山口 明

Abstract The thermal expansion of a diluted Ce system La_1-xCe_xCu₆ for (0.6 ≤ x ≤ 1) has been measured between 10 and 150 K to reveal the change from the coherent heavy Fermion state (0.9 ≤ x ≤ 1) to the incoherent Kondo state (0 < x ≤ 0.73). The large Ce concentration x dependence of the linear thermal expansion coefficient along b-axis α_b(T) suggests that the coupling between the 4f¹ electron and the lattice strain is the largest along the b-axis in the three crystallographic axes. The maximum of the magnetic contribution to the volume thermal expansion coefficient β_m(T) at T = 50 K is retained in the x range of 0.6 ≤ x ≤ 1, suggesting the crystalline electric field (CEF) level for x = 1 doesn’t change by the substitution. Furthermore, the upturn in β_m(T) below 25 K, which should be a precursor of the maximum at T = 2.5 K reported for x = 1, is retained when we decrease x from 1 to 0.6. Because the ground state for x = 0.6 is the incoherent Kondo state, the robustness of the maximum at T = 50 K and upturn in the current x value implies that β_m(T) in 10 ≤ T ≤ 150 K is attributed to the CEF and Kondo effects rather than the formation of the heavy Fermion state.

Abstract We performed electrical resistivity measurements of Ce₃TiSb₅ under pressure. From the pressure dependences of antiferromagnetic ordering temperature T_N and lower anomalous temperature T*, temperature–pressure phase diagram of Ce₃TiSb₅ up to 2.3 GPa has been constructed. T* rapidly decreases with increasing pressure and disappears around 1 GPa. T_N increases with increasing pressure, however a hump structure of electrical resistivity below T_N becomes small. Hence, some change for the magnetic ordered state is expected to occur in higher pressure region.

Two-dimensional metal-organic frameworks (2D MOFs) have attracted much attention, as they are the crystalline materials that exhibit both conductivity and microporosity. Numerous efforts have been made to advance their application as chemiresistive sensors or electrochemical capacitors. However, the intrinsic physical properties and spin states of these materials remain poorly understood. Most of these 2D MOFs possess a honeycomb lattice, with a Kagome lattice arrangement of metal cations. These structural characteristics suggest that these MOFs would be candidates for geometrically frustrated spin systems with unprecedented magnetic phenomena. Herein, by performing magnetic susceptibility and specific heat measurements at an ultralow temperature down to 38mK on a 2D semiconductive MOF, Cu-3(HHTP)(2), a quantum spin liquid state that arises from the geometrical frustration was suggested. This result illustrates the potential of strongly correlated MOFs as systems with emergent phenomena induced by unusual structural topologies.

In this study, we investigate the magnetoelectric (ME) effect of the newly discovered antiferromagnetic (AFM) compound Ce3TiBi5 with a hexagonal structure (space group P6(3)/mcm). In this system, Ce ions form zig-zag chains that extend along the c -axis. We focus on the lack of the local inversion symmetry at the Ce site, although the crystal structure has an inversion center. A theoretical study has predicted that magnetizationcan be induced by an electric current in the AFM ordering on the zig-zag chain. We conducted magnetization measurements on Ce3TiBi5 under an applied constant electric current and a static magnetic field around the AFM ordering temperature of T-N = 5.0 K. We successfully observed of the current-induced magnetization below T-N. The magnitude of the current-induced magnetization has a linear electric current dependence and exhibits no magnetic field dependence. This behavior is consistent with the theoretical prediction of the ME effect in the ferrotoroidal ordered state.

We have performed parallel measurements of dc-magnetization and ac-magnetic susceptibility for a ferromagnetic superconductor, UGe2, 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.

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 χ′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 χ′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.

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 (H-ab). The Curie temperature (T-C) is suppressed to zero by the application of H-ab, and the ac-susceptibility measured along the c-axis chi(c)' under H-ab continuously shows a critical-divergence-like anomaly at T-C, indicating the presence of a field-induced ferromagnetic quantum critical point in this compound. On the other hand, the magnitude of chi(c)' at T-C rapidly decreases with increasing H-ab up to around 0.5 T. This result suggests that the magnetic properties around T-C rapidly change with H-ab in the relatively lower H-ab range.

A cryostat was constructed for high-resolution X-ray Compton scattering measurements at temperature down to 1.7 K, in order to investigate superfluid helium-4. Compton profiles of helium were measured using synchrotron X-rays for gas and liquid phases, respectively. In the measurement of the liquid phase, we succeeded in measuring the Compton profile of the superfluid helium at 1.7 K. Comparison of the results with theoretical calculation reveals importance of many-body effects beyond the mean-field treatment of electron systems.

