.

We have been developing a micro superconducting quantum interference device (μ-SQUID) magnetometer for the study of quantum effects in μm- or nm-sized magnetic materials. In the μ-SQUID magnetometer, the sample is placed on the SQUID loop directly and the flux is detected by the loop itself. The close proximity between the sample and the SQUID loop is of great advantage for highly sensitive detection of magnetization from the tiny magnetic materials. However, the heat release during the operation of the SQUID could cause a great problem in experiments below 1 K because of the close proximity of the SQUID and sample. Here we report a digital circuit for the μ-SQUID magnetometer which can reduce the heat release in the low temperature operation and whose parts are commercially available. The circuit consists of a FPGA-digital board with analog-to-digital and digital-to-analog converters, and an independent pre-amplifier. By using this circuit we succeeded in measuring quantum tunneling of magnetization in the single molecule magnet, Mn12Ph at 0.7 K.

We report development of micro superconducting quantum interference device (μ-SQUID) magnetometers for investigation of quantum tunneling of magnetization in μm- and nm-size magnetic materials. Both high- and low-temperature superconductor (HTS and LTS) based μ-SQUID magnetometers were fabricated and a three dimensional magnetic coil system was constructed for this purpose. The HTS-μ-SQUIDs with a hole of 4×9 μm2 work at temperatures between 4.2 and 70 K and in magnetic fields up to 120 mT. A magnetization measurement of a ferrimagnetic micro-crystal was carried out at 35 K with an accuracy of 10−9 emu. The development of LTS-μ-SQUIDs has been started in order to study much smaller magnetic materials in a mK temperature range. We present a preliminary result on the LTS-μ-SQUID with a hole of 1×1 μm2. The critical current as a function of applied magnetic field shows the SQUID modulation at 4.2 K and up to 30 mT. The heat release associated with the present measurement method is estimated to be on the order of several microwatts.

Among many two-dimensional (2D) high T(C) superconductors, graphite intercalation compounds (GICs) are the most famous intercalation family, which are classified as typical electron-phonon mediated superconductors. We show unambiguous experimental facts that BaC(6), the superconductivity of which has been missing for many years so far among various alkaline earth metal (Ca, Sr, and Ba) intercalted GICs, exhibits superconductivity at T(C)=65 mK. By adding this finding as the additional experimental point, a complete figure displaying the relationship between T(C) and interlayer distance (d) for GICs is now provided, and their possible superconducting mechanisms raised so far are revisited. The present study settles a long-running debate between theories and experiments on the superconductivity in the first stage GICs.

Josephson critical current I c between a single crystal CePt 3 Si and Al has been measured for the junctions on the CePt 3 Si surface perpendicular to the a and c axes. In the magnetic-field dependence of I c , a dominant maximum peak at zero field has been observed only for the junction on the a -axis face thus far, indicating that the Josephson effect is allowed at least for current flow parallel to the a axis. This result places a restriction on the representations of the order parameter in the noncentrosymmetric superconductor CePt 3 Si.

Josephson critical current density J c between a single crystal URu 2 Si 2 and an s -wave superconductor has been investigated for URu 2 Si 2 –Cu–Nb(SNS') and URu 2 Si 2 –Nb(SS') junctions. In cont...

The CeIrIn5-Ag junctions of about 2×10−9 cm−2 area have been made using microfabrication techniques, and the surface superconducting state of CeIrIn5, which has two characteristic temperatures T0 and Tc, has been investigated, where T0 and Tc are the transition temperature to zero-resistivity state and the bulk, thermodynamic transition temperature, respectively. The temperature, below which superconducting anomalies are observed, varies from junction to junction, and yet it is always well above Tc=0.4 K. This result, together with no indication of transition at Tc, suggests that at least the surface of CeIrIn5 is in the superconducting state above Tc. The data on the critical current Ic in superconducting anomalies point to the possibility to define a local transition temperature for each junction.

We performed dc-magnetization and ac-susceptibility measurements to clarify how the magnetic properties of CeAgSb2, which shows ferromagnetic order along the c-axis below 9.6 K, evolve with the application of a transverse magnetic field (Hab). The Curie temperature (TC) is suppressed to zero by the application of Hab, and the ac-susceptibility measured along the c-axis \(\chi '_{c}\) under Hab continuously shows a critical-divergence-like anomaly at TC, indicating the presence of a field-induced ferromagnetic quantum critical point in this compound. On the other hand, the magnitude of \(\chi '_{c}\) at TC rapidly decreases with increasing Hab up to around 0.5 T. This result suggests that the magnetic properties around TC rapidly change with Hab in the relatively lower Hab range.

