Field-temperature Phase Diagram Research Articles

Magnetotransport and electronic structure of the antiferromagnetic semimetal YbAs

A number of rare-earth monopnictides have topologically non-trivial band structures together with magnetism and strong electronic correlations. In order to examine whether the antiferromagnetic (AFM) semimetal YbAs ($T\rm_N$ = 0.5 K) exhibits such a scenario, we have grown high-quality single crystals using a flux method, and characterized the magnetic properties and electronic structure using specific heat, magnetotransport and angle-resolved photoemission spectroscopy (ARPES) measurements, together with density functional theory (DFT) calculations. Both ARPES and DFT calculations find no evidence for band inversions in YbAs, indicating a topologically trivial electronic structure. From low-temperature magnetotransport measurements, we map the field-temperature phase diagram, where we find the presence of a field stabilized phase distinct from the AFM phase at low temperatures. An extremely large magnetoresistance (XMR) for both YbAs and the nonmagnetic counterpart LuAs, is also observed, which can consistently be accounted for by the presence of electron-hole compensation. Moreover, an angle-dependent study of the Shubnikov-de Haas effect oscillations reveals very similar Fermi surfaces between YbAs and LuAs, with light effective masses down to at least 0.5 K, indicating that the Yb-$4f$ electrons are well localized, and do not contribute to the Fermi surface. However, the influence of the localized Yb-$4f$ electrons on the magnetotransport of YbAs can be discerned from the distinct temperature dependence of the XMR compared to that of LuAs, which we attribute to the influence of short-ranged spin correlations that appear well above $T\rm_N$.

Temperature-field phase diagrams of one-way quantum work deficit in two-qubit XXZ spin systems

The spin-1/2 XXZ chain in a uniform magnetic field at thermal equilibrium is considered. For this model, we give a complete classification of all qualitatively different phase diagrams for the one-way quantum work (information) deficit. The diagrams can contain regions (phases, fractions) with both stationary and variable (state-dependent) angles of optimal measurement. We found cases of phase diagrams in which the sizes of regions with the variable optimal measurement angle are large and perhaps such regions can be detected experimentally. We also established a relationship between the behavior of optimal measurement angles near the boundaries separated different regions and Landau's theory of phase transitions of the second and first kind.

Spontaneous relaxor to ferroelectric transition in lead-free relaxor piezoceramics and the role of point defects

The occupationally disordered structures and associated local polar fluctuations in lead-free relaxors determine their electrical properties that are also sensitive to external stimuli. These stimuli can lead to phase transitions, and the associated enhancement in the electro-mechanical responses necessitate a better understanding of these transitions. Here we report a non-canonical spontaneous phase transition from a relaxor to a ferroelectric phase in (Na1/2Bi1/2)TiO3-BaTiO3 with temperature. With the help of experiments (dielectric permittivity, diffraction, differential scanning calorimetry, polarization and Raman spectroscopy), a complete picture of the temperature evolution of relaxor behavior leading to this spontaneous phase transition has been reported. Furthermore, it has been shown that internal chemical pressure from oxygen vacancies can be utilized to tailor these phase transitions. Finally, an electric field-temperature phase diagram has been proposed with an emphasis on the influence of the defect chemistry. This work provides new insights into the origin of these spontaneous phase transitions.

Structural and magnetic properties of antiferromagneticCe2IrGa12

We report a study of the structural and magnetic properties of single crystals of Ce$_2$IrGa$_{12}$. Ce$_2$IrGa$_{12}$ crystallizes in a layered tetragonal structure, and undergoes an antiferromagnetic transition below 3.1 K. We characterize the temperature-field phase diagrams of Ce$_2$IrGa$_{12}$ for fields both within the $ab$-plane and along the $c$-axis, where the presence of a field-induced magnetic phase is found for in-plane fields. The ordering temperature is moderately enhanced upon the application of pressures up to 2.3~GPa, suggesting that Ce$_2$IrGa$_{12}$ corresponds to the well localized region of the Doniach phase diagram.

Robust Perpendicular Skyrmions and Their Surface Confinement.