The Josephson effect between a single-crystal UPt3 and a conventional superconductor Al has been investigated under pressure for the junction on the UPt3 surface perpendicular to the hexagonal c[0001] axis. Simultaneously measured magnetization has revealed that the Meissner fraction approaches a minimum value of similar to 1% at the critical pressure of P-c similar to 0.4 GPa. The critical temperature T-c and the temperature T-J where a measurable Josephson critical current I-c first appears decrease with increasing pressure; the decreasing rate of vertical bar dT(c)/dP vertical bar is almost constant, whereas vertical bar dT(J)/dP vertical bar shows an abrupt increase above P-c, indicating that the Josephson effect is suppressed above P-c. These results, together with the decrease in penetration depth lambda derived from the magnetic-field dependence of I-c, suggest that the superconducting phase is modified above P-c by the disappearance of the symmetry-breaking antiferromagnetic order.

We measured the shear modulus of solid He-confined between two parallel plates with gap distance comparable to the dislocation length in solid He. We constructed two sets of paired transducers with gaps of 11 and 290 m, and measured the dependence of shear modulus on the temperature and stress amplitude using polycrystalline He. The relative change of the shear modulus with decreasing temperature in the 11 m gap was 28 % of that in the 290 m gap. The stress required to unpin He in the 11-m gap solid was at least ten times larger than that in the 290-m gap solid. The results are attributed to the difference in the network distribution of solid He in the two gaps. The network length is not directly controlled by the pinning at the walls, but the solid quality difference induced by the growth difference in the thin slab gap causes the network distribution difference.

The magnetization change Delta M at the superconducting transition of LaIrSi3 has been measured in various magnetic fields H. The observed Delta M 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 mu SR experiments, in which no spontaneous magnetic field has been observed in the superconducting state.

To investigate the elastic properties of solid He-4 at low temperatures, the shear modulus of solid He-4 confined in a narrow gap, comparable to the length of the dislocation network of solid He-4 was measured. Two sets of parallel plate transducers were prepared. One set has a narrow gap of 11 pm and the other has a 290 pm gap as a reference of the bulk solid measurement. The temperature and strain dependences of the shear modulus were measured for solid He-4 in both cases. The increase of the shear modulus from 200 mK down to 14 mK was found to be smaller by 0.3 in the narrow-gap solid compared with the case of the bulk solid. By measuring the strain dependence of the shear modulus, the stress required to unbind He-3 in the narrow-gap solid was an order of magnitude larger than that in the bulk solid. These gap dependences can be related to the dislocation-network difference between two solids. The maximum length of the dislocation segment in the narrow-gap solid He-4 was found to be shorter than 6 mu m, which was one order of magnitude smaller than that in the bulk. The difference of the network distribution is considered to originate in the difference of the crystal quality which is caused by the confinement effect in the slab geometry. (C) 2015 Elsevier B.V. All rights reserved.

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 Delta M below the superconducting transition temperature T-c similar to 2.2 K was smaller than 10(-5) G in zero magnetic field, suggesting that Delta M originates in the Meissner effect induced by the possible residual field. When CePt3Si was cooled in zero magnetic field, Delta M along the c axis first decreased below T-c(+) similar to 0.75 K (high-T-c phase) and then increased below T-c(-) similar to 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.

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.

The anomalous behavior of the heavy-fermion superconductor UBe13, which appears below the superconducting transition temperature T-c similar to 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 0.7K between UBe13 and conventional superconductors. The zero-bias resistance of the point contacts between UBe13 and Pt decreases continuously below T-c 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.

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 BaC6, 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 nowprovided, 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.

A novel star-shaped high-spin Fe(III) tetranuclear cluster was unexpectedly obtained by the reaction of an [Fe-III(phsal)(2)](-) anion with an Fe(III) coordination cation (H(2)phsal: N-(2-hydroxyphenyl)salicylaldimine). Despite a star-shaped Fe(III) tetranuclear structure similar to reported Fe(III) single molecule magnets, the present compound exhibited a bulk antiferromagnetic transition at 0.6 K due to a magnetic exchange coupling through intercluster pi-stacking interactions.

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 mu SR measurements. (C) 2015 The Japan Society of Applied Physics

The transverse acoustic impedance of superfluid He-3 was measured in the A(1) and A(2) phases up to 13 T to investigate the surface states in nonunitary superfluids. The temperature dependence of the impedance was much larger in the A(1) phase than in the A(2) phase. This nonsymmetric behavior indicates that momentum exchange with walls for spin-down surface states is quite different from that for spin-up surface states. The spin-dependent response might be a reflection of an essential feature of the nonunitary states where gap amplitudes depend on spin states. Weak-coupling theories ignore any spin-dependent processes and do not account for the nonsymmetric behavior.

