Low-field Phase Research Articles

Two energy scales in YbInCu4 from specific heat in high magnetic fields

Metallic YbInCu4 undergoes a first-order valence transition at 42K, where the specific volume increases by 0.5% upon cooling. It is believed that the valence transition is driven by band structure effects that help quench Yb localized moments via a Kondo mechanism. The known phase diagram indicates that the transition can be suppressed with external magnetic fields. We used an adiabatic calorimeter to measure the specific heat in the high-field phase of the material. Data obtained in magnetic fields up to 50T show an enhanced Sommerfeld coefficient and anomalies in the specific heat that are compatible with a much depressed Kondo temperature. Our data indicate quite different energy scales in the low-field and high-field phases of YbInCu4.

Vortex creep characteristics via magnetic relaxation of Bi 2Sr 2CaCu 2O 8+ δ crystals

The vortex activation energy U( B, J)∝ B ν J − μ ∼ H ν M − μ , where J and M are the current and the magnetization, respectively, in Bi 2Sr 2CaCu 2O 8+ δ is investigated via magnetic relaxation measured by a miniature Hall sensor. The exponent μ at the low-field solid phase is definitively observed to be ∼1/7, which is consistent with the single-vortex collective creep. With increasing H, plastic creep is observed just above H on if the measurement is performed after a sufficiently long waiting time. On the other hand, with increasing T below H on, the μ decreases abruptly at 4 πM dp∼5 G near the depinning temperature T dp. The kink at M dp is attributed to a change in the creep character from that controlled by the bulk pinning into that controlled by the surface pinning.

Field-induced phase transition in a type III FCC antiferromagnet: K 2IrCl 6

The field-dependent antiferromagnetic structures in K 2IrCl 6 are investigated using magnetisation and neutron diffraction experiments. A field-induced transition from a collinear low-field phase to a non-collinear structure is observed. The phase diagram is constructed using magnetisation data. It is anisotropic with respect to the applied field direction. The high-field ordering vector was determined in neutron diffraction experiments.

Sound velocity measurement of nuclear-ordered U2D2 solid 3He along the melting curve

The sound velocity of a single-domain 3He crystal was measured in the nuclear-ordered low-field phase and the paramagnetic phase along the melting curve, using 10.98 MHz longitudinal sound. The temperature dependence of the sound velocity along the melting curve was explained by a nuclear spin contribution and the molar volume change along the melting curve. By comparing the measured velocity with thermodynamic quantities, we extracted the Grüneizen constant for the exchange energy. The anisotropy of the velocity in the ordered phase was investigated for three samples and was found to be smaller than 2×10 −5 in Δ v/ v. The attenuation coefficient of the sound was much smaller than 0.2 cm −1 .

Magnetic structures of GdMn6Ge6 - a nuclear magnetic resonance analysis

The origin of the interesting magnetic properties of GdMn6Ge6 in the low-temperature, low-field phase is a spiral arrangement of the magnetic moments. This has been established by pulsed nuclear magnetic resonance experiments. Spin-echo spectra taken in an external magnetic field, and the observed enhancement of the radio-frequency field, suggest the presence of a spiral structure. This information is crucial for a quantitative analysis of the complicated 55Mn spectra. The anisotropy of the hyperfine field at the 55Mn site is about 10% and the enhancement has been found to be highly anisotropic, as well. Comparing the temperature dependencies of the Zeeman splitting at the individual lattice sites, Mn has been identified as the source of the transferred hyperfine fields at the 73Ge nuclei. An analysis of the 73Ge and 155/157Gd spectra yields the size and relative orientation of the hyperfine fields and electric field gradients at the nuclei in agreement with 155Gd Mössbauer spectroscopy results.

Physical and Dielectric Properties of (1–x)PbZrO3·xBaTiO3 Thin Films Prepared by Chemical Solution Deposition

Physical and dielectric properties of (1–x)PbZrO3·x BaTiO3 thin films prepared by a chemical coating process have been investigated as a function of BaTiO3 (x) content (0≤x≤0.2). Changing the molar ratio between propylene glycol and water prior to the deposition optimized the chemical precursors. (1–x)PbZrO3‐x BaTiO3 thin films that contained a majority of perovskite phase, but also contained large amounts of other phases, were fabricated. These films could withstand fields of 250 kV/cm at 1 kHz. The microstructure of the thin films was found to depend on the BaTiO3 content. The phase transition from antiferroelectric to ferroelectric was gradually induced as the BaTiO3 content increased. A maximum dielectric constant of ∼809 was obtained at the composition of x= 0.1. A maximum dielectric constant of ∼809 was obtained at the composition of x= 0.1. A thin film at the low‐field antiferroelectric‐ferroelectric phase boundary with x= 0.05 exhibited the highest Psat and Pr values. The maximum values of these were 45 and 31 μC/cm2, respectively.

