Low-temperature Phase Diagram Research Articles

Quasicondensation and coherence in the quasi-two-dimensional trapped Bose gas

We simulate a trapped quasi-two-dimensional Bose gas using a classical field method. To interpret our results we identify the uniform Berezinskii-Kosterlitz-Thouless (BKT) temperature $T_{BKT}$ as where the system phase space density satisfies a critical value. We observe that density fluctuations are suppressed in the system well above $T_{BKT}$ when a quasi-condensate forms as the first occurrence of degeneracy. At lower temperatures, but still above $T_{BKT}$, we observe the development of appreciable coherence as a prominent finite-size effect, which manifests as bimodality in the momentum distribution of the system. At $T_{BKT}$ algebraic decay of off-diagonal correlations occurs near the trap center with an exponent of 0.25, as expected for the uniform system. Our results characterize the low temperature phase diagram for a trapped quasi-2D Bose gas and are consistent with observations made in recent experiments.

Cluster Monte Carlo and numerical mean field analysis for the water liquid–liquid phase transition

Using Wolff's cluster Monte Carlo simulations and numerical minimization within a mean field approach, we study the low temperature phase diagram of water, adopting a cell model that reproduces the known properties of water in its fluid phases. Both methods allow us to study the thermodynamic behavior of water at temperatures, where other numerical approaches – both Monte Carlo and molecular dynamics – are seriously hampered by the large increase of the correlation times. The cluster algorithm also allows us to emphasize that the liquid–liquid phase transition corresponds to the percolation transition of tetrahedrally ordered water molecules.

Magnetic properties of PrCu2 at high pressure

We report a study of the low-temperature high-pressure phase diagram of the intermetallic compound PrCu2, by means of molecular-field calculations and 63,65Cu nuclear-quadrupole-resonance (NQR) measurements under pressure. The pressure-induced magnetically-ordered phase can be accounted for by considering the influence of the crystal electric field on the 4f electron orbitals of the Pr3+ ions and by introducing a pressure-dependent exchange interaction between the corresponding local magnetic moments. Our experimental data suggest that the order in the induced antiferromagnetic phase is incommensurate. The role of magnetic fluctuations both at high and low pressures is also discussed.

Magnetotransport in the CeIrIn5 System: The Influence of Antiferromagnetic Fluctuations

We present an overview of magnetotransport measurements on the heavy-fermion superconductor CeIrIn5. Sensitive measurements of the Hall effect and magnetoresistance are used to elucidate the low-temperature phase diagram of this system. The normal-state magnetotransport is highly anomalous, and experimental signatures of a pseudogap-like precursor state to superconductivity, as well as evidence for two distinct scattering times governing the Hall effect and the MR, are observed. Our observations point out the influence of antiferromagnetic fluctuations on the magnetotransport in this class of materials. The implications of these findings, both in the context of unconventional superconductivity in heavy-fermion systems and in relation to the high-temperature superconducting cuprates, are discussed.

Liquid Water-Ice I Phase Diagrams under High Pressure: Sodium Chloride and Sucrose Models for Food Systems

The knowledge of high pressure and low temperature phase diagrams of aqueous systems is required in fields such as food sciences, biology, cryo-microscopy and geology, to reduce processing costs, improve treatments results or advance in physical phenomena understanding. The phase transition curve between liquid water and ice I for sucrose and sodium chloride solutions has been obtained for concentrations ranging from 16% to 36% and from 1.63% to 16.09% (w/w), respectively. An accurate experimental method, based on the pressurization of an ice-solution mixture, adequate to build the entire phase transition curve at constant concentration, has been developed. Simon-like equations have been used to empirically describe the phase transition curves, so that they allow easy data interpolation.

A low-temperature phase diagram for ilmenite-rich compositions in the system Fe2O3-FeTiO3