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 T-K similar to 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 have carried out point-contact spectroscopy measurements on CeCu6 and CeAl3 to study these heavy fermion states. Temperature and magnetic field dependences of the point-contact spectra were observed in detail. At the lowest temperature of 0.4 K, differential resistance spectra of CeCu6 and CeAl3 as function of bias voltage exhibit each distinct minimum structure and additional small peak structure. Temperature evolutions of these spectra started at temperatures related to the Kondo effect, although the minimum and the peak structure emerged in reverse between both spectra of two compounds. In magnetic fields, both structures monotonically change with no distinct change without a change of curvature of magnetic field dependence of magnitude of differential resistance at zero bias voltage at critical magnetic field of meta-magnetization. This behavior in magnetic field is different from that of CeAl2, which shows antiferromagnetic ordering. This result may suggest that PCS is able to discriminate between heavy fermion state and antiferromagnetic state.

We have measured the magnetization of single crystals of the non-centrosymmetric superconductor LaNiC2, which is considered to break time-reversal symmetry by mu SR measurements. Below the transition temperature T-c similar to 3K, 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 contribution of solid He-4 contained in the torsion rod of a torsional oscillator to its resonant frequency is investigated using a modified torsional oscillator. The purpose of the present study is to discriminate between two possible contributions to the resonant frequency of the torsional oscillator, which abruptly increases below 3 0.2 K. The first possible contribution is the elastic property change of solid He-4 in the torsion rod (i.e., shear modulus increase), and the second possible contribution is mass decoupling by the supersolid transition of solid He-4 in the oscillating body (i.e., a decrease in the momentum inertia). A torsional oscillator that has no hollow space for solid He-4 in the body was constructed, and its resonant frequency and Q-factor were compared to those of the conventional oscillator. The shear modulus contributions of solid He-4 in the torsion rod were identical, but the momentum inertias of solid He-4 in the oscillating body were quite different between the two oscillators. Surprisingly, the observed frequency behavior and magnitude of the two oscillators were comparable. This finding indicates that the resonant frequency increase observed in the torsional oscillator originates from the increase in the shear modulus of solid He-4 in the torsion rod, rather than from the momentum inertia decrease of the solid He-4 in the oscillating body. The temperature dependence of the increase in the shear modulus of solid He-4 was well reproduced by the dislocation-vibration model with a much lower He-3 impurity concentration than that of commercially available He-4 gas.

Learning from serendipitous assembly, we have prepared a new family of designed 3d-4f Mn(6)Ln complexes. The dynamics of relaxation of the magnetization via alternating-current magnetic susceptibility for the new Mn(6)Ln complexes 1 (Ln = La), 2 (Ln = Tb), and 4 (Ln = Dy) have been studied down to 0.2 K.

Experimental exploration of highly spin-polarized states of liquid He-3 by applying external magnetic field is limited by the availability of static magnetic field. In the "ferromagnetic" superfluid A(1) phase of liquid He-3 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 A(1) 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 He-3. 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 He-3.

The superfluid He-3 A(1) phase, containing a spin-polarized condensate allows us to explore the dynamics of superfluid spin current. In the mechano-spin effect (MSE), a mechanically applied pressure gradient and a superleak-spin filter enable one to directly boost spin polarization of He-3 in a small chamber. We are developing new apparatus for achieving greater enhancement of spin density. A development of a new-type He-3-hydraulic actuator has been already reported. We present here the construction of new-type of superleak-spin-filter made of packed powder aluminum oxide (referred as PAP-SL). The PAP-SL is popular in the study of superfluid He-4, but has not been established for that of the superfluid He-3. The attempt to construct the PAP-SL for the spin pump experiment was made by using aluminum oxide powder with nominal 1 mu m powder diameter and with packing fraction of 40 %. Before executing the experiment, the nuclear demagnetization cryostat of ISSP, Univ. Tokyo which has been used for this experimental activity, was heavily damaged by the 2011 Great East Japan (Higashi Nihon) Earthquake. The repair work and earthquake damage protection strengthening has just been accomplished.

We have developed a new method to perform point-contact spectroscopy PCS under pressure, in which a thin-film deposition technique is used for making a point contact. We investigated a pressure dependence of the point-contact spectra of the heavy-fermion superconductor URu2Si2 single crystal. In PCS of URu2Si2, two characteristic components evolve at low temperatures: one is a symmetric structure with respect to zero bias voltage suggesting a gap of the density of states at the Fermi surface, and the other is an anti-symmetric structure. We observed little change of two components between hidden order and antiferromagnetic phases.

