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Pressure is the cleanest way to tune heavy fermion systems to a quantum phase transition in order to study the rich physics and competing phases, and the comparison between ytterbium and cerium systems is particularly fruitful. We briefly review the mechanisms in play and show some examples of expected and unexpected behaviour. We emphasize the importance of the valence changes under pressure and show how modern synchrotron techniques can accurately determine this, including at low temperature., Comment: To appear in proceedings of International Conference on Superconductivity and Magnetism ICSM 2012 (Istanbul)

We report the results of the low-temperature specific heat measurements of the single crystalline β -pyrochlore oxide superconductors AOs2O6 (A=K, Rb, and Cs) under high pressure up to 13 GPa. We find that superconducting transition temperature (Tc) monotonically increases for CsOs2O6 and RbOs2O6, while the one for KOs2O6 decreases by applying the pressure. With further increasing the pressure, Tc is suddenly suppressed at the same lattice volume for all compounds, concomitant with the first-order structural phase transition.

The intermediate valent systems TmSe and SmB6 have been investigated up to 16 and 18 GPa by ac microcalorimetry with a pressure (p) tuning realized in situ at low temperature. For TmSe, the transition from an antiferromagnetic insulator for p3 GPa), Tn is found to increase linearly with p. A similar linear p increase of Tn is observed for the quasitrivalent compound TmS which is at ambiant pressure equivalent to TmSe at p=7 GPa. In the case of SmB6 long range magnetism has been detected above p=8 GPa, i.e. at a pressure slightly higher than the pressure of the insulator to metal transition. However a homogeneous magnetic phase occurs only above 10 GPa. The magnetic and electronic properties are related to the renormalization of the 4f wavefunction either to the divalent or the trivalent configurations. As observed in SmS, long range magnetism in SmB6 occurs already far below the pressure where a trivalent Sm3+ state will be reached. It seems possible, to describe roughly the physical properties of the intermediate valence equilibrium by assuming formulas for the Kondo lattice temperature depending on the valence configuration. Comparison is also made with the appearance of long range magnetism in cerium and ytterbium heavy fermion compounds., 22 pages including figures

Inelastic neutron-scattering measurements of the dispersion and width of transverse acoustic branches are reported for SixGe1−x, particularly for x=0.51. These alloys form high-quality single crystals. They are strongly disordered dynamically owing to the large Ge/Si mass ratio. This allows to study in function of disorder (x) the relations between the inhomogeneous broadening of acoustic waves, the Boson peak, and low-temperature thermal anomalies common to glasses. For x⩽0.51 we find no boson peak, apparently no plateau in the thermal conductivity, but at large x a Rayleigh law on the phonon width, Γ∝Q4, where Q is the scattering vector.

The temperature distribution in a parallelepipedic crystal irradiated with a Gaussian repartition of light is calculated by solving the three-dimensional heat equation and taking into account the losses through the sides of the samples. The theoretical and experimental maps of temperature are satisfactorily compared. For barium titanate crystals, the resistance to intense cw laser (25.7 kW/mm2 at 514.5 nm) and to Nd:YAG pulsed laser (peak power of 9.2 MW at 532 nm) is investigated. The defects induced by the irradiation are analysed. Under pulsed illumination, the damage threshold is found to be 0.54 GW/cm2 in a nominally undoped BaTiO3 and 0.44 GW/cm2 in a rhodium-cobalt doped BaTiO3 crystal.

We describe a versatile modular device built to generate forces up to 25 kN at cryogenic temperatures in order to achieve very fine in situ hydrostatic pressure tuning in the range 0–21 GPa. This device was designed to save time during measurements and protect the experimental setup by avoiding warming and cooling cycles to vary the pressure. The force is generated by a bellow operated with pressurized 4He and amplified mechanically. Diamond and sapphire anvil cells are used to perform electrical resistivity, magnetic susceptibility, and specific heat measurements under highly hydrostatic conditions by using helium as the transmitting medium. The pressure is determined by the ruby fluorescence technique. The performance of the device is illustrated by measurements of the superconducting transition of Pb and the magnetic transitions of CeRu2Ge2 in the range 0–10 GPa. This device is currently being adapted in a dilution fridge in order to be operated down to 50 mK.