aB stract An approximate low-temperature, metastable phase diagram is drawn for the system (1 – X) Fe2O3-(X)FeTiO3. It is based on published and new magnetic data from nine synthetic samples with bulk compositions in the range 0.6 < X < 1.0. Fields are plotted for (1) the paramagnetic phase (PM); the Fe2O3-rich ferrimagnetic phase (FM); (2) the FeTiO3-rich antiferromagnetic phase (AF); and (3) a re-entrant spin-glass phase (RSG). In addition, two subfields are plotted: (1) FM′, a subfield of the FM-phase, which occurs below a characteristic temperature TK, at which the magnetic susceptibility drops sharply on cooling, and (2) PM′, a subfield of the PM-phase (traditionally called superparamagnetic) forms below a sharp rise in susceptibility at TS, and exhibits measurable dispersion in the magnetic susceptibility at T < TS. The diagram is drawn with a bicritical point, Tλλ′, at X ≈ 0.87, T ≈ 39 K, which is the intersection of second-order magnetic phase boundaries for the paramagnetic → ferrimagnetic [PM(PM′) → FM] transition, TC(X), and the PM(PM′) → AF transition, TN(X). In addition, the RSG phase is plotted as one of four stable phases at Tλλ′, a construction that is not required by the phase rule, but is strongly favored by the physics of competition between the incompatible magnetically ordered structures of the FM- and AF-phases. These phase relations are at such low temperature as to be of little consequence for terrestrial magnetism, however, they may well be essential for interpreting the magnetism of the Moon, Mars, and other cold planets. These phase relations are also essential for the characterization of fine natural and synthetic intergrowths, and for understanding magnetic materials for low-temperature technologi cal applications.

Duality, magnetic space group and their applications to quantum phases and phase transitions on bipartite lattices in several experimental systems

By using a dual vortex method, we study phases such as superfluid, solids, supersolids and quantum phase transitions in a unified scheme in extended boson Hubbard models at and slightly away from half filling on bipartite optical lattices such as honeycomb and square lattice. We also map out its global phase diagram at T = 0 of chemical potential versus the ratio of kinetic energy over the interaction. We stress the importance of the self-consistence condition on the saddle point structure of the dual gauge fields in the translational symmetry breaking insulating sides, especially in the charge density wave side. We find that in the translational symmetry breaking side, different kinds of supersolids are generic possible states slightly away from half filling. We propose a new kind of supersolid: valence bond supersolid (VB-SS). In this VB-SS, the density fluctuation at any site is very large indicating its superfluid nature, but the boson kinetic energies on bonds between two sites are given and break the lattice translational symmetries indicating its valence bound nature. We show that the quantum phase transitions from solids to supersolids driven by a chemical potential are in the same universality class as that from a Mott insulator to a superfluid, therefore have exact exponents z = 2 , ν = 1 / 2 , η = 0 with a logarithmic correction. Comparisons with previous quantum Monte Carlo (QMC) simulations on a square lattice are made. Implications on possible future QMC simulations in both bipartite lattices are given. All these phases and phase transitions can be potentially realized in ultra-cold atoms loaded on optical bipartite lattices. Then we apply our results to investigate the reentrant “superfluid” in a narrow region of coverages in the second layer of 4He adsorbed on graphite and the low temperature phase diagram of hydrogen physisorbed on krypton-preplated graphite (H 2/Kr/graphite) near half filling. We suggest that 4He and H 2 lattice supersolids maybe responsible for the experimental signals in the two systems. Finally, we suggest Cooper supersolid is repressible for the phase diagram of La 2− x Ba x CuO 4 near x = 1 / 8 .

First-principles based calculation of binary and multicomponent phase diagrams for titanium carbonitride

In this paper we have used a combined first principles and Calphad approach to calculate phase diagrams in the titanium–carbon–nitrogen system, with particular focus on the vacancy-induced ordering of the substoichiometric carbonitride phase, TiC x N y ( x + y ≤ 1 ). Results from earlier Monte Carlo simulations of the low-temperature binary phase diagrams are used in order to formulate sublattice models for TiC x N y within the compound energy formalism (CEF) that are capable of describing both the low temperature ordered and the high-temperature disordered state. We parameterize these models using first-principles calculations and then we demonstrate how they can be merged with thermodynamic descriptions of the remaining Ti–C–N phases that are derived within the Calphad method by fitting model parameters to experimental data. We also discuss structural and electronic properties of the ordered end-member compounds, as well as short range order effects in the TiC x N y phase.

Phase diagram of an anisotropic bosonict−Jmodel

We have studied by quantum Monte Carlo simulations the low temperature phase diagram of a mixture of isotopic, hard core bosons, described by the $t\text{\ensuremath{-}}{J}_{z}\text{\ensuremath{-}}{J}_{\ensuremath{\perp}}$ model, with ${J}_{\ensuremath{\perp}}=\ensuremath{\alpha}{J}_{z}$. The coexistence of superfluid hole-rich and insulating, antiferromagnetically ordered hole-free phases is observed at sufficiently low hole density for any $\ensuremath{\alpha}&lt;1$. A two-component checkerboard supersolid phase is not observed. The experimental relevance and possible broader implications of these findings are discussed.

