Magnetic Field Phase Diagram Research Articles

Kondo-Anderson transitions

Dilute magnetic impurities in a disordered Fermi liquid are considered close to the Anderson metal-insulator transition (AMIT). Critical power-law correlations between electron wave functions at different energies in the vicinity of the AMIT result in the formation of pseudogaps of the local density of states. Magnetic impurities can remain unscreened at such sites. We determine the density of the resulting free magnetic moments in the zero-temperature limit. While it is finite on the insulating side of the AMIT, it vanishes at the AMIT, and decays with a power law as function of the distance to the AMIT. Since the fluctuating spins of these free magnetic moments break the time-reversal symmetry of the conduction electrons, we find a shift of the AMIT, and the appearance of a semimetal phase. The distribution function of the Kondo temperature ${T}_{K}$ is derived at the AMIT, in the metallic phase, and in the insulator phase. This allows us to find the quantum phase diagram in an external magnetic field $B$ and at finite temperature $T$. We calculate the resulting magnetic susceptibility, the specific heat, and the spin relaxation rate as a function of temperature. We find a phase diagram with finite-temperature transitions among insulator, critical semimetal, and metal phases. These new types of phase transitions are caused by the interplay between Kondo screening and Anderson localization, with the latter being shifted by the appearance of the temperature-dependent spin-flip scattering rate. Accordingly, we name them Kondo-Anderson transitions.

Pomeranchuk Instability of Spin-Antisymmetric Channel in Sr3Ru2O7

We study a possible Pomeranchuk instability in the “nematic phase” in Sr 3 Ru 2 O 7 by calculating the Landau interaction function on the basis of an extended d – p model with the on-site Coulomb interaction U p p between 2 p electrons at the O site. We find that the spin-antisymmetric p -wave type instability becomes dominant within the second-order perturbation theory with respect to the on-site Coulomb interactions U p p and U d d which is the conventional interaction between 4 d electrons at the Ru site. Furthermore, within the mean field theory, we obtain a temperature vs magnetic field phase diagram, which is consistent with that observed in Sr 3 Ru 2 O 7 .

Phase diagram in an external magnetic field beyond a mean-field approximation

The phase structure of the Polyakov loop-extended chiral quark-meson model is explored in a nonperturbative approach, beyond a mean-field approximation, in the presence of a magnetic field. We show that by including meson fluctuations one cannot resolve the qualitative discrepancy on the dependence of the crossover transition temperature in a nonzero magnetic field between effective model predictions and recent lattice results [1]. We compute the curvature of the crossover line in the $T\ensuremath{-}{\ensuremath{\mu}}_{B}$ plane at a nonzero magnetic field and show that the curvature increases with increasing magnetic field. On the basis of QCD inequalities, we also argue that, at least in the large ${N}_{c}$ limit, a chiral critical end point and, consequently, a change from crossover to a first-order chiral phase transition are excluded at zero baryon chemical potential and nonzero magnetic field.

Quantum phase transitions in alternating spin-(, ) Heisenberg chains

The ground state spin-wave excitations and thermodynamic properties of two types of ferrimagnetic chains are investigated: the alternating spin-1/2 spin-5/2 chain and a similar chain with a spin-1/2 pendant attached to the spin-5/2 site. Results for magnetic susceptibility, magnetization and specific heat are obtained through the finite-temperature Lanczos method with the aim of describing the available experimental data, as well as comparison with theoretical results from the semiclassical approximation and the low-temperature susceptibility expansion derived from Takahashi’s modified spin-wave theory. In particular, we study in detail the temperature versus magnetic field phase diagram of the spin-1/2 spin-5/2 chain, in which several low-temperature quantum phases are identified: the Luttinger liquid phase, the ferrimagnetic plateau and the fully polarized phase, and the respective quantum critical points and crossover lines.