The Josephson effect for a heavy-fermion superconductor CeIrIn5, which has two characteristic temperatures T0 and Tc, has been investigated for CeIrIn5-Cu-Nb junctions, where T0 and Tc are the temperature of the transition to the zero-resistivity state and the bulk, thermodynamic transition temperature, respectively. For all the junctions fabricated on the (110) and (001) planes of CeIrIn5, the temperature below which the Josephson effect is observed is near T0 = 0.8 K and much higher than Tc = 0.3 K. This result suggests that at least the surface of CeIrIn5 is in a superconducting state below T0, in which phase coherence between CeIrIn5 and Nb is possible.

The Josephson effect between a single-crystal CeIrIn 5 and an s -wave superconductor has been investigated for current flows parallel to the [1 0 0] and [1 1 0] axes of CeIrIn 5 . The magnetic-field modulation of the Josephson critical current has shown a nearly ideal Fraunhofer pattern in the [1 0 0] direction and a small deviation from that pattern in the [1 1 0] direction. The clear anisotropy, which has been reported for YBa 2 Cu 3 O 7 - δ indicating the d -wave superconductivity, was not observed in this study.

We have studied the thermal history dependence of the superconducting properties below and above room temperature (RT) in the ceramic ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4+\ensuremath{\delta}}$ with excess oxygen, which were prepared by annealing in high-pressure oxygen gas or electrochemical oxidation. The diffusion of the excess oxygen is concluded not to occur below RT. In the sample with small \ensuremath{\delta}, the low ${T}_{c}$ (32 or 36 K) superconducting phase appears, of which ${T}_{c}$ is changed by the annealing at about 200 K. This change is presumably due to some phase transition without the diffusion of the excess oxygen. On the other hand, when \ensuremath{\delta} reaches some threshold, another single superconducting phase with high ${T}_{c}$ (45 K), which is stable below 373 K at least, appears. This sample was prepared by the electrochemical oxidation at 333 K for more than 48 h, where the excess oxygen can diffuse. In the sample electrochemically oxidized at RT, both the low- and high-${T}_{c}$ superconducting phases were formed only near the surface area of the sample. These phases were homogenized by annealing at 373 K.

Josephson critical current I c between a single crystal UPt 3 and Al has been measured for a corner junction on the UPt 3 faces perpendicular to the a [11 2 0] and b [10 1 0] axes. The magnetic field dependence of I c has shown a global maximum at zero magnetic field like the junctions on a single face. We have not observed a behavior indicating the phase shift of the order parameter between the a - and b -axis directions thus far.

The thickness of the Meissner region ρ in Au of Au-clad Nb wire has been measured down to 50 mK, in order to investigate the superconductivity in a normal metal induced by the proximity effect. The magnetic field dependence of ρ and the Josephson critical current I C through a SNS junction made of an Au-clad Nb wire have been also measured to estimate the coherence length ξ N in Au. Above 150 mK, ρ and ξ N increase in proportion to T -1 , and are independent of the mean free path l N in Au. Although Au is in the clean limit ( l N > ξ N ), these results can be qualitatively interpreted by the dirty limit theory with a modification using ξ N derived in the clean limit.

On annealing in chlorine trifluoride gas (ClF 3 ), the Iow- T c phase YBa 2 Cu 3 O x ( x ~ 6.6, T c ~ 60 K) is readily converted into a high- T c phase exhibiting 90 K superconductivity. YBa 2 Cu 3 O 6.60 ( T onset = 63 K) began to react with ClF 3 at 423 K. The initial reaction rate was proportional to the ClF 3 pressure and the activation energy was calculated to be 17.1 kJ mol −1 . The reaction process was represented by the rate equation, [1 − (1 − x )1/2] =Kt, derived from Jander's model for a gas/solid reaction, the reaction proceeding preferentially in a direction parallel to the CuO 2 network in the YBa 2 Cu 3 O x structure. Optimum annealing conditions required to obtain the high- T c single-phase structure were ClF 3 pressure = 2.7 kPa, temperature = 513 K, reaction time = 60 min. Under these conditions the crystal structure changed from pseudo-orthorhombic YBa 2 Cu 3 O 6.60 to the orthorhombic single phase of chemical composition YBa 2 Cu 3 O 6.17 F 0.78 Cl 0.52 . The T onset value of this product was 94.5 K.