Spin-density wave in polycrystalline Cr films from infrared reflectivity

The spin-density wave properties of polycrystalline chromium thin films were determined by using infrared reflectivity to determine the gap energies. The incommensurate spin-density wave of bulk chromium is highly sensitive to perturbations from stress, disorder, and finite size effects, such as those found in polycrystalline films. Films prepared under various conditions display three different types of spin-density wave behavior: incommensurate, commensurate, and mixed. Unexpectedly, the mixed phase includes the incommensurate spin-density wave and two different forms of commensurate spin-density wave. A phenomenologically determined low-temperature phase diagram is created to describe the spin-density wave in chromium in the stress-disorder plane. The effects of stress and disorder on the spin-density wave of chromium films are analogous to the effects of dilute alloying in bulk chromium. In this case, tensile stress has a similar effect to Mn impurities, while disorder has a similar effect to Al.

Ab initio prediction of the low-temperature phase diagrams in the systems MBr–MCl (M = Li, Na, K)

We have studied the low-temperature phase diagrams of the systems MBr–MCl (M = Li, Na or K), via global exploration of the enthalpy landscapes for many different compositions, leading to candidates for solid solution-like and ordered crystalline phases. For all these candidates the free enthalpies are computed on ab initio level, and the low-temperature phase diagrams of the three chemical systems are derived. We find not only the expected stable solid solution in the rocksalt structure type, but also a metastable solid solution based on the CsCl-type for the KBr–KCl system, which becomes thermodynamically stable at elevated pressures. Furthermore, we calculated the P– T– x phase diagram up to the transition pressure for the rocksalt modification, and for 5 GPa for the CsCl-modification, for the KBr–KCl system.

A block effect of external field in the one-dimensional ferromagnetical Ising model with long-range interaction

We consider the one-dimensional ferromagnetical Ising model with long-range interaction under external field blocks of equal length with alternating signs and investigate the low-temperature phase diagram of this model. It turns out that when the absolute value of the external field is sufficiently small, the set of Gibbs states substantially depends on block size: at small block sizes there are at least two Gibbs states and at large block sizes there is a unique Gibbs state.

Ab initio prediction of the low-temperature phase diagrams in the systems CsX–LiX (X = F, Cl, Br, I)

We have calculated the low-temperature phase diagrams for the ternary alkali halides CsX–LiX (X = F, Cl, Br, I) at an ab initio level without any recourse to experimental information. The starting point of our general approach is the global exploration of the enthalpy landscapes for many different compositions in these systems. Candidates for both ordered stoichiometric modifications and crystalline solid-solution phases are identified, and their free enthalpies are computed at an ab initio level. From this the low-temperature phase diagrams are derived. We find that in all systems under investigation only crystalline ordered phases should be present, in agreement with available experimental data. Furthermore, we predict several new thermodynamically stable and metastable phases in these systems.

Ab initio prediction of the low-temperature phase diagrams in the systems KBr–NaBr, KX–RbX, and LiX–RbX (X=Cl,Br)

The authors have calculated the low-temperature phase diagrams for the ternary alkali halides KBr-NaBr, KX-RbX, and LiX-RbX (X=Cl,Br) systems on the ab initio level without any recourse to experimental information. Via global exploration of the enthalpy landscapes for many different compositions in these systems, candidates for both ordered stoichiometric modifications and crystalline solid solution phases have been identified. Next, their free enthalpies were computed on ab initio level, and the respective low-temperature phase diagram has been derived. They find miscibility gaps in the systems KBr-NaBr and KX-RbX (X=Cl,Br), while in LiX-RbX (X=Cl,Br) only crystalline ordered phases should be present, in agreement with available experimental data. Furthermore, they predict several new thermodynamically stable and metastable phases in these systems.

Ab Initio Computation of the Low-Temperature Phase Diagrams of the Alkali Metal Iodide−Bromides: MBrxI1-x (0 ≤ x ≤ 1), Where M = Li, Na, K, Rb, or Cs

We have studied the low-temperature phase diagrams of the systems MBr-MI (M = Li, Na, K, Rb, or Cs) via global exploration of the enthalpy landscapes for many different compositions, leading to candidates for solid solution-like and ordered crystalline phases. For all of these candidates the free enthalpies are computed at the ab initio level, and the low-temperature phase diagrams of the five chemical systems are derived. We find not only the expected stable solid solution in the rocksalt structure type but also metastable solid solutions based on the CsCl type for the RbBr-RbI and CsCl-CsI systems. Furthermore, additional metastable structure candidates exhibiting ordered crystalline structures exist for several compositions. In the case of the LiBr-LiI system, the metastable solid solution based on the wurtzite type was generated, and the location of the miscibility gap was predicted.

