Phase Diagram Of Lattice Research Articles

Metallic local-moment magnetocalorics as a route to cryogenic refrigeration

Commercial adiabatic demagnetisation refrigerators still employ the same hydrated salts that were first introduced over 85 years ago. The inherent limitations of these insulating magnetocalorics – poor thermal conductivity at sub-Kelvin temperatures, low entropy density, corrosiveness – can be overcome by a new generation of rare-earth based metallic magnetocalorics. Here, we present the metallic magnetocaloric YbNi1.6Sn as an attractive alternative to conventional refrigerants. YbNi1.6Sn retains high entropy into the 100 mK regime and avoids the noble metal constituents of alternative refrigerants. Demagnetisation tests demonstrate that YbNi1.6Sn enables economical and durable alternatives to traditional cooling devices for temperatures reaching below 120 mK. We find that the magnetocaloric properties of this material are facilitated by unusually small Kondo and RKKY interactions, which position YbNi1.6Sn in the extreme local moment limit on the generalised Kondo lattice phase diagram.

Effects of magnetic and non-magnetic doping on the vortex lattice in MgB2

Small-angle neutron scattering has been used to study the vortex lattice in superconducting MgB2 doped with either manganese or carbon to achieve a similar suppression of the critical temperature. Measurements were performed with the magnetic field applied along the c axis, where the vortex lattice in pure MgB2 is known to undergo a field- and temperature-driven 30° rotation transition. For Mn doping, the vortex lattice phase diagram remains qualitatively similar to that of pure MgB2, indicating only a modest effect on the vortex–vortex interaction. In contrast, the vortex lattice rotation transition is completely suppressed in the C-doped case, probably due to a change in the electronic structure which affects the two-band/two-gap nature of superconductivity in MgB2. The vortex lattice longitudinal correlation length shows the opposite behavior, remaining roughly unchanged between pure and C-doped MgB2 while it is significantly reduced in the Mn-doped case. However, the extensive vortex lattice metastability and related activated behavior, observed in conjunction with the vortex lattice transition in pure MgB2, are also seen in the Mn-doped sample. This shows that the vortex lattice disordering is not associated with a substantially increased vortex pinning.

Effects of the Order Parameter Anisotropy on the Vortex Lattice in UPt3

We have used small-angle neutron scattering to determine the vortex lattice phase diagram in the topological superconductor UPt3 for the applied magnetic field along the crystalline c-axis. A triangular vortex lattice is observed throughout the superconducting state, but with an orientation relative to the hexagonal basal plane that changes with field and temperature. At low temperature, in the chiral B phase, the vortex lattice undergoes a non-monotonic rotation with increasing magnetic field. The rotation amplitude decreases with increasing temperature and vanishes before reaching the A phase. Within the A phase an abrupt ±15° vortex lattice rotation was previously reported by Huxley et al., Nature 406, 160-164 (2000). The complex phase diagram may be understood from competing effects of the superconducting order parameter, the symmetry breaking field, and the Fermi surface anisotropy. The low-temperature rotated phase, centered around 0.8 T, reported by Avers et al., Nature Physics 16, 531-535 (2020), can be attributed directly to the symmetry breaking field.

Pressure-tuned Frustration of Magnetic Coupling in Elemental Europium.

Applying linear response and the magnetic force theorem in correlated density functional theory, the intersublattice exchange constants of antiferromagnetic Eu are calculated and found to vanish near the pressure of P_{c}=82 GPa, just where magnetic order is observed experimentally to be lost. The Eu 4f^{7} moment remains unchanged at high pressure, again in agreement with spectroscopic measurements, leaving the picture of perfect frustration of interatomic Ruderman-Kittel-Kasuya-Yoshida couplings in a broad metallic background, leaving a state of electrons strongly exchange coupled to arbitrarily oriented, possibly quasistatic local moments. This strongly frustrated state gives way to superconductivity at T_{c}=1.7 K, observed experimentally. These phenomena, and free energy considerations related to correlations, suggest an unusual phase of matter that is discussed within the scenarios of the Doniach Kondo lattice phase diagram, the metallic spin glass class, and itinerant spin liquid or spin gas systems.

SU(2) with fundamental fermions and scalars

We present preliminary results on the lattice simulation of an SU(2) gauge theory with two fermion flavors and one strongly interacting scalar field, all in the fundamental representation of SU(2). The motivation for this study comes from the recent proposal of “fundamental” partial compositeness models featuring strongly interacting scalar fields in addition to fermions. Here we describe the lattice setup for our study of this class of models and a first exploration of the lattice phase diagram. In particular we then investigate how the presence of a strongly coupled scalar field affects the properties of light meson resonances previously obtained for the SU(2) model. Preprint: CP3-Origins-2017-047 DNRF90

Infrared conformality and bulk critical points: SU(2) with heavy adjoint quarks

Abstract The lattice phase structure of a gauge theory can be a serious obstruction to Monte Carlo studies of its continuum behaviour. This issue is particularly delicate when numerical studies are performed to determine whether a theory is in a (near-)conformal phase. In this work we investigate the heavy mass limit of the SU(2) gauge theory with N f = 2 adjoint fermions and its lattice phase diagram, showing the presence of a critical point ending a line of first order bulk phase transition. The relevant gauge observables and the low-lying spectrum are monitored in the vicinity of the critical point with very good control over different systematic effects. The scaling properties of masses and susceptibilities open the possibility that the effective theory at criticality is a scalar theory in the universality class of the four-dimensional Gaussian model. This behaviour is clearly different from what is observed for SU(2) gauge theory with two dynamical adjoint fermions, whose (near-)conformal numerical signature is hence free from strong-coupling bulk effects.