The magnetization change ΔM at the superconducting transition of LaIrSi3 has been measured in various magnetic fields H. The observed ΔM in H = 0 is within the range of the Meissner effect induced by the possible residual field, suggesting that the spontaneous magnetization, if it exists, is below the experimental accuracy. This result is in agreement with the results of μSR experiments, in which no spontaneous magnetic field has been observed in the superconducting state.

We have measured the magnetization of two noncentrosymmetric superconductors: Ir2Ga9 and CePt3Si using a SQUID magnetometer, neither of which has been tested for the time-reversal symmetry in the superconducting state. For Ir2Ga9, the magnetization change ΔM below the superconducting transition temperature Tc ~ 2.2 K was smaller than 10-5 G in zero magnetic field, suggesting that ΔM originates in the Meissner effect induced by the possible residual field. When CePt3Si was cooled in zero magnetic field, ΔM along the c axis first decreased below Tc+ ~ 0.75 K (high-Tc phase) and then increased below Tc– ~ 0.45 K (bulk superconducting phase). Since this behavior cannot be explained by the Meissner effect, it may be a sign of a spontaneous magnetization.

The maximum supercurrent I c passing through the S(Nb)-N(Cu doped with Mn)-S(Nb) junction made of Cu-clad Nb wire has been measured down to 30 mK, in order to investigate the relation between the superconducting proximity effect and the Kondo effect. The maximum supercurrent becomes small with an increase in Mn concentration and remarkably with a decrease in electronic mean free path in Cu. The electronic mean free path dependence of I c may be explained qualitatively in consideration of the spin flip scattering and the RKKY interaction of Mn impurities. The Meissner effect induced by the proximity effect in Cu-clad Nb wire can be understood through the behavior of the coherence length ξ N in Cu which is derived from I c .

Annealing in ClF3 gas is first found to be effective for the incorporation of fluorine and chlorine into a partially reduced YBa2Cu3O7−δ (YBCOx) of which superconducting transition temperature (Tc is less than 90 K and for the remarkable increase in Tc. ClF3 reacts effectively with YBCOx at a temperature ranging from 250 to 300°C. As a result of the ClF3 treatment of YBCO6.61, the pseudo-orthorhombic structure changed into the single phase of orthorhombic-I with the chemical composition of YBa2Cu3O6.17F0.78Cl0.52. Tc (onset) was enhanced from 62 to 94 K, while the diamagnetic susceptibility is still half of the YBCO6.88.

The proximity effect in Cu clad Nb wires of which Cu is doped with dilute magnetic impurities, has first been investigated in relation to the Kondo effect. As for Cu doped with dilute Fe, Co and Ni impurities, the probability of the spin flip scattering of conduction electrons, α, by Fe, Co and Ni impurities decreases linearly with decreasing temperature. As for Cu doped with dilute Mn impurities, the probability, α, by Mn impurities is concluded to be suppressed by the RKKY interaction between Mn impurities which depends on the electronic mean free path. The temperature dependence of the resistivity of the Mn alloy may be understood as a result of the coexistence of the dilute Kondo effect and the RKKY interaction between Mn impurities.

We have investigated the pressure dependence of ac and dc susceptibilities of the heavy-fermion superconductor CePt3Si (Tc= 0.75 K) that coexists with antiferromagnetism (TN = 2.2 K). As hydrostatic pressure is increased, Tc first decreases rapidly, then rather slowly near the critical pressure Pc = 0.6 GPa and shows a stronger decrease again at higher pressures, where Pc is the pressure at which TN becomes zero. A transition width and a difference in the two transition temperatures defined in the form of structures in the out-of-phase component of ac susceptibilities also become small near Pc, indicating that a double transition observed in CePt3Si is caused by some inhomogeneous property in the sample that leads to a spatial variation of local pressure. A sudden increase in the Meissner fraction above Pc suggests the influence of antiferromagnetism on superconductivity., 4 pages with 5 figures. This paper will be published in J. Phys. Soc. Jpn

The anomalous behavior of the heavy-fermion superconductor UBe13, which appears below the superconducting transition temperature Tc ∼ 0.9 K, has been investigated by point-contact spectroscopy and the Josephson effect. An abrupt increase in the temperature dependence of the Josephson critical current is observed below 0.7 K between UBe13 and conventional superconductors. The zero-bias resistance of the point contacts between UBe13 and Pt decreases continuously below Tc and shows an abrupt drop at 0.5–0.6 K. These results suggest that the anomaly originates from superconductivity rather than magnetism, which has been considered as one of the origins of the anomaly.