Magnetic skyrmions are two-dimensional magnetization swirls that stack in the form of tubes in the third dimension and which are proposed as prospective information carriers for nonvolatile memory devices due to their unique topological properties. From resonant elastic X-ray scattering measurements on Cu2OSeO3 with an in-plane magnetic field, we find that a state of perpendicularly ordered skyrmions forms, in stark contrast to the well-studied bulk state. The surface state is stable over a wide temperature range, unlike the bulk state in out-of-plane fields which is confined to a narrow region of the temperature-field phase diagram. In contrast to ordinary skyrmions found in the bulk, the surface state skyrmions result from the presence of magnetic interactions unique to the surface which stabilize them against external perturbations. The surface guiding makes the robust state particular interesting for racetracklike devices, ultimately allowing for much higher storage densities due to the smaller lateral footprint of the perpendicular skyrmions.

Quadrupolar susceptibility and magnetic phase diagram of PrNi2Cd20 with non-Kramers doublet ground state

ABSTRACTIn this study, ultrasonic measurements were performed on a single crystal of cubic PrNi2Cd20, down to a temperature of 0.02 K, to investigate the crystalline electric field ground state and search for possible phase transitions at low temperatures. The elastic constant (C11−C12)/2, which is related to the Γ3-symmetry quadrupolar response, exhibits the Curie-type softening at temperatures below ∼30 K, which indicates that the present system has a Γ3 non-Kramers doublet ground state. A leveling-off of the elastic response appears below ∼0.1 K toward the lowest temperatures, which implies the presence of level splitting owing to a long-range order in a finite-volume fraction associated with Γ3-symmetry multipoles. A magnetic field–temperature phase diagram of the present compound is constructed up to 28 T for H || [110]. A clear acoustic de Haas–van Alphen signal and a possible magnetic-field-induced phase transition at H ∼26 T are also detected by high-magnetic-field measurements.

Low-field vortex melting in a single crystal of Ba0.6K0.4Fe2As2

Theoretically, the vortex melting phenomenon occurs at both low and high magnetic fields at a fixed temperature. While the high field melting has been extensively investigated in high Tc cuprates, the low field melting phenomena in the presence of disorder hasn't been well explored. Using bulk magnetization measurement and high-sensitivity differential magneto-optical imaging technique, we detect a low-field vortex melting phenomenon in a single crystal of Ba0.6K0.4Fe2As2. The low field melting is accompanied by a significant change in local magnetization ~ 3 G, which decreases with increasing applied field. The observed vortex melting phenomena is traced on a field temperature phase diagram and which lies very close to theoretically predicted Lindemann criteria based low field melting line. Our analysis shows a Lindemann number cL = 0.14 associated with the low field melting. Imaging of low-field vortex melting features shows the process nucleates via formation of extended finger-like projections which spreads across the sample with increasing field or temperature, before entering into an interaction-dominated vortex solid phase regime. Magnetization scaling analysis shows that the dimensionality of melting vortex state is close to one. Angular dependence of bulk magnetization hysteresis loop in our sample shows the presence of extended defects. From our studies, we propose the sample contains a peculiar geometry of extended defects arranged in a plane in the sample, with these planes extending through the sample thickness. In the weak intervortex interaction limit, we argue that reduced vortex dimensionality due to pinning by these peculiar extended defect planes strongly enhances thermal fluctuations. It is these extended defects planes, which we propose are promoting low dimensional vortex melting in the pnictide system.

Complex phase transitions and associated electrocaloric effects in different oriented PMN-30PT single crystals under multi-fields of electric field and temperature

The phase composition and their evolution in ferroelectrics are very complex near the morphotropic phase boundary (MPB), especially under multiple fields of electric field and temperature, which results in versatile behaviors of electrocaloric effect (ECE). This paper systematically studied the phase transitions and associated ECEs in the 〈001〉, 〈011〉 and 〈111〉 oriented 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-30PT) single crystals with the chemical composition in the MPB area. Based on the dielectric and polarization responses under multi-fields, the refined electric field-temperature phase diagrams are established, wherein the ECE properties are closely related to the electric-field-induced phase transitions and exhibit a complex evolution across positive and negative values. The negative ECE originates from the transition from a monoclinic phase to a tetragonal or orthogonal phase under the noncollinear electric field, so it only appears in the 〈001〉 or 〈011〉 oriented crystals. Conversely, the transition from the tetragonal phase to the rhombohedral or orthogonal phase in the 〈111〉 or 〈011〉 oriented crystals induces a positive ECE peak with ∆Tmax = 0.40 K (@10 kV/cm). Just above Tm, the positive ECE reaches ∆Tmax = 0.61 K, 0.63 K and 0.68 K in the 〈001〉, 〈011〉 and 〈111〉 oriented crystals, respectively. In addition, there is an abnormal net endothermic phenomenon around the monoclinic-tetragonal or monoclinic-orthogonal phase boundaries due to the electric-field-induced irreversible transition.