We have implemented AC calorimetric measurements in diamond anvil pressure cells. The range of pressure (0–20 GPa), the excellent hydrostatic conditions of solid 4He used as the pressure transmitting medium associated with the possibility of continuously tuning the pressure at low temperature makes this experiment ideally suited for investigating phase diagrams under pressure and in particular for studying the thermodynamics of correlated metals around a Quantum Critical Point (QCP) where magnetic order is suppressed at absolute zero temperature. We present here results on the heavy fermion compound CeRu2Ge2 up to 8 GPa and down to 2 K. The specific heat anomalies associated with the different magnetic phase transitions are clearly resolved, allowing a precise determination of the magnetic phase diagram. On approaching the QCP close to 7 GPa, we observe a striking broadening of the antiferromagnetic transition associated with an anomalous behavior of the signal phase which could correspond to a change in the nature of this transition.

The thermal conductivity K(T) of pure and Si-doped CuGeO3 single crystals has been measured in the temperature range 2–40 K, in order to study the influence of the spin-Peierls transition on the heat transport. Anisotropic effects along the three main crystallographic directions have been found. Changes in K(T) due to cyclings at room temperature have been observed. The results are qualitatively interpreted by assuming that phonons and pseudofermions contribute to the heat transport.

We report on a compact loading cell for the continuous variation of uniaxial forces up to a few tons in low temperature investigations. The cell is based on a He activated bellow and a mechanical force amplification system. Due to its modular design, it may be used in studies of the effect of uniaxial pressure on solids or the in situ variation of hydrostatic pressure in an anvil cell. High pressure He is supplied by a fine capillary, so that bellow, force amplifier, and experiment may be cooled uniformly to mK temperatures even at high magnetic fields. The pressure in the bellow is controlled by a simple proportional-integral regulator at room temperature. Characteristic features of a piezoelectric sensor, used to measure the load at low temperature, are presented in detail. With the method described here, we observe long-term stability to better than ±0.2 N in sweeps over a wide range of temperatures and fields. The performance of the loading device is illustrated by means of measurements into the superconducting state of URu2Si2.

The low-temperature anomalies of glass forming systems are analyzed focusing on the concept of fragility. It is shown that two extremes on the fragility plot, Si${\mathrm{O}}_{2}$ and C${\mathrm{a}}_{0.4}$${\mathrm{K}}_{0.6}$(N${\mathrm{O}}_{3}$${)}_{1.4}$, show a strong difference in specific heat in the range of excess vibrational contribution. This difference cannot be explained in the framework of the soft potential model in its present state. A relation between the excess vibrational contribution and the amount of structural fluctuations is suggested. It is shown that the suggested relation explains the found difference in low-temperature specific heat between strong and fragile glass formers.

The low-energy excitations of a very fragile glass-former (in the sense of Angell), poly(vinyl chloride), have been studied by low-frequency Raman scattering and by measurements of the low-temperature heat capacity and thermal conductivity. Two different samples were investigated : one with a crystallinity of about 15%, the other quenched and amorphous, as measured by small-angle neutron scattering. The boson peak in Raman scattering was observed in both samples even at room temperature. A clear correspondance between the Raman boson peak, the excess of heat capacity and the plateau of thermal conductivity was shown. It is confirmed that the boson peak or the heat capacity excess are relatively small in this very fragile glass-former. However it is deduced that the high concentration of tunnelling systems in the amorphous sample is the result of a rapid quenching rather than an intrinsic property of fragile glass-formers.