We have investigated the differential resistance of the point contacts between heavy-fermion superconductor UPt3 and a normal metal Pt, which were fabricated using a commercial piezo-electric actuator, and retried the observation of the energy gap of UPt3. A V-shaped dip is observed in both normal and superconducting states and disappeared around TK ~ 20 K, suggesting that it is related to the Kondo effect. Below the superconducting transition temperature, a shallow double-minimum structure, which indicates the energy gap, has been observed for the contacts on the faces perpendicular to the a-, b- and c-axes of UPt3.

We report magnetization of a non-centrosymmetric superconductor, LaPt3Si below 0.6 K up to 200 Oe. The home-made SQUID and Hall sensor magnetometers that operate below 1 K were constructed for this purpose. Although the SQUID magnetometer is more sensitive than the Hall sensor's one, it was found not to work correctly for the rapid magnetization change of LaPt3Si below 0.4 K. The Hall sensor magnetometer, in contrast, can properly detect magnetization jumps in the M–H curve of the superconducting state. The observed flux jumps are probably related to the interfusion of the mixed state of the LaPt3Si that is observed in the μSR measurements.

We have measured the magnetization of single crystals of the non-centrosymmetric superconductor LaNiC2, which is considered to break time-reversal symmetry by μSR measurements. Below the transition temperature Tc ∼ 3 K, the emergence of magnetization on the order of 10−5 G is observed in zero magnetic field along the c-axis but not along the a-axis. This spontaneous magnetization is larger than the Meissner effect induced by the possible residual field and did not change after applying magnetic fields of up to 15 mOe or after warming to room temperature. The results that the direction is related to the lack of inversion symmetry and that the magnitude is sensitive to the crystallinity of the sample suggest some analogy with pyroelectricity.

The Meissner effect in Cu doped with Mn, Fe, Co and Ni which is induced by the proximity effect of Nb is measured. The thickness ϱ of the Meissner region in Cu is found to be a linear function of T −1 2 except for Mn-impurity sample. The ϱ of Mn-impurity sample is suppressed by the magnetic scattering around T K and recovered with decreasing temperature below T K . This result indicates that the pair breaking probability due to Mn impurities rapidly decreases below T K . The dc magnetic field dependence of ϱ shows anomalous behavior around T K .

The Josephson effect between a single crystal CeTIn 5 (T = Co, Ir) and an s -wave superconductor has been investigated for a current flow along the c -axis of CeTIn 5 . For CeCoIn 5 , the magnetic-field modulation of the Josephson critical current has shown an oscillating pattern, which does not contradict with d x 2 - y 2 superconductivity, while a Fraunhofer-like pattern has been observed for CeIrIn 5 , suggesting that an s -wave component is induced on the (0 0 1) face of CeIrIn 5 .

We have investigated the pressure dependence of dc susceptibility χ dc of heavy-fermion superconductor CePt 3 Si for a single crystal and a poly-crystal that show a similar specific heat anomaly at T c ~ 0.4 K. When coexisting antiferromagnetism disappears at pressures above P c ~ 0.6 GPa, the Meissner fraction of the single crystal becomes twice or third times as large as that below P c depending on the magnetic field direction. The poly-crystal, on the other hand, has not shown such an anomaly, possibly because the distribution of local pressure that seems wider than that in the single crystal causes the remains of antiferromagnetic phase even above P c .

The diamagnetism of a normal metal (N: Cu or Au), which is induced by the proximity effect of a superconductor (S: Nb), has been investigated for N-S double layers, which are formed by a thin-film deposition process. Detailed studies of samples, which have different electronic mean-free path \ell_N in N, suggest that \ell_N should be controlled by the impurity concentration rather than the mechanical imperfections in the lattice in order to clarify the \ell_N dependence of the proximity effect. Both the screening distance \rho in N and the parameter \nu in \rho \propto T^-\nu increase with an increase in \ell_N. This result can be understood on the assumption that the normal metal changes its behavior from the "dirty" limit (\xi_N>\ell_N) to the "clean" limit (\xi_N, Comment: 5 pages, 6 Postscript figures, to be published in J. Phys. Soc. Jpn