We report the superconducting properties of LaPt3Si poly and single crystals with non-centrosymmetric structure. We have measured ac magnetic susceptibility, chi(ac) - chi' + i chi'', under sweeping magnetic fields. On the chi', a peak ascribed to the differential paramagnetic effect DPE is observed just below critical fields, where the superconducting state of LaPt3Si collapses completely by an external magnetic field. However this DPE behavior disappears below 0.40 K. Additionally, in this temperature range, the width of the magnetic field of collapsing superconductivity becomes broader and the superconducting domains survive even in higher magnetic fields.

The Josephson critical current I-c between a single-crystal UPt3 and Al has been measured for the junctions on the UPt3 surface perpendicular to the hexagonal a[11 (2) over bar0], b[10 (1) over bar0], and c[0001] axes. The magnetic field dependence of I-c shows a Fraunhofer diffraction pattern for a current flow parallel to the c-axis, suggesting that the Josephson coupling is uniform throughout the junction. Small modifications applied to the Fraunhofer pattern have been observed for a current flow parallel to the a- and b-axes. These results contradict the E-2u scenario for the odd-parity order parameter in UPt3, in which the Josephson effect is forbidden in the three directions, whereas the recently proposed E-1u scenario permits, at least, the a- and c-axis Josephson couplings.

We have measured the specific heat of poly- and single-crystalline LaPt3Si samples in various magnetic fields. In zero magnetic field, we observed distinct superconducting transitions at T-c similar to 0.64 K and 0.61 K for the poly- and single crystals, respectively. Temperature T dependences of the specific heat C of both samples around T-c resembled each other and could be well described by an exponential equation for a conventional superconductor at low temperatures. In a magnetic field, a characteristic peak of C/T appeared while the T-c was considerably suppressed. These trends were pronounced for the single crystal. The transition of the polycrystal became broad above 40 Oe and a characteristic tail appeared at temperatures above the peak of C/T. We suggest that the tail is induced by domains that have crystallographic disorders of the non-centrosymmetry and a higher critical magnetic field than that of the bulk. These domains became superconducting at temperatures above the peak of C/T, and then formed the tail. Both the poly- and single-crystalline LaPt3Si samples have been found to show characteristics that are in-between those of type-I and type-II superconductors.

The superfluid He-3 A(1) phase contains a spin-polarized condensate. This property allows novel superfluid spin current experiments. In the mechano-spin effect of the A(1) phase a mechanically applied pressure gradient and a superleak-spin filter enable to directly boost spin polarization of He-3 in a small chamber. Using a flexible membrane as an electrostatically actuated pump, we carried out such experiments and observed 50% enhancement of spin density. Here we report on a new He-3-hydraulic actuator for achieving greater enhancement of spin density. The actuator consists of two liquid He-3 chambers located at a 4.2 K plate and in the interior of the cell. The pressure in the 4.2 K chamber is heater-controlled and it transmits a force onto a membrane in the cell. The motion of the membrane induces spin-polarized current into an accumulation chamber.

We have measured specific heat of poly and single crystalline LaPt3Si samples under various magnetic fields. For zero magnetic field, we observed a distinct peak and jump in the specific heat at the super-conducting transition temperature T-c similar to 0.64 K and 0.61 K for the poly and single crystals, respectively. Temperature dependences of the specific heat of both samples below T-c can be well-described by a exponential equation for a conventional superconductor at zero magnetic field. The jump at T-c of single crystal is still distinct under magnetic fields. On the other hand, the peak of the poly crystal becomes broad above 40 Gauss. The broad additional specific heat tail appears at higher temperatures than the T-c expected from the results of the single crystal. We show, for the first time, the temperature dependence of superconducting critical magnetic field H-c(T) obtained from the specific heat measurement. Small anisotropy of H-c(T) was observed for the single crystal, with H-c,(0) = 66 and 61 Gauss for H//a and H//c, respectively.

We have fabricated a 3-axes magnetometer that can reduce the residual magnetic field below 50 mu G for each direction. Josephson critical current I-c between a noncentrosymmetric superconductor (CePt3Si or LaPt3Si) and Al has been investigated inside it to avoid the magnetic-flux trapping during the superconducting transition and found to show a magnetic field H dependence similar to the previous one obtained using one-axis magnetometer; asymmetric patterns with respect to H = 0 are commonly observed for CePt3Si and LaPt3Si. This suggests that the residual-field components perpendicular to H applied for I-c(H) measurements does not basically affect the I-c(H) patterns.