We report a low-temperature loss anomaly in several oxidic perovskites such as ${\mathrm{KTaO}}_{3}$, ${\mathrm{KTaO}}_{3}$:Nb, ${\mathrm{SrTiO}}_{3}$, ${\mathrm{SrTiO}}_{3}$:Ca, ${\mathrm{PbTiO}}_{3}$:La, Cu, and ${\mathrm{BaTiO}}_{3}$:La. We show that this anomaly arises from a low-frequency dielectric relaxation. The activation energy and the relaxation time of this process are nearly the same for all the investigated perovskites disregarding their composition, texture, and ferroelectric properties. We thus ascribe the loss anomaly to the localization of polarons on residual defects. Although the dielectric losses in ${\mathrm{SrTiO}}_{3}$ and ${\mathrm{SrTiO}}_{3}$:Ca are qualitatively similar to other perovskites, the loss anomaly occurs at much lower temperatures: 10 instead of 40 K.

The changes in the light, neutron and X-ray diffracted intensities observed when an electric field is applied to crystals of alpha -LiIO3 are not yet fully understood. In the present paper, after summarizing the already known diffraction topographic experimental results, we present new ones obtained mainly by the use of section topographs. The discussion tries to establish connections between these data, and considers their origins. In particular, the present investigation shows that the majority of the enhanced diffracted intensity originates from small areas, which seem to correspond to growth bands. The influence of the nature and inhomogeneities of the electrodes is also considered. Finally, we describe the results of topographic experiments and conductivity measurements under an alternating electric field.

The thermal conductivity of nominally pure KTaO3 exhibits an unexpected strong phonon scattering around 8 K. The question arises whether this scattering is related to the distorted microregions observed by other experiments. We have investigated a set of undoped single crystals grown by different methods. By comparison with doped specimens, it is suggested that acoustic phonons are coupled to the soft optic mode via the local disorder. From complementary dielectric permittivity and electron paramagnetic resonance experiments, oxygen vacancies and rhombic Fe3+ appear to be incontestable candidates to account for the distorted microregions.

We discuss the sensitivity of iron based superconductors to the hydrostaticity of pressure conditions, which results in discrepancies between different studies. To clarify the situation we present the phase diagram of several systems obtained on single crystals, using the highly hydrostatic pressure conditions of argon in a diamond anvil cell, and an ac susceptibility method to detect superconductivity.

Single crystals of Sr14−xCaxCu24O41 (x=0 and 12) are grown by the travelling solvent floating zone technique using an image furnace. The grown crystals are characterized for their single crystallinity by the X-ray and Neutron Laue method. The magnetic susceptibility measurements in Sr14Cu24O41 show considerable anisotropy along the main crystallographic axes. Low-temperature specific heat measurement and DC susceptibility measurement in Ca-doped crystal showed antiferromagnetic ordering at 2.8 K at ambient pressure. High-pressure AC susceptibility measurement on Ca-doped crystal showed a sharp superconducting transition at 2 K under 40 kbars. Tc onset reached a maximum value of 9.9 K at 54 kbars. The bulk superconductivity of the sample is confirmed by the high-pressure AC calorimetry with Tc max=9.4 K and TN=5 K at 56 kbars.

The itinerant quasi-ferromagnetic metal MnSi has been studied by detailed thermal expansion measurements under pressures and magnetic fields. A sudden decrease of the volume at the critical pressure Pc ~1.6 GPa has been observed and is in good agreement with the pressure variation of the volume fraction of the spiral magnetic ordering. This confirms that the magnetic order disappears by a first order phase transition. The energy change estimated by the volume discontinuity on crossing Pc is of similar order as the Zeeman energy of the transition from the spiral ground state to a polarized paramagnetic one under magnetic field. In contrast to the strong pressure dependence of the transition temperature, the characteristic fields are weakly pressure dependent, indicating that the strength of the ferromagnetic and the Dzyaloshinskii-Moriya interactions do not change drastically around Pc. The evaluated results of the thermal expansion coefficient and the magnetostriction are analyzed thermodynamically. The Sommerfeld coefficient of the linear temperature term of the specific heat is enhanced just below Pc. The magnetic field-temperature phase diagrams in the ordered and paramagnetic phases are also compared. Comparison is made with other heavy fermion compounds with first order phase transition at 0 K., Comment: 9 pages, 13 figures, accepted to be published in JPSJ