Van der Waals interaction between flux lines in high- superconductors

In anisotropic or layered superconductors thermal fluctuations as well as impurities induce a van der Waals (vdW) attraction between flux lines, as has recently been shown by Blatter and Geshkenbein in the thermal case [Phys. Rev. Lett. 77, 4958 (1996)] and by Mukherji and Nattermann in the disorder dominated case [Phys. Rev. Lett. 79, 139 (1997)]. This attraction together with the entropic or disorder induced repulsion has interesting consequences for the low field phase diagram. We present two derivations of the vdW attraction, one of which is based on an intuitive picture, the other one following from a systematic expansion of the free energy of two interacting flux lines. Both the thermal and the disorder dominated case are considered. In the thermal case in the absence of disorder, we use scaling arguments as well as a functional renormalization of the vortex-vortex interaction energy to calculate the effective Gibbs free energy on the scale of the mean flux line distance. We discuss the resulting low field phase diagram and make quantitative predictions for pure BiSCCO (Bi_2-Sr_2-CaCu_2-O_8). In the case with impurities, the Gibbs free energy is calculated on the basis of scaling arguments, allowing for a semi-quantitative discussion of the low-field, low-temperature phase diagram in the presence of impurities.

Van der Waals interaction between flux lines in high- superconductors: A variational approach

In pure anisotropic or layered superconductors thermal fluctuations induce a van der Waals attraction between flux lines. This attraction together with the entropic repulsion has interesting consequences for the low field phase diagram; in particular, a first order transition from the Meissner phase to the mixed state is induced. We introduce a new variational approach that allows for the calculation of the effective free energy of the flux line lattice on the scale of the mean flux line distance, which is based on an expansion of the free energy around the regular triangular Abrikosov lattice. Using this technique, the low field phase diagram of these materials may be explored. The results of this technique are compared with a recent functional RG treatment of the same system.

Van der Waals attraction of vortices in superconductors

We show that in anisotropic and layered superconductors the fluctuations of vortex lines produces an attractive long-range vortex — vortex interaction of the van der Waals type. This attraction follows from the anisotropic screening properties of the material and has profound consequences for the low-field phase diagram of these materials.

Vortex-matter phase transitions inBi2Sr2CaCu2O8:Effects of weak disorder

The vortex matter phase diagram in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} crystals is investigated by introducing very low doses of point and correlated disorder. We conclude that at least three distinct phases are present. The ordered low-field quasilattice phase has a finite shear modulus which vanishes at the first-order melting or sublimation transition at elevated temperatures. At lower temperatures the quasilattice transforms into a disordered solid as field is increased above the second magnetization peak. This disorder-driven transition shifts to lower fields with increased point disorder. The first-order transition displays corresponding downward curvature in the vicinity of the critical point. {copyright} {ital 1997} {ital The American Physical Society}

Small Angle Neutron Scattering Studies of the Vortex Lattice in theUPt3Mixed State: Direct Structural Evidence for theB→CTransition

Small angle neutron scattering studies of the flux line lattice (FLL) in UPt3 for fields H⊥c provide direct microscopic evidence for the 5 kOe B→C transition. We find a pronounced maximum in the longitudinal correlation length of the FLL at the transition and an abrupt change in the field dependence of the scattered intensity which can be interpreted as a 15% decrease in the coherence length and a 9% increase in the penetration depth, consistent with discontinuities in the critical fields. Finally, in the low field phase, the FLL distortion evolves roughly linearly with field, while in the high field phase it appears to be less field dependent.

Temperature and magnetic-field dependence of the lattice constant in the spin-Peierls cuprateCuGeO3sstudied by capacitance dilatometry in fields up to 16 T

We present high-resolution measurements of the thermal-expansion coefficient and the magnetostriction along the a axis of ${\mathrm{CuGeO}}_{3}$ in magnetic fields up to 16 T. From the pronounced anomalies of the lattice constant a, occurring for both temperature- and field-induced phase transitions, clear structural differences between the uniform, dimerized, and incommensurate phases are established. A precise field temperature phase diagram is derived and compared in detail with existing theories. Although there is fair agreement with the calculations within the Cross-Fisher theory, some significant and systematic deviations are present. In addition, our data yield a high-resolution measurement of the field and temperature dependence of the spontaneous strain scaling with the spin-Peierls order parameter. Both the zero-temperature values as well as the critical behavior of the order parameter are nearly field independent in the dimerized phase. A spontaneous strain is also found in the incommensurate high-field phase, which is significantly smaller and shows a different critical behavior than that in the low-field phase. The analysis of the temperature dependence of the spontaneous strain yields a pronounced field dependence within the dimerized phase, whereas the temperature dependence of the incommensurate lattice modulation compares well with that of the dimerization in zero magnetic field.

Role of electromagnetic coupling in the low-field phase diagram ofBi2.15Sr1.85CaCu2O8+δ

A detailed study has been made of a transition in the flux lattice of the superconductor ${\mathrm{Bi}}_{2.15}$${\mathrm{Sr}}_{1.85}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ using the technique of muon-spin rotation. The results are in excellent agreement with a recent theoretical analysis on the low-field phase diagram of highly anisotropic superconductors. In particular, the important role of electromagnetic interactions between vortex segments is demonstrated. The form of the phase boundary suggests that the transition may be identified with a simultaneous melting and decoupling of the vortices in adjacent layers.

Indication of confirmation of transition and formation boundary from ordered to disordered flux vortex chain state in high-Tcsuperconductors Y1Ba2Cu3O7-δand Bi2Sr2Ca2Cu3O10and new low-field data delineating magnetic transistion in Gd1Ba2(Fe0.02Cu0.98)3O7-δ

Data are presented herein that support a phase boundary or quasi-phase-boundary delineating in Y 1 Ba 2 Cu 3 O 7- δ and in Bi 2 Sr 2 Ca 2 Cu 3 )O 10 ceramic materials a transition from a vortex solid lattice to a line-flux disordered state that has been referred to as representing flux lattice melting to a flux liquid, but herein is interpreted not in terms of a liquid but in the form of a less-mobile ‘polymer’-like or entangled chain species. These data are derived from electrical resistance ( r ) versus applied magnetic field ( H ) measurements at various isotherms ( T ) corresponding to the zero resistance state of yttrium-barium-cuprate, and the mixed state foot regime of bismuth-strontium-calcium-cuprate. We interpret significant slope changes in r versus B at constant T in these materials to be indicative of the H-T conditions for a second-order or weakly first-order phase transition delineating the disordering of a flux lattice vortex solid. We believe that this technique is in ways more direct and at least as accurate as the conventional mechanical oscillator and vibrating magnetometer method to study the flux state. Additional very-low-field studies in Gd 1 Ba 2 (Fe 0.02 Cu 0.98 ) 3 O 7- δ , from 1 to 1000 mT, yield indication for what appears to be a magnetic transition at ca. 77 K at 575 mT, and possibly a second transition at 912 mT (also at ca . 77 K). These data points correspond well with the extrapolated low-field experimental magnetic phase transition boundary curve described at higher field herein (and by others using the mechanical technique), and also correspond well to theoretically predicted work regarding transition involving the vortex state.

Van der Waals Attraction of Vortices in Anisotropic and Layered Superconductors.

We show that in anisotropic and layered superconductors the fluctuations of vortex lines produce an attractive long-range vortex-vortex interaction of the van der Waals type. This attraction follows from the anisotropic screening properties of the material and has profound consequences for the low-field phase diagram of these materials.

The low-temperature phase of : I. Angle and temperature dependence of the Shubnikov - de Haas and de Haas - van Alphen oscillations

The magnetoresistance and magnetization of single crystals of the organic charge-transfer salt (where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene) have been studied in fields of up to 30 T and at temperatures as low as 20 mK. Five separate series of quantum oscillations have been observed in the low-temperature, low-field phase of this material and have been studied as a function of tilt angle of the field. It is proposed that two of these frequencies are the result of Stark quantum interference while the others are Shubnikov - de Haas (SdH) and de Haas - van Alphen (dHvA) oscillations due to closed Fermi surface pockets or conventional magnetic breakdown. The unconventional temperature dependence observed for some of these oscillations and the applicability of current models of the Fermi surface of are discussed.

Low-field phase diagram of layered superconductors: The role of electromagnetic coupling.

We determine the position and shape of the melting line in a layered superconductor taking the electromagnetic coupling between layers into account. In the limit of vanishing Josephson coupling we obtain a new generic reentrant low-field melting line. Finite Josephson coupling pushes the melting line to higher temperatures and fields and a new line shape $B_{{\rm m}} \propto (1-T/T_c)^{3/2}$ is found. We construct the low-field phase diagram including melting and decoupling lines and discuss various experiments in the light of our new results.

Nuclear spin dynamics of nuclear-ordered solid3He in the low field phase

Pulsed NMR was used to investigate nuclear spin dynamics of nuclear-ordered solid3He in the low field phase. The nuclear spin motion became unstable under certain conditions. Under stable conditions the spin motion can be described by the OCF equations. The tipping-angle-dependent frequency shift and multiple spin echoes were observed, which are similar to the case of superfluid3He. The onset of the instability of spin motion is attributed to the stimulated emission mechanism through the three-magnon relaxation process, which is similar to the Suhl instability in electronic magnetism. We derived the magnon life time from the analysis of the instability. During the instability, a largenegative frequency shift was observed. This negative shift is explained by the extension of Fomin-Ohmi's theory to include the state of decayed magnon and this explanation is consistent with the instability model.

Spin-flop phase and heavy fermion behavior in CePb3.

The magnetoresistance of a high-quality single crystal of the antiferromagnetic heavy fermion compound Ce${\mathrm{Pb}}_{3}$ has been measured as a function of field up to 18 T, as a function of temperature between 8 mK and 8 K, and as a function of field orientation. Below the N\'eel temperature, two field-induced phase transitions are observed, which occur at approximately 5 and 9.5 T with the field parallel to (110), but which merge into a single transition when the field is rotated to (100). The behavior is consistent with a spin-flop phase lying between the low-field antiferromagnetic phase and the high-field ferromagnetically polarized phase. Close to the spin-flop transition the ${T}^{2}$ coefficient of resistivity is enhanced, and the Fermi-liquid regime suppressed.

Low-field vortex dynamics over seven time decades in aBi2Sr2CaCu2O8+δsingle crystal for temperatures13<~T<~83K

Using a custom-made dc superconducting quantum interference device (dc-SQUID) magnetometer, we have measured the time relaxation of the remanent magnetization ${M}_{\mathrm{rem}}$ of a ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ single crystal from the fully critical state for temperatures $13 \mathrm{K}<~T<~83 \mathrm{K}$. The measurements cover a time window of seven decades ${10}^{\ensuremath{-}2} \mathrm{s}\ensuremath{\lesssim}t\ensuremath{\lesssim}{10}^{5} \mathrm{s}$, so that the current density $j$ can be studied from values very close to ${j}_{c}$ down to values considerably smaller than ${j}_{c}$. From the data we have obtained (i) the flux-creep activation barriers $U$ as a function of current density $j$, (ii) the current-voltage characteristics $E(j)$ in a typical range of ${10}^{\ensuremath{-}7}$-${10}^{\ensuremath{-}15}$ V/cm, and (iii) the critical current density ${j}_{c}(0)$ at $T=0$. Three different regimes of vortex dynamics are observed: For temperatures $T\ensuremath{\lesssim}20$ K the activation barrier $U(j)$ is logarithmic, no unique functional dependence $U(j)$ could be found for the intermediate-temperature interval $20 \mathrm{K}\ensuremath{\lesssim}T\ensuremath{\lesssim}40 \mathrm{K}$, and finally for $T\ensuremath{\gtrsim}40$ K the activation barrier $U(j)$ follows a power-law behavior with an exponent $\ensuremath{\mu}\ensuremath{\simeq}0.6$. From the analysis of the data within the weak collective pinning theory for strongly layered superconductors, it is argued that for temperatures $T\ensuremath{\lesssim}20$ K pancake vortices are pinned individually, while for temperatures $T\ensuremath{\gtrsim}40$ K pinning involves large collectively pinned vortex bundles. A description of the vortex dynamics in the intermediate-temperature interval $20 \mathrm{K}\ensuremath{\lesssim}T\ensuremath{\lesssim}40 \mathrm{K}$ is given on the basis of a qualitative low-field phase diagram of the vortex state in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$. Within this description a second peak in the magnetization loop should occur for temperatures between 20 and 40 K, as has been observed in several magnetization measurements in the literature.