Electric field-temperature phase diagram of Bi1/2(Na0.8K0.2)1/2TiO3 relaxor ferroelectrics with Fe doping

Electrically and thermally induced transitions between ferroelectric and relaxor states are of great importance for Bi1/2Na1/2TiO3 (BNT)-based materials because of their close relevance to electromechanical properties. In this study, the electric field-temperature (E-T) phase diagrams of Fe doped Bi1/2(Na0.8K0.2)1/2TiO3 (BNKT) ceramics are constructed via an experiment and theory combined approach. A novel phenomenological model based on the Landau-Devonshire theory and the Preisach model is proposed to describe the electric field induced phase transitions. Using this model, an approximate free-energy landscape is obtained by fitting the experimental double polarization-electric field loop, and then the electric field representing ferroelectric and relaxor two-phase equilibrium is calculated from the free-energy landscape for the construction of an E-T phase diagram. The constructed E-T phase diagrams meet basic thermodynamics requirements, for example, the Clausius-Clapeyron relation, and, therefore, overcome some of the shortcomings of previously reported E-T phase diagrams for BNT-based ceramics. The relationship between the E-T phase diagram and electromechanical properties is also established. From the E-T phase diagrams, it is predicted that Fe doping could lower the threshold field of triggering giant strains of BNKT ceramics at a high temperature. This prediction is successfully verified by experimental measurement of the electric field induced strain. At the optimized temperature for strain property, the threshold field of triggering giant strain is estimated to be about 2.6 kV/mm for a 3.0% Fe doped sample, significantly lower than 3.5 kV/mm for the undoped sample. This shows that the E-T phase diagram can provide valuable guidance for the improvement of electromechanical properties of BNT-based ceramics.

Magnetocapacitance effect and magnetostriction by the field-induced spin-crossover in [MnIII(taa)

We report the magnetocapacitance effect and the magnetostriction in the spin-crossover (SCO) complex [MnIII(taa)] (H3taa = tris(1-(2-azolyl)-2-azabuten-4-yl)amine). It is shown that huge enhancements of dielectric constant and lattice volume are caused by the field-induced SCO transition from the low-spin to the high-spin states in [MnIII(taa)]. In addition, the temperature-field phase diagram of the field-induced SCO in thermal equilibrium state is clarified up to 28 T. Our analysis suggests that a large magnetocaloric effect can also be caused by the field-induced SCO. These results indicate the possibility of multiple control of various material properties by the field-induced SCO.

Anisotropic temperature-field phase diagram of single crystalline β−Li2IrO3 : Magnetization, specific heat, and Li7 NMR study

Detailed magnetization, specific heat, and $^7$Li nuclear magnetic resonance (NMR) measurements on single crystals of the hyperhoneycomb Kitaev magnet $\beta$-Li$_2$IrO$_3$ are reported. At high temperatures, {\cred anisotropy of the magnetization is reflected by the different Curie-Weiss temperatures for different field directions}, in agreement with the combination of a ferromagnetic Kitaev interaction ($K$) and a negative off-diagonal anisotropy ($\Gamma$) as two leading terms in the spin Hamiltonian. At low temperatures, magnetic fields applied along $a$ or $c$ have only a weak effect on the system and reduce the N\'eel temperature from 38 K at 0 T to about 35.5 K at 14 T, with no field-induced transitions observed up to 58 T on a powder sample. In contrast, the field applied along $b$ causes a drastic reduction in the $T_N$ that vanishes around $H_c=2.8$ T giving way to a crossover toward a quantum paramagnetic state. $^7$Li NMR measurements in this field-induced state reveal a gradual line broadening and a continuous evolution of the line shift with temperature, suggesting the development of local magnetic fields. The spin-lattice relaxation rate shows a peak around the crossover temperature 40 K and follows power-law behavior below this temperature.