We have measured the thermal conductivity {kappa} of ceramic Nd{sub 1.853}Ce{sub 0.147}CuO{sub {ital y}{minus}{delta}}. At low temperatures we observe an increase of {kappa} below the Nd magnetic ordering temperature due to an additional contribution of magnons to the heat current. At high temperatures the behavior is more nearly that of CuO than that of hole-doped high-temperature superconductors. The latter behavior is tentatively attributed to the absence of apical oxygen atoms.

It has been shown that “pure” incipient ferroelectric KTaO3 single crystals exhibit temperature size-dependent distorted micro-regions, which have been invoked to account for the sample-dependent phonon scattering observed in thermal conductivity K(T). A frequency-dependent dielectric loss peak (observed at 44 K for 1 kHz) is found to be present in all samples. The correlation between the thermal conductivity at 6.5 K and the intensity of the dielectric loss factor peak strongly supports the assumption that, below 20 K, K(T) is governed by the interaction of acoustic phonons with both the transverse optic soft mode and a dielectrically-active entity, the nature of which is still unknown.

The behaviour of the elastic constant C66 of Tm VO4 is investigated experimentally and theoretically at low temperature under applied magnetic field larger than the critical one

The interplay of magnetism and unconventional superconductivity (d singlet wave or p triplet wave) in strongly correlated electronic system (SCES) is discussed with recent examples found in heavy fermion compounds. A short presentation is given on the formation of the heavy quasiparticle with the two sources of a local and intersite enhancement for the effective mass. Two cases of the coexistence or repulsion of antiferromagnetism and superconductivity are given with CeIn3 and CeCoIn5. A spectacular example is the emergence of superconductivity in relatively strong itinerant ferromagnets UGe2 and URhGe. The impact of heavy fermion matter among other SCES as organic conductor or high Tc oxide is briefly pointed out., Comment: 22pages, 15 figures

Thermal and magneto-thermal conductivity measurements on undoped semi-insulating GaAs show the presence of paramagnetic phonon scattering centres. The data suggest that this may be due to traces of Cr3+ rather than to EL2 as earlier seemed likely.

The crucial question of whether the superconducting phase found in the spin ladder compound Sr14-xCaxCu24O41 is due to the natural pair forming mechanism predicted for isolated ladders is still open. A recent experimental breakthrough enables us to perform calorimetric measurements under hydrostatic pressure in a diamond anvil cell. We report here measurements on the compound Sr25Ca11.5CU24O41 which show that the superconductivity is a bulk property and that antiferromagnetic order and superconductivity coexist over a wide pressure range. At the pressure where the sample becomes superconducting, the variation of the Neel temperature is continuous. However at high pressure there is a sharp increase of TN, and TN and Tc become comparable. The possible nature of the superconducting phase is discussed in the light of these results and previous measurements of the upper critical field

We report the results of the low temperature specific heat of the β -pyrochlore oxide superconductors RbOs 2 O 6 and KOs 2 O 6 under high pressure. The superconducting transition temperature T c of RbOs 2 O 6 monotonically increases with increasing pressure and then abruptly vanishes at 6.0 GPa. Similar abrupt vanishment is also observed in KOs 2 O 6 at 5.2 GPa. Under high pressure above 5.8 GPa, other two anomalies are found instead of that of the superconductivity for RbOs 2 O 6 . It is suggested that the abrupt destruction of the superconductivity and the appearance of anomalies under high pressure are important to understand the origin of the superconductivity in this system.

0038-1098; Recent results on laser induced anisotropy in terbium gallium garnet are extended to the dynamic regime. We observed that the characteristic conoscopic pattern formation time presents a quadratic dependence on the beam size. The observed pattern intensity is accounted for by a simple analytical formula. The transient refractive index change due to thermal stress in the terbium gallium garnet is determined. (C) 1999 Elsevier Science Ltd. All rights reserved.

