Quantum effects on the competition between antiferromagnetism and superconductivity in heavy-fermion systems

We study a Ginzburg-Landau theory of two coupled fields describing superconductivity and antiferromagnetism in a metal. A coupling between the two-components superconductor and the antiferromagnetic (AF) field is included in the classical action. The classical results are improved calculating the quantum corrections to one-loop order with the method of the effective potential near the AF phase, but in the paramagnetic side. We discuss the influence of these corrections, including the possibility of fluctuation induced first order transitions. A scaling approach is used to obtain the critical and shift exponents at a quantum bicritical point.     

Study on the competition between density waves, singlet, and triplet pairing superconductivity in organic conductors (TMTSF)2X

We study the effect of dimerization of TMTSF molecules and the effect of magnetic field (Zeeman splitting) on the phase competition in quasi one-dimensional organic superconductors (TMTSF)₂X by applying the random phase approximation method. As for the dimerization effect, we conclude that due to the decrease of the dimerization, which corresponds to applying the pressure and cooling, spin and charge density wave states are suppressed and give way to a superconducting state. As for the magnetic field effect, we find generally that spin-triplet pairing mediated by a coexistence of 2k_F spin and 2k_F charge fluctuations can be strongly enhanced by applying magnetic field rather than triplet pairing due to a ferromagnetic spin fluctuations. Applying the above idea to (TMTSF)₂X compounds, a magnetic field induced singlet-triplet transition is consistent with above mechanism in (TMTSF)₂ClO₄.     

Anisotropic thermal expansion and evaluation of uniaxial pressure dependence of superconducting and magnetic transitions in ErNi2B2C

We present anisotropic thermal expansion measurements on single crystalline ErNi₂B₂C. All three, superconducting, antiferromagnetic and weak ferromagnetic phase transitions are unambiguously distinguished in the data. Anisotropic uniaxial pressure dependencies of the transitions are estimated based on the Ehrenfest relation, leading to a conclusion, in particular, that weak ferromagnetic states may be suppressed by small, order of few kbar, hydrostatic pressure. Additionally, magnetostriction is shown to be a useful probe for rich and complex H-T diagram in this material.     

Zero-temperature Kosterlitz-Thouless transition in a two-dimensional quantum system

We construct a local interacting quantum dimer model on the square lattice, whose zero-temperature phase diagram is characterized by a line of critical points separating two ordered phases of the valence bond crystal type. On one side, the line of critical points terminates in a quantum transition inherited from a Kosterlitz-Thouless transition in an associated classical model. We also discuss the effect of a longer-range dimer interaction that can be used to suppress the line of critical points by gradually shrinking it to a single point. Finally, we propose a way to generalize the quantum Hamiltonian to a dilute dimer model in presence of monomers and we qualitatively discuss the phase diagram.     

Magnetic and magnetocaloric properties of the intermetallic compound TbNiAl

Magnetization measurements have been carried out on the intermetallic compound TbNiAl in applied fields up to 120 kOe. Temperature dependence of magnetization under zero-field-cooled and field-cooled conditions shows thermomagnetic irreversibility, which is attributed to magnetic frustration. With the increase of field, the irreversibility decreases and vanishes completely at high fields. Magnetocaloric effect has been calculated in terms of isothermal magnetic entropy change using magnetization isotherms obtained at various temperatures. The maximum entropy change is 13.8 J kg⁻¹ K⁻¹ near the ordering temperature for a field change of 50 kOe. The refrigerant capacity is found to be 494 J kg⁻¹ for the same field change and for a temperature difference of 52 K between the cold and the hot sinks.     

Electrical and magnetic properties of TmCoIn5 and YbCoIn5 single crystals

Electrical and magnetic properties of TmCoIn₅ and YbCