Magnetic Quantum Phase Transition Research Articles

Pressure versus magnetic-field tuning of a magnetic quantum phase transition

The low temperature properties of CeCu6−xAux show different behavior at the quantum phase transition depending on how the long-range antiferromagnetic order is suppressed. Tuning by hydrostatic pressure p results in a temperature dependence of the specific heat C(T) and the electrical resistivity ρ(T) as C/T=aln(T0/T) and ρ≈ρ0+A′T, while C/T=γ0−a′T0.5 and ρ≈ρ0+A″T1.5 when tuned by applying a magnetic field B. The magnetic (p,T) and (B,T) phase diagrams as inferred from specific heat, electrical resistivity and neutron diffraction measurements are distinctly different. These findings suggest that the spectra of the low-lying magnetic fluctuations near these quantum phase transitions differ.

Low-temperature properties of YbZn2Sb2

We report synthesis and properties of the ternary Yb compound YbZn2Sb2. The resistivity, magnetization and specific heat at low temperature and high magnetic field of YbZn2Sb2 are typical of a strongly diamagnetic semimetal. A comparison with isostructural YbZn2As2 suggests that YbZn2Sb2 will undergo a nonmagnetic to magnetic quantum phase transition at high pressure that is driven by a valence transition.

Uniaxial stress experiments at the quantum phase transition of CeCu6−xAux

The resistivity ρ(T) of the heavy-fermion system CeCu6−xAux shows that for x=0.2 the Néel temperature TN rises under uniaxial stress σa parallel to the a-axis, in qualitative agreement with previous studies of the specific heat. For x=0.1, near the magnetic quantum phase transition, no hints of magnetic order are observed up to the highest σa of 4.5kbar. A detailed discussion of the dependence of ρ0 on hydrostatic and uniaxial pressure is given.

Scaling Analysis of Magnetic-Field-Tuned Phase Transitions in One-Dimensional Josephson Junction Arrays

We have studied experimentally the magnetic field-induced superconductor-insulator quantum phase transition in one-dimensional arrays of small Josephson junctions. The zero bias resistance was found to display a drastic change upon application of a small magnetic field; this result was analyzed in context of the superfluid-insulator transition in one dimension. A scaling analysis suggests a power law dependence of the correlation length instead of an exponential one. The dynamical exponents $z$ were determined to be close to 1, and the correlation length critical exponents were also found to be about 0.3 and 0.6 in the two groups of measured samples.

Pressure versus magnetic-field tuning of a magnetic quantum phase transition

Specific heat $C(T)$ and electrical resistivity $\ensuremath{\rho}(T)$ of ${\mathrm{CeCu}}_{6\mathrm{\ensuremath{-}}x}{\mathrm{Au}}_{x}$ at very low temperatures T show distinctly different behavior depending on whether long-range antiferromagnetic order is suppressed by hydrostatic pressure p or an applied magnetic field B. p tuning yields $C/T=a\mathrm{ln}{(T}_{0}/T)$ and $\ensuremath{\rho}\ensuremath{\approx}{\ensuremath{\rho}}_{0}{+A}^{\ensuremath{'}}T,$ while B tuning shows $C/T={\ensuremath{\gamma}}_{0}\ensuremath{-}{a}^{\ensuremath{'}}{T}^{0.5}$ and $\ensuremath{\rho}\ensuremath{\approx}{\ensuremath{\rho}}_{0}{+A}^{\ensuremath{''}}{T}^{1.5}.$ This suggests that the spectrum of low-lying excitations that determines the behavior near these quantum phase transitions differs.