Josephson vortex dynamics and Fulde-Ferrell-Larkin-Ovchinnikov superconductivity in the layered organic superconductor β″−(BEDT−TTF)2SF5CH2CF2SO3

Interlayer resistance is reported in a layered organic superconductor ${\ensuremath{\beta}}^{\ensuremath{''}}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{SF}}_{5}{\mathrm{CH}}_{2}{\mathrm{CF}}_{2}{\mathrm{SO}}_{3}$ with ${T}_{\mathrm{c}}\ensuremath{\approx}5$ K. At 0.7 K in parallel magnetic fields, the interlayer resistance has nonzero values in a wide field region from 2 T to the upper critical field $(\ensuremath{\sim}13\phantom{\rule{0.16em}{0ex}}\mathrm{T})$, which can be ascribed to the Josephson vortex (JV) dynamics. Significant hysteresis between 2 T and 8 T and a steplike structure between 4 T and 8 T are found, both of which become less evident with increasing temperature. In the second field-derivative curves, successive small dips are observed between 10 T and 12 T, which are ascribed to the commensurability (CM) effect between the JV lattice and the wavelength ${\ensuremath{\lambda}}_{\mathrm{FFLO}}$ of the order parameter oscillation in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase. The CM effect is observed only in nearly parallel fields, showing that the FFLO phase is stabilized only in a limited field angle region. The temperature-field phase diagram and ${\ensuremath{\lambda}}_{\mathrm{FFLO}}$ values are obtained.

Tailored Tunnel Magnetoresistance Response in Three Ultrathin Chromium Trihalides.

Materials that demonstrate large magnetoresistance have attracted significant interest for many decades. Extremely large tunnel magnetoresistance (TMR) has been reported by several groups across ultrathin CrI3 by exploiting the weak antiferromagnetic coupling between adjacent layers. Here, we report a comparative study of TMR in all three chromium trihalides (CrX3, X = Cl, Br, or I) in the two-dimensional limit. As the materials exhibit different transition temperatures and interlayer magnetic ordering in the ground state, tunneling measurements allow for an easy determination of the field-temperature phase diagram for the three systems. By changing sample thickness and biasing conditions, we then demonstrate how to maximize and further tailor the TMR response at different temperatures for each material. In particular, near the magnetic transition temperature, TMR is nonsaturating up to the highest fields measured for all three compounds owing to the large, field-induced exchange coupling.

Statistics of geometric clusters in the Ising model on a Bethe lattice

The statistical mechanics method is developed for determination of generating function of like-sign spin clusters’ size distribution in Ising model as modification of Ising–Potts model by K. K. Murata (1979). It is applied to the ferromagnetic Ising model on Bethe lattice. The analytical results for the field-temperature percolation phase diagram of + spin clusters and their size distribution are obtained. The last appears to be proportional to that of the classical non-correlated bond percolation with the bond probability, which depends on temperature and Ising model parameters.

Effects of domain structures on vortex state of two-dimensional superconducting Mo2C crystals

Understanding of superconducting phase and vortex states in two-dimensional (2D) superconductors is fundamentally and technologically important. Here we report on low-temperature transport measurements on high-quality ultrathin superconducting Mo2C crystals containing well-defined line-shaped domain boundaries. We observe pronounced two-stage resistive transition accompanied by an appearance of kink structure well below the superconducting transition temperature, which is related to the development of one dimensional (1D) superconducting phase within the domain boundaries. The resistive transition under the magnetic fields is found to be well described by the thermally assisted collective flux creep model. The magnetic field dependence of activation energy barrier exhibits an enhancement at low fields, indicating a dimensional crossover from 2D to 1D vortex states in Mo2C crystals. We obtain the entire magnetic field-temperature phase diagram of ultrathin Mo2C crystals, revealing the existence of additional 1D vortex state induced by the domain boundaries. Our results demonstrate that the domain structures play an important role in determining the superconducting phase and vortex dynamics of 2D superconductors.