Orthogonal technicolor with isotriplet dark matter on the lattice

We study the gauge dynamics of an SO(4)-gauge theory with two Dirac Wilson fermions transforming according to the vector representation of the gauge group. We determine the lattice phase diagram by locating the strong coupling bulk phase transition line and the zero quark mass line. We present results for the spectrum of the theory obtained at a fixed value of the lattice spacing. In particular we measure the pseudoscalar, vector and axial meson masses. The data are consistent with a chiral symmetry breaking scenario rather than a conformal one. When used to break the electroweak symmetry dynamically the model leads to a natural dark matter candidate.

Minimal technicolor on the lattice

We present results from a lattice study of SU(2) gauge theory with 2 flavors of Dirac fermions in adjoint representation. This is a candidate for a minimal (simplest) walking technicolor theory, and has been predicted to possess either an IR fixed point (where the physics becomes conformal) or a coupling which evolves very slowly, so-called walking coupling. In this initial part of the study we investigate the lattice phase diagram and the excitation spectrum of the theory.

Vortex phase diagram and two-dimensional bosons

We study the phase diagram of the flux lines using the mapping to two-dimensional bosons interacting via the Yukawa potential in the hypernetted-chain approximation. The pair-correlation function, static structure factor, and spectrum of the excited energies are calculated over a wide range of the parameter. These quantities are used for studying the freezing transition from the entangled vortex liquid into the Abrikosov lattice. The resulting $B\ensuremath{-}T$ phase diagram is in good agreement with the known estimates for the vortex lattice phase diagram and the Monte Carlo simulations.

Square vortex lattices for two-component superconducting order parameters

I investigate the vortex lattice structure of the Ginzburg Landau free energy for a two component order parameter in the weak-coupling clean-limit when the field is along the high symmetry axis in a tetragonal crystal. It is shown that the vortex lattice phase diagram as a function of the Ginzburg Landau free energy parameters includes phases with a hexagonal, centered rectangular, rectangular, and square unit cells. It is also shown that the square vortex lattice has the largest region of stability. The field distribution of the square vortex lattice near $H_{c2}$ is determined and the application of this model to Sr$_2$RuO$_4$ is discussed.

Thermodynamic consideration of the tetragonal lattice distortion of the L1 0 ordered phase

In an attempt to understand the thermodynamic consequences of the tetragonal distortion accompanying the L1 0 ordering, we have conducted a theoretical investigation by treating the problem of a rigid lattice free energy model. The problem is treated as an elasticity problem and a generalized formulation is proposed in a form which can be directly utilized to study incoherent two phase equilibria. By use of the Static Concentration Wave (SCW) mean field model as a rigid lattice free energy model, the formulation is applied to the case of our interest. We show that the thermodynamic stability of the L1 0 phase may be significantly influenced by the tetragonal lattice distortion, depending on the magnitude of the associated strain energy relative to the competing positive entropic contribution in the free energy of the stress free L1 0 phase. In association with this, we suggest that the neglect of tetragonal distortion (i.e. the use of a rigid lattice free energy model) could be a source of serious errors particularly for alloys with lower L1 0 ordering transition temperatures (e.g. CuAu and InMg). A prototype phase diagram of the 〈001〉 f.c.c. ∗ special point structures, calculated with the tetragonal distortion taken into account, has indeed displayed topological features which are fundamentally different from those of a rigid lattice phase diagram and, furthermore, has reproduced the main topological features of the AuCuAu 3Cu and the InMgIn 3Mg sides of their respective phase diagrams.

Criticality of the discrete N-vector ferromagnet in planar self-dual lattices

The authors study an extended version of the discrete N-vector (or cubic) ferromagnetic model within a real space renormalisation group approach which preserves the two-spin correlation function. The N-evolution (for real values of N) of the Wheatstone-bridge hierarchical lattice phase diagram, which presents paramagnetic intermediate (nematic-like) and ferromagnetic phases, as well as of the thermal ( nu ) and crossover ( phi ) critical exponents, is presented. The self-avoiding walk problem is recovered in the N to O limit, and the so-called 'corner rule' is re-obtained in a larger context. The Ising, N- and 2N-state Potts ferromagnets are recovered as particular cases. An interchange of stability occurs at N=N* approximately=6.9 in such a way that the 2N-state Potts special point (where all three existing phases join) is multicritical if N(N*, but only critical if N)N* (consistently phi (N*)=O). For the cubic model, nu (N) presents a maximum at N=Nmax approximately=1.5. The results are exact, for all N, for the Wheatstone-bridge hierarchical lattice, and approximate, for N<or=2, for the square lattice. Last but not least, they discuss the connection between the present approach and the phenomenological renormalisation group.

Lattice lambda phi4 theory with Yukawa couplings to staggered fermions.

An investigation of the \ensuremath{\lambda}${\ensuremath{\varphi}}^{4}$ theory coupled via Yukawa couplings to fermions has been initiated on the lattice. Dynamical fermions are taken fully into account, using hybrid-molecular-dynamic algorithms. Different ways of transcribing Yukawa couplings onto the lattice are discussed. It is found that the lattice phase diagram is very sensitive to the manner in which this interaction is regularized. Some results for scalar and fermionic correlation functions are also presented.