On Low-Temperature Structural Phase Transitions

We comment on zero- and low-temperature structural phase transitions, expecting that these comments might be relevant not only for this structural case. We first consider a textbook model whose classical version is the only model for which the Landau theory of phase transitions and the concept of “soft mode” introduced by Ginzburg are exact. Within this model, we reveal the effects of quantum fluctuations and thermal ones at low temperatures. To do so, the knowledge of the dynamics of the model is needed. However, as already was emphasized by Ginzburg et al. in eighties, a realistic theory for such a dynamics at high temperatures is lacking, what also seems to be the case in the low-temperature regime. Consequently, some theoretical conclusions turn out to be dependent on the assumptions on this dynamics. We illustrate this point with the low-temperature phase diagram, and discuss some unexpected shortcomings of the continuous medium approaches.

Superconductivity in inhomogeneous Hubbard models

We present a controlled perturbative approach to the low-temperature phase diagram of highly inhomogeneous Hubbard models in the limit of small coupling, ${t}^{\ensuremath{'}}$, between clusters. We apply this to the dimerized and checkerboard models. The dimerized model is found to behave like a doped semiconductor, with a Fermi-liquid groundstate with parameters (e.g., the effective mass) which are smooth functions of the Hubbard interaction, $U$. By contrast, the checkerboard model has a Fermi liquid phase at large $U&gt;{U}_{c}=4.58$, a $d$-wave superconducting state with a full gap for ${U}_{c}&gt;U&gt;0$, and a narrow strip of an intermediate $d$-wave superconducting phase with gapless ``nodal'' quasiparticles for $\ensuremath{\mid}U\ensuremath{-}{U}_{c}\ensuremath{\mid}&lt;\mathcal{O}({t}^{\ensuremath{'}})$.

Field-Induced Order and Spin Waves in the Pyrochlore AntiferromagnetTb2Ti2O7

High resolution time-of-flight neutron scattering measurements on Tb(2)Ti(2)0(7) reveal a rich low temperature phase diagram in the presence of a magnetic field applied along [110]. In zero field at T = 0.4 K, Tb(2)Ti(2)0(7) is a highly correlated cooperative paramagnet with disordered spins residing on a pyrochlore lattice of corner-sharing tetrahedra. Application of a small field condenses much of the magnetic diffuse scattering, characteristic of the disordered spins, into a new Bragg peak characteristic of a polarized paramagnet. At higher fields, a magnetically ordered phase is induced, which supports spin wave excitations indicative of continuous, rather than Ising-like, spin degrees of freedom.

Ab initio computation of low-temperature phase diagrams exhibiting miscibility gaps

A new methodology for the computation of the low-temperature part of phase diagrams without recourse to any experimental information is presented. A central element is a procedure for deciding whether formation of crystalline solid solution phases can take place in the chemical system. Via global exploration of the enthalpy landscapes for many different compositions in the system, candidates for ordered stoichiometric and crystalline solid solution phases are identified. Next, their free enthalpies are computed at ab initio level and a low-temperature phase diagram is derived. As examples, the low-temperature phase diagrams for the ternary alkali halides NaCl/LiCl NaBr/LiBr and NaCl/KCl are presented.

Thermal stability of the interfaces between Co-, Ni-, and Fe-based ferromagnets in contact with selected nitrides MN (M=Al, B, Nb, Ta, Ti, and V)

Nitride tunnel barriers have potential applications in magnetic tunnel junctions (MTJs). Thermal stability of the interfaces between Co-, Ni-, and Fe-based ferromagnets and these nitride tunnel barriers is critical to device performance. With guidance from low-temperature ternary isothermal phase diagrams of the Co–M–N, Ni–M–N, and Fe–M–N systems (M=Al, B, Nb, Ta, Ti, and V), the interfaces in Co∕MN, Ni∕MN, and Fe∕MN structures were evaluated in terms of two criterions: the phases in contact must (1) be in equilibrium with each other (i.e., connected by a stable tie line) and (2) have negligible mutual solubility in the phase diagram at the temperatures of interest. Of the investigated interfaces, Co∕AlN, Co∕BN, Co∕NbN, Co∕TaN, Co∕TiN, Ni∕BN, Ni∕TaN, Fe∕BN, Fe∕NbN, Fe∕TaN, and Fe∕TiN were found to be thermodynamically stable. However, in light of some simplifications made in this analysis, the current evaluation of interfacial stability serves as a useful step in preselecting candidate nitride-based MTJ tunnel barrier materials for detailed study.