Supersolid phase and magnetization plateaus observed in the anisotropic spin-32Heisenberg model on bipartite lattices

We study the spin-3/2 Heisenberg model including easy-plane and exchange anisotropies in one and two dimensions. In the Ising limit, when the off-diagonal exchange interaction J is zero, the phase diagram in magnetic field is characterized by magnetization plateaus that are either translationally invariant or have a two-sublattice order, with phase boundaries that are macroscopically degenerate. Using a site factorized variational wave function and perturbational expansion around the Ising limit, we find that superfluid and supersolid phases emerge between the plateaus for small finite values of J. The variational approach is complemented by a Density Matrix Renormalization Group study of a one-dimensional chain and exact diagonalization calculations on small clusters of a square lattice. The studied model may serve as a minimal model for the layered Ba2CoGe2O7 material compound, and we believe that the vicinity of the uniform 1/3 plateau in the model parameter space can be observed as an anomaly in the measured magnetization curve.

A first-order magnetic phase transition near 15 K with novel magnetic-field-induced effects in Er5Si3

We present magnetic characterization of a binary rare-earth intermetallic compound Er5Si3, crystallizing in Mn5Si3-type hexagonal structure, through magnetization, heat capacity, electrical resistivity and magnetoresistance measurements. Our investigations confirm that the compound exhibits two magnetic transitions with decreasing temperature, the first one at 35 K and the second one at 15 K. The present results reveal that the second magnetic transition is a disorder-broadened first-order transition, as shown by thermal hysteresis in the measured data. Another important finding is that, below 15 K, there is a magnetic-field-induced transition with a hysteretic effect with the electrical resistance getting unusually enhanced at this transition and the magnetoresistance is found to exhibit intriguing magnetic-field dependence, indicating novel magnetic phase coexistence phenomenon. It thus appears that this compound is characterized by interesting magnetic anomalies in the temperature–magnetic-field phase diagram.

Symmetry and magnetic field driven transitions in the 2D triangular lattice compound RbFe(MoO4)2

The temperature versus magnetic field phase diagram of the 2D triangular lattice and multiferroic compound RbFe(MoO4)2 is analysed from the point of view of symmetry. The paramagnetic space group and its irreducible representations are used in order to obtain the magnetic symmetry of the possible modulated phases and characterize the restrictions imposed by this symmetry on the corresponding magnetic structures. Superspace symmetry is considered in the case of incommensurate phases. It is shown that the experimentally observed phases correspond to different isotropy subgroups originating in the same irreducible representation of the paramagnetic symmetry group. The relevant couplings between the primary transverse spin modulation and the electric polarization, the in-plane magnetization and the secondary longitudinal magnetic modulation are discussed. The mechanisms for the destabilization of the improper ferroelectric chiral phase and the origin of the different orientation of the spins with respect to the external field in the two collinear phases are analysed from a symmetry based perspective.

On the Hidden Order in URu2Si2 – Antiferro Hexadecapole Order and Its Consequences

An antiferro ordering of an electric hexadecapole moment is discussed as a promising candidate for the long standing mystery of the hidden order phase in URu$_{2}$Si$_{2}$. Based on localized $f$-electron picture, we discuss the rationale of the selected multipole and the consequences of the antiferro hexadecapole order of $xy(x^{2}-y^{2})$ symmetry. The mean-field solutions and the collective excitations from them explain reasonably significant experimental observations: the strong anisotropy in the magnetic susceptibility, characteristic behavior of pressure versus magnetic field or temperature phase diagrams, disappearance of inelastic neutron-scattering intensity out of the hidden order phase, and insensitiveness of the NQR frequency at Ru-sites upon ordering. A consistency with the strong anisotropy in the magnetic responses excludes all the multipoles in two-dimensional representations, such as $(O_{yz},O_{zx})$. The expected azimuthal angle dependences of the resonant X-ray scattering amplitude are given. The $(x^{2}-y^{2})$-type antiferro quadrupole should be induced by an in-plane magnetic field along $[110]$, which is reflected in the thermal expansion and the elastic constant of the transverse $(c_{11}-c_{12})/2$ mode. The $(x^{2}-y^{2})$-type [$(xy)$-type] antiferro quadrupole is also induced by applying the uniaxial stress along $[110]$ direction [$[100]$ direction]. A detection of these induced antiferro quadrupoles under the in-plane magnetic field or the uniaxial stress using the resonant X-ray scattering provides a direct redundant test for the proposed order parameter.

High-field magnetic phase transitions and spin excitations in magnetoelectric LiNiPO4

The magnetically ordered phases and spin dynamics of magnetoelectric LiNiPO${}_{4}$ have been studied in fields up to 17.3 T along the $c$ axis. Using neutron diffraction, we show that a previously proposed linearly polarized incommensurate (IC) structure exists only for temperatures just below the N\'eel temperature ${T}_{\text{N}}$. The ordered IC structure at the lowest temperatures is shown instead to be an elliptically polarized canted spiral for fields larger than 12 T. The transition between the two IC phases is of second order and takes place about 2 K below ${T}_{\text{N}}$. For ${\ensuremath{\mu}}_{0}H>16$ T and temperatures below 10 K, the spiral structure is found to lock in to a period of five crystallographic unit cells along the $b$ axis. Based on the neutron-diffraction data, combined with detailed magnetization measurements along all three crystallographic axes, we establish the magnetic phase diagrams for fields up to 17.3 T along $c$ and for fields up to 16 T along $a$ and $b$. The spin excitations in the high-field IC spiral phase have been studied in detail by inelastic neutron scattering. A mean-field analysis shows that the spin Hamiltonian derived previously from the low-temperature spin waves at zero field predicts the transition between the linear and elliptical polarization of the IC structure, and that a generalization of the spin-wave theory, assuming the random-phase approximation, accounts for the inelastic scattering data obtained in the commensurable uniform phase at fields below 12 T as well as those obtained in the high-field IC spiral phase.

Ab initiomodeling of Bose-Einstein condensation in Pb2V3O9

We apply density functional theory band structure calculations and quantum Monte Carlo simulations to investigate the Bose-Einstein condensation in the spin-1/2 quantum magnet Pb2V3O9. In contrast to previous conjectures on the one-dimensional nature of this compound, we present a quasi-two-dimensional model of spin dimers with ferromagnetic and antiferromagnetic interdimer couplings. Our model is well justified microscopically and provides a consistent description of the experimental data on the magnetic susceptibility, high-field magnetization, and field vs. temperature phase diagram. The Bose-Einstein condensation in the quasi-two-dimensional spin system of Pb2V3O9 is largely governed by intralayer interactions, whereas weak interlayer couplings have a moderate effect on the ordering temperature. The proposed computational approach is an efficient tool to analyze and predict high-field properties of quantum magnets.

Magnetic susceptibility and specific heat of a spinel MnV2O4single crystal

We investigated the magnetic and the thermodynamic properties of a spinel MnV${}_{2}$O${}_{4}$ single crystal. The magnetization and susceptibility show the signature of ferromagnetic ordering at $~$61 K. A sharp peak in the magnetization and the specific heat indicate the structural phase transition at 58 K, consistent with the scattering experiments. The specific-heat data exhibits the signature of double transitions. In addition to the higher-temperature transition at ${T}_{S}$, the second transition occurs at a lower temperature ${T}^{*}$, which divides the phase below ${T}_{S}$ into a low-magnetic-field and a high-magnetic-field phase. Based on the magnetic susceptibility and the specific heat, a wide-range temperature and magnetic-field phase diagram was constructed. The low-magnetic-field phase shows the characteristics of a spin-glass-like state. A comparison with the magnetic and thermodynamic characteristics of similar materials suggests that the low temperature and low-magnetic-field, spin-glass-like phase in MnV${}_{2}$O${}_{4}$ may originate from the competition between the interchain and the intrachain exchange interaction owing to the fluctuating V-ion orbitals.

Magnetic Field and Pressure Phase Diagrams of Uranium Heavy-Fermion Compound U2Zn17

We have performed magnetization measurements at high magnetic fields of up to 53 T on single crystals of a uranium heavy-fermion compound U$_2$Zn$_{17}$ grown by the Bridgman method. In the antiferromagnetic state below the N\'{e}el temperature $T_{\rm N}$ = 9.7 K, a metamagnetic transition is found at $H_c$ $\simeq$ 32 T for the field along the [11$\bar{2}$0] direction ($a$-axis). The magnetic phase diagram for the field along the [11$\bar{2}$0] direction is given. The magnetization curve shows a nonlinear increase at $H_m$ $\simeq$ 35 T in the paramagnetic state above $T_{\rm N}$ up to a characteristic temperature $T_{{\chi}{\rm max}}$ where the magnetic susceptibility or electrical resistivity shows a maximum value. This metamagnetic behavior of the magnetization at $H_m$ is discussed in comparison with the metamagnetic magnetism of the heavy-fermion superconductors UPt$_3$, URu$_2$Si$_2$, and UPd$_2$Al$_3$. We have also carried out high-pressure resistivity measurement on U$_2$Zn$_{17}$ using a diamond anvil cell up to 8.7 GPa. Noble gas argon was used as a pressure-transmitting medium to ensure a good hydrostatic environment. The N\'{e}el temperature $T_{\rm N}$ is almost pressure-independent up to 4.7 GPa and starts to increase in the higher-pressure region. The pressure dependences of the coefficient of the $T^2$ term in the electrical resistivity $A$, the antiferromagnetic gap $\Delta$, and the characteristic temperature $T_{{\rho}{\rm max}}$ are discussed. It is found that the effect of pressure on the electronic states in U$_2$Zn$_{17}$ is weak compared with those in the other heavy fermion compounds.

Phase diagram of hot QCD in an external magnetic field: Possible splitting of deconfinement and chiral transitions

The structure of the phase diagram for strong interactions becomes richer in the presence of a magnetic background, which enters as a new control parameter for the thermodynamics. Motivated by the relevance of this physical setting for current and future high-energy heavy ion collision experiments and for the cosmological QCD transitions, we use the linear sigma model coupled to quarks and to Polyakov loops as an effective theory to investigate how the chiral and the deconfining transitions are affected, and present a general picture for the temperature--magnetic field phase diagram. We compute and discuss each contribution to the effective potential for the approximate order parameters, and uncover new phenomena such as the paramagnetically-induced breaking of global Z_3 symmetry, and possible splitting of deconfinement and chiral transitions in a strong magnetic field.

Vortex glass melting in Mg-deficientMgB2

In this paper we verify that clustering of Mg-deficient regions in superconducting ${\text{Mg}}_{1\ensuremath{-}x}{\text{B}}_{2}$ samples, introduces random disorder into the system, sufficient to stabilize a vortex glass phase for small enough values of $x$. Employing harmonic susceptibility, among other experimental techniques, we have conducted a systematic investigation of the glass-liquid frontier in the magnetic field versus temperature phase diagram of vortex matter for three samples, corresponding to $x=0.025$, 0.075, and 0.100. Interestingly, the studied melting line presents a kink in the low-field/high-temperature region of the diagram, indicating that thermal disorder might become important at larger temperatures, anticipating the melting process of the vortex glass phase.

Phase transition of a triangular lattice Ising antiferromagnetFeI2

The phase transition of ${\text{FeI}}_{2}$ is revisited. This material has the hexagonal crystal structure and shows an antiferromagnetic order at ${T}_{\text{N}}=9.3\text{ }\text{K}$ in zero field. When an external magnetic field is applied along the $c$ axis below ${T}_{\text{N}}$, successive metamagnetic transitions occur. The magnetism of ${\text{FeI}}_{2}$ has been interpreted based on the further neighbor interaction model on the triangular net. However, the exchange interaction constants derived from the analysis do not seem realistic. We have measured the magnetic susceptibility, magnetization, and specific heat on single crystals of ${\text{FeI}}_{2}$. From the specific-heat measurement, the temperature versus magnetic field phase diagram is constructed, in which five distinct magnetic phases, namely, the antiferromagnetic and four ferrimagnetic ones, exist. The magnetization measurement reveals that magnetization steps appear at $\frac{1}{3}$, $\frac{12}{25}$, $\frac{13}{25}$ and between $\frac{16}{25}$ and $\frac{17}{25}$ of the saturation magnetization, ${M}_{\text{s}}=3.5\text{ }{\ensuremath{\mu}}_{\text{B}}/\text{Fe}$. Based on an experimental evidence, we argue that a lattice distortion occurs below ${T}_{\text{N}}$ and the exchange interaction between spins on a triangle becomes anisotropic. We discuss the phase transition using the anisotropic triangular lattice model with ${J}_{1}$, ${J}_{2}$, and ${J}_{3}$. From the analysis of the metamagnetic transition fields, we obtain, $2.5l{J}_{1}/{k}_{\text{B}}l3.0\text{ }\text{K}$, $\ensuremath{-}16.2l{J}_{2}/{k}_{\text{B}}l\ensuremath{-}15.2\text{ }\text{K}$, and $\ensuremath{-}16.4l{J}_{3}/{k}_{\text{B}}l\ensuremath{-}14.4\text{ }\text{K}$.