Anisotropic phase diagram of ferroquadrupolar ordering in the trigonal chiral compoundDyNi3Ga9

The rare-earth chiral compound ${\mathrm{DyNi}}_{3}{\mathrm{Ga}}_{9}$ with trigonal structure shows successive phase transitions at 10, 9, 5.5, and 2.5 K. We previously reported that the transverse elastic modulus ${C}_{66}$ exhibits significant softening toward ${T}_{\mathrm{Q}}=10$ K and the phase transition at ${T}_{\mathrm{Q}}$ is ferroquadrupolar ordering due to the quasidegenerate quartet under a crystal electric field (CEF). To investigate the order parameter and magnetic field-temperature phase diagrams of the ferroquadrupolar ordering, we performed ultrasonic measurements under magnetic fields. ${T}_{\mathrm{Q}}$ decreases drastically at low fields in the magnetic field in the $c$ plane. In contrast, ${T}_{\mathrm{Q}}$ remains even at high fields in the field along the $c$ axis. We found anisotropic and complex phase diagrams, and this anisotropy of the diagrams originates from the field dependences of the CEF energies and a quadrupole interaction. From our calculated results, we propose that the order parameter of the ferroquadrupolar ordering will be the electric quadrupole ${O}_{xy}$. There are first-order successive phase transitions in the field sweep in the $c$ plane, and a magnetic-field-induced phase appears in the phase diagram for the $a$ axis.

Stabilization of the vortex-liquid state by strong pairing interaction

We theoretically investigate qualitative features of the field-temperature ($H$-$T$) phase diagram of superconductors with strong attractive interaction lying in the BCS-BEC crossover regime. Starting with a simple attractive Hubbard model, we estimate three kinds of characteristic fields, i.e., the pair-formation field $H^*$, the vortex-liquid-formation field $H_\mathrm{c2}$, and the vortex-lattice-formation field $H_\mathrm{melt}$. The region between $H_\mathrm{c2}$ and $H_\mathrm{melt}$, as well as that between $H_\mathrm{c2}$ and $H^*$, is found to be enlarged as the interaction is stronger. In other words, a strong attractive interaction can stabilize both the vortex-liquid and preformed-pair regions. We also point out the expected particle-density dependence of the $H$-$T$ phase diagram.

Tomonaga\u2013Luttinger liquid behavior and spinon confinement in YbAlO3

Low dimensional quantum magnets are interesting because of the emerging collective behavior arising from strong quantum fluctuations. The one-dimensional (1D) S = 1/2 Heisenberg antiferromagnet is a paradigmatic example, whose low-energy excitations, known as spinons, carry fractional spin S = 1/2. These fractional modes can be reconfined by the application of a staggered magnetic field. Even though considerable progress has been made in the theoretical understanding of such magnets, experimental realizations of this low-dimensional physics are relatively rare. This is particularly true for rare-earth-based magnets because of the large effective spin anisotropy induced by the combination of strong spin–orbit coupling and crystal field splitting. Here, we demonstrate that the rare-earth perovskite YbAlO3 provides a realization of a quantum spin S = 1/2 chain material exhibiting both quantum critical Tomonaga–Luttinger liquid behavior and spinon confinement–deconfinement transitions in different regions of magnetic field–temperature phase diagram.

The half magnetization plateau in Ni3V2O8 studied by electron spin resonance

Ni3V2O8, regarded as an S = 1 kagome staircase lattice antiferromagnet, possesses a novel magnetic field–temperature phase diagram. Specifically, a half plateau region is observed in the high field magnetization curve for magnetic fields in the range of 11–19 T. This experimental observation is theoretically unexpected for a standard kagome lattice antiferromagnet, and consequently, the underlying magnetic structure is still unclear. Multi-frequency electron spin resonance results in this study strongly support a collinear magnetic arrangement at the half plateau region. The resonant modes can be well fit by only considering the antiferromagnetic interactions on a four-spin sublattice of the spine sites.

Mean-field theory of isotropic-uniaxial nematic-biaxial nematic phase transitions in an external field

We present a simple mean-field theory to study the effects of an external magnetic field on biaxial nematic liquid crystals. Isotropic-uniaxial nematic-biaxial nematic phase sequence in an external magnetic field is described. We determine the temperature-field phase diagrams. The model predicts two new phase diagrams. It is shown that in the presence of the magnetic field, the first order uniaxial nematic to biaxial nematic and isotropic to uniaxial nematic phase transitions become second order transition at a tricritical point.