We develop a physical understanding for how anhamonicity of lattice vibration affects transition temperature, \(T_{\text{c}}\), of superconductivity mediated by phonons, via specific heat measurements on β-pyrochlore oxide superconductors under high pressure and in magnetic fields. We find that \(T_{\text{c}}\) demonstrates unimodal dependence on pressure, where \(T_{\text{c}}\) increases more than three times by pressure. On the other hand, an electron–phonon coupling constant, λ, is monotonically enhanced as pressure is applied, leading to the enhancement of effective mass of conduction electrons. From the analysis based on the McMillan equation, moreover, an average phonon frequency, ωlog, is found to reflect a downward trend on all the pressure regions, which is consistent with the results of the previous inelastic neutron scattering and specific heat measurements. The downward trend of ωlog means that the application of pressure enhances a magnitude of anhamonicity, contrary to a common sense of pres...

We report the results of the low-temperature specific heat measurements on the β-pyrochlore oxide superconductor RbOs 2 O 6 ( T c =6.3 K) in magnetic field under high pressure. We investigated the upper critical field H c2 at several pressures and found that the slope of H c2 at T c monotonically increases with increasing pressure in low-pressure superconducting phase SC1, indicating that the effective mass m * is enhanced by the pressure. Moreover, the jump of the specific heat at T c is also enhanced by the application of the pressure in SC1, suggesting that the pressure gives rise to the enhancement of the electron–phonon coupling and consequently enhances T c .

We report the result of the specific heat and the powder X-ray diffraction measurements on the single crystalline β-pyrochlore oxide superconductors A Os 2 O 6 under high pressure. We found from the specific heat measurements that the superconducting phase in the low pressure region (SC1) suddenly vanishes at the critical pressure P * and an alternative superconducting phase (SC2) with lower T c compared to that of SC1 appears above P * . In addition, we found from the X-ray diffraction measurements that a first-order phase transition of the crystal structure takes place at ∼ P * , accompanied with reduction of the crystal symmetry. This result suggests that the sudden suppression of T c arises from the structural phase transition.

We report the results of the specific heat measurements for β-pyrochlore oxide superconductor RbOs2O6 with superconducting transition temperature Tc ~ 6.3 K in magnetic field and under pressure. We find that Tc rises up to 9.3 K by applying the pressure concomitant with an increase of an initial slope of the upper critical field Hc2 and a jump of the specific heat at Tc. From the analysis of our data, the effective mass of conduction electrons and the electron-phonon coupling appear to increase under the pressure.

We present a detailed study of the high pressure phase diagram of the non-centrosymmetric antiferromagnet CeCoGe 3 by measurements of the ac calorimetry in a diamond anvil pressure cell. Under high pressure the magnetic ordering temperature shows a step like decrease due to various successive magnetic phases under high pressure. The magnetic order disappears at a critical pressure of p c ≈5.5 GPa. Around the critical pressure bulk superconductivity has been detected in the ac calorimetric measurement.

We report on the synthesis of superconducting single crystals of FeSe, and their characterization by X-ray diffraction, magnetization and resistivity. We have performed ac susceptibility measurements under high pressure in a hydrostatic liquid argon medium up to 14 GPa and we find that TC increases up to 33-36 K in all samples, but with slightly different pressure dependences on different samples. Above 12 GPa no traces of superconductivity are found in any sample. We have also performed a room temperature high pressure X-ray diffraction study up to 12 GPa on a powder sample, and we find that between 8.5 GPa and 12 GPa, the tetragonal PbO structure undergoes a structural transition to a hexagonal structure. This transition results in a volume decrease of about 16%, and is accompanied by the appearance of an intermediate, probably orthorhombic phase.