Competition and/or coexistence of antiferromagnetism and superconductivity in CeRhIn5 and CeCoIn5

AbstractThe Ce compounds CeCoIn5 and CeRhIn5 are ideal model systems to study the competition of antiferromagnetism (AF) and superconductivity (SC). Here we discuss the pressure–temperature and magnetic field phase diagrams of both compounds. In CeRhIn5 the interesting observation is that in zero magnetic field a coexistence AF + SC phase exist inside the AF phase below the critical pressure $p_{\rm c}^{\rm *} \approx 2$ GPa. Above $p_{\rm c}^{\rm *} $ AF is suppressed in zero field but can be re‐induced by applying a magnetic field. The collapse of AF under pressure coincides with the abrupt change of the Fermi surface. In CeCoIn5 a new phase appears at low temperatures and high magnetic field (LTHF) which vanishes at the upper critical field Hc2. In both compounds the paramagnetic pair breaking effect dominates at low temperature. We discuss the evolution of the upper critical field under high pressure of both compounds and propose a simple picture of the glue of reentrant magnetism to the upper critical field in order to explain the interplay of antiferromagnetic order and SC.

Effect of magnetic field on the two valence transitions in EuPtP

The hexagonal layered compound EuPtP exhibits two valence transitions at T1 = 240 K and T2 = 198 K. We present field-induced shift of temperature of the two valence transitions and magnetic field versus temperature phase diagram in EuPtP. Magnetic field dependence of the valence transition temperatures, T1 and T2, is similar to that reported in Eu(Pd1−xPtx)2Si2 and YbIn1−xAgxCu4.

Neutron diffraction study of the high magnetic field phase diagram of La-doped PrB6

The high magnetic field phase diagram of lanthanum-doped PrB6 was investigated by single-crystal neutron diffraction. In this compound, a peculiar incommensurate phase denoted IC2 was observed in magnetic fields applied along directions close to [111]. Unlike the double-k IC1 phase [kIC1 = (1/4 − δ, ±1/4,1/2)] formed in zero field below TN = 7 K, IC2 is characterized by a propagation vector with two incommensurate components, kIC2 = (1/4−δ′, 1/4 − δ′, 1/2) (δ′ ≊ 0.03). The field and temperature evolutions of domain populations further indicate that, contrary to the low-field phases C and IC1, the magnetic structure in phase IC2 is single-k.

Separation of Energy Scales in Undoped YbRh2Si2Under Hydrostatic Pressure

The temperature ($T$)-magnetic field ($H$) phase diagram of YbRh$_2$Si$_2$ in the vicinity of its quantum critical point is investigated by low-$T$ magnetization measurements. Our analysis reveals that the energy scale $T^\star(H)$, previously related to the Kondo breakdown and terminating at 0.06 T for $T\to 0$, remains unchanged under pressure, whereas the antiferromagnetic critical field increases from 0.06 T ($p=0$) to 0.29 T ($p=1.28$ GPa), resulting in a crossing of $T_N(H)$ and $T^\star(H)$. Our results are very similar to those on Yb(Rh$_{1-x}$Co$_x$)$_2$Si$_2$, proving that the Co-induced disorder can not be the reason for the detachment of both scales under chemical pressure.

Interplay of anisotropy and frustration: Triple transitions in a triangular-lattice antiferromagnet

The classical Heisenberg antiferromagnet on a triangular lattice with the single-ion anisotropy of the easy-axis type is studied theoretically. The phase diagram in an external magnetic field is constructed from the mean-field analysis. Three successive Berezinskii-Kosterlitz-Thouless transitions are found by Monte Carlo simulations in zero field. Two upper transitions are related to the breaking of the discrete ${\mathbb{Z}}_{6}$ symmetry while the lowest transition is associated with a quasilong-range ordering of transverse components. The intermediate collinear phase between the first and second transition is the critical phase predicted by Jos\'e et al. [Phys. Rev. B 16, 1217 (1977)].