Magnetic fluctuations in CeCu 6− xAu x at the nonmagnetic–magnetic quantum phase transition

When the heavy-fermion system CeCu 6 is doped with Au, long-range antiferromagnetic order develops above a threshold concentration x=0.1, with T N reaching 2.3 K for x=1. The magnetic ordering vector of CeCu 6− x Au x as determined from elastic neutron scattering is almost independent of x for x=0.15, 0.2 and 0.3, Q≈(0.63 0 0.26), and jumps to the a ∗ axis for x=0.5. At the quantum critical point x=0.1, the specific heat C exhibits a divergence of C/ T∼−ln( T/ T 0) towards low temperatures T, and the electrical resistivity varies linearly with T. Inelastic neutron scattering experiments were carried out at the ILL Grenoble and at Risø in order to investigate whether the critical fluctuations for x=0.1 can account for this anomalous behavior. We review some salient features of the both data sets. The observed reduced dimensionality of the critical fluctuations indeed leads to the above anomalous behavior. A detailed study of the energy and temperature dependence of the dynamic structure factor reveals unusual dynamics. The orthorhombic–monoclinic distortion in CeCu 6 is suppressed rapidly with increasing x and is shown to be unrelated to the quantum phase transition.

High-field spin dynamics of antiferromagnetic quantum spin chains

The characteristic internal order of macroscopic quantum ground states in one-dimensional spin systems is usually not directly accessible, but reflected in the spin dynamics and the field dependence of the magnetic excitations. In high magnetic fields quantum phase transitions are expected. We present recent work on the high-field spin dynamics of the S=1 antiferromagnetic Heisenberg chains NENP (Haldane ground state) and CsNiCl 3 (quasi-1D HAF close to the quantum critical point), the uniform S= 1 2 chain CTS, and the spin-Peierls system CuGeO 3.

Low-temperature thermal conductivity and thermopower of CeCu 6− xAu x near the magnetic quantum phase transition

The thermal conductivity κ and thermopower S in the heavy-fermion alloy CeCu 6− x Au x at the critical concentration x=0.1 separating magnetically ordered and non-magnetic ground states, has been measured in the range 0.1–2.5 K at zero magnetic field and B=6 T. κ( T) is found to be linear at high T in close agreement with pure CeCu 6. In the limit of low T, κ is strongly suppressed. In contrast to x=0, for x=0.1 κ increases only very weakly in a magnetic field of 6 T. The (positive) thermopower is enhanced with respect to x=0, dropping off only well below 0.5 K. The magnetic field reduces strongly the value of S and leads to a collapse of S( T) for x=0 and 0.1 onto identical curves.

Field dependence of the magnetic quantum phase transition in MnSi

The dependence on pressure (P) and field (H) of the magnetic phase diagram of MnSi has been investigated by means of measurements of the AC susceptibility up to 16 kbar and 7 T at temperatures down to 30 mK. For ambient pressure, we report on a peak in above at a characteristic temperature that rises quickly with field. The pressure dependence of is found to be strongly analogous to that of the zero-field transition to long-range order at . Features of in the field versus temperature (T) phase diagram may be viewed as `fingerprint' evidence of a field-induced crossover at of the itinerant magnetism from a non-polarized regime at high T and low H to a polarized regime at low T and high H. The long-wavelength spin spiral, present at low H, appears to be supported only in the polarized regime, so in the immediate vicinity of the critical pressure for which , a small pocket exists as a re-entrant state.

Magnetic quantum phase transition in MnSi under hydrostatic pressure

The crossover from a spin-polarized to nonpolarized state as a function of pressure ( p) at low temperature (T) has been investigated in MnSi via high-precision measurements of the electrical resistivity r and magnetic susceptibility x. In the magnetic phase ( p, pc.14.6 kbar!, r}T 2 at low T as expected for a Fermi liquid in a weakly polarized state. In the nonmagnetic phase ( p. pc), r vs T is consistent with the predictions for a marginal Fermi liquid model in which nearly critical spin fluctuations of long wavelength lead to a singular quasiparticle interaction. The transition is second order for p, p*.12 kbar and weakly firstorder in the range p*, p, pc , where the transition temperature Tc lies below a peak of x vs T. The variation of Tc with p and of both r and x with T and p may be understood in terms of a model of quantum critical phenomena. @S0163-1829~97!04213-6# I. INTRODUCTION

Miniature ac susceptometers for use inside clamp type pressure cells

The design and performance of miniature ac susceptometers are described that are small enough to fit inside clamp type pressure cells. The susceptometers are comprised of a primary and a balanced pair of secondaries with numbers of turns as high as 1000 to optimize the coupling to the sample and hence measurement sensitivity. Two methods of balancing the vacuum signal of the secondaries are presented, making susceptibility measurements even in clamp cells generating large stray signals possible. With the technique presented here a comprehensive series of measurements was carried out into the pressure induced magnetic quantum phase transition of MnSi.

Field dependence of the magnetic quantum phase transition in MnSi

We report on precision measurements of the AC susceptibility in high-purity samples of MnSi as a function of pressure up to 16 kbar, field up to 7 T and temperature down to 30 mK. In zero field it has previously been shown, that the weakly spin polarised (ordered) phase collapses above a critical pressure P c = 14.6 kbar. The normal metallic state at this quantum phase transition was thereby reported to be consistent with the presence of a highly dispersive spectrum of magnetic fluctuations. Our new results reveal that a field polarised state is restored above P c by application of a magnetic field, which can be considered as a further sensitive control parameter to tune a quantum critical regime. The crossover between the non-polarised and polarised regimes is characterised by a strongly field and pressure dependent maximum in χ characteristic of itinerant electron metamagnetism.

An application of the coupled-cluster method to the S= 1/2 triangular-lattice antiferromagnet

The coupled-cluster method is applied to the spin- 1/2 triangular-lattice anisotropic antiferromagnet, a noncollinear and frustrated magnet with an anisotropy parameter first introduced by Singh and Huse (1989). Various physical quantities including the ground-state energy, the anisotropy susceptibility, the sublattice magnetization, and their critical exponents associated with the magnetic order-disorder quantum phase transition, are calculated by the systematic inclusion of two-spin correlations. These results are then compared and contrasted with those obtained from other methods such as series expansions and variational calculations. The feasibility of carrying out quantum Monte Carlo simulations using the results presented here is also discussed by drawing parallels with the coupled-cluster calculations on the square-lattice antiferromagnets.

Crossover scaling functions and an extended minimal subtraction scheme

A field theoretic renormalization-group method is presented which is capable of dealing with crossover problems associated with a change in the upper critical dimension. The method leads to flow functions for the parameters and coupling constants of the model which depend on the set of parameters that characterize the fixed point landscape of the underlying problem. Similar to Nelson's trajectory integral method, any vertex function can be expressed as a line integral along a renormalization-group trajectory, which in the field theoretic formulation are given by the characteristics of the corresponding Callan-Symanzik equation. The field theoretic renormalization automatically leads to a separation of the regular and singular parts of all crossover scaling functions. The method is exemplified for the crossover problem in magnetic phase transitions, percolation problems and quantum phase transitions. The broad applicability of the method is emphasized.

Critical behaviour of ZrZn 2

The nature of the metallic state of the low temperature itinerant ferromagnet ZrZn 2 in the vicinity of a magnetic quantum phase transition has been studied in a series of susceptibility and resistivity measurements under high pressures. A pressure-temperature phase diagram has been constructed on the basis of the susceptibility data. The magnetic transition temperature is observed to follow T c 4 3 ∼ (p c − p) down to temperatures of order 1 K. No evidence for a first order transition was found at any pressure, but a clearly defined and moderately pressure-dependent maximum was observed in the susceptibility in the paramagnetic regime, i.e. for p > p c. The temperature dependence of the resistivity is found to differ from that of the conventional Fermi liquid model over a wide range of temperature and pressure. The shape of the phase diagram and the observed behaviour of the resistivity have been interpreted in terms of a quantum Ginzburg Landau model for the magnetic fluctuation spectrum.

Critical behaviour at the transition from a magnetic to a nonmagnetic metallic state in MnSi as a function of hydrostatic pressure

Abstract We report a study of the temperature and pressure dependence of the resistivity and magnetic susceptibility of MnSi which sheds light on the nature of the normal metallic state in the vicinity of a magnetic quantum phase transition. This state might be described in terms of a mean-field theory, the first approximation of which is a simple form of a marginal Fermi liquid for a pure metal with a highly dispersive magnetisation fluctuation spectrum.