Thermal conductivity measurements have been made from 50 mK to 100 K of GaP and InP samples doped with Ni or V. The GaP samples both show strong resonant phonon scattering at 500 and 380 GHz respectively which is attributed to the T1-T2 tunnelling splitting associated with a dynamic orthorhombic Jahn-Teller distortion of the T1 orbital ground state of Ni2+ and V2+. Low-frequency scattering in both systems is tentatively attributed to these same ions in complex with a defect. The assignment of the scattering to V2+ is supported by studies of photo-induced changes in the V2+ concentration. Uniaxial stress measurements on GaP:V suggest strong coupling to E modes but the strength of the coupling to T2 modes is less clear. The absence of resonance scattering in a n-type sample of InP:Ni suggests the Ni is all in the Ni+ state (2T2) as expected and the absence of scattering in n-type InP:V is consistent with the instability of V2+ in that system.

Low temperature phonon scattering in the incipient ferroelectric KTaO3 is discussed in the light of measurements carried out on Nb, Na and Li-doped KTaO3 crystals.

The thermal conductivity of GaP(Cr) and InP(Cr) has been found to be very sensitive to uniaxial stress. The strong phonon scattering attributable to Cr3+. The sensitivity observed to both E and T2 strains indicates that Cr3+ in GaP is an orthorhombic Jahn-Teller system. This may also be the case for InP but the evidence is less complete.

The effects of γ, electron and neutron irradiation on the thermal conductivity (70 mK ÷ 100 K) of Ni-doped Al2O3 and pure Al2O3 single crystals have been studied. The results after neutron irradiation show that the behaviour of the Ni3+ ions in the perturbed crystal is similar to that of TLS in structural glasses.

Ion channeling measurements have been performed on KTaO 3 :Nb (KTN) in order to check the position of Nb relative to the tantalum site. No evidence for “off-center” Nb was found in the limit of experimental accuracy. In addition, the carbon content of the samples was measured and large concentrations were found (⋍300 p.p.m. weight). Possible relationship between Nb location and carbon presence is discussed.

RXO 4 compounds (R = rare earth and X = V, As or P) exhibit a crystallographic phase transition sensitive to the magnetic field. They furthermore have a large thermal conductivity which is increased by application of the field. The possibility of using these compounds for magnetic cooling is examined. TmVO 4 and DyVO 4 are studied with particular interest.

The near-infrared fluorescence of pure and doped KTaO3 samples is studied. In the neighbourhood of 14570 cm-1, 11 peaks are observed at 16 K and are associated with the fluorescence of a Ta3+ ion near an O vacancy. In the ferroelectric KTa0.982Nb0.0l8O3 crystal the fluorescence is not affected below Tc approximately=25 K, indicating that the Ta3+ ion in the KTaO3 material is located in a ferroelectric microdomain.

Thermal conductivity measurements have been made from 50 mK to 100 K of Ni-doped GaAs samples containing different proportions of Ni+, Ni2+ and Ni3+. Resonant phonon scattering at 300 GHz is assigned to Ni2+ (3T1) and this is supported by studies of photoinduced changes in the Ni2+ concentration. The scattering is attributed to tunnelling within the ground state of an orthorhombic Jahn-Teller system. Uniaxial stress measurements show that the levels are very sensitive to both E and T2 strains and provide values of VEb and VTb. Data for Cr3+(4T1) in GaAs, GaP and InP are re-examined in the light of these results. It is proposed that the rich spectrum of resonance frequencies is due to multiple-ion effects caused by ion-ion coupling via the strain field.

Phonon scattering by 3d impurities in GaAs, GaP and InP has been investigated by measuring the thermal conductivity between 50 mK and 100 K. Seven of the systems studied conform to the expectation that orbital triplets should scatter more strongly than doublets. It seemed likely that this should also be the case for isolated V2+ in GaAs but in fact only weak scattering is observed at high frequencies ( nu 0>100 GHz). This is interpreted as evidence that V2+ in GaAs has a low-spin configuration leading to a doublet ground state, 2E. Two strong phonon resonances are observed at low frequencies, approximately=6 and approximately=20 GHz. These are sensitive to sub-band-gap illumination and are assigned to a centre previously seen by TDEPR and APR and attributed to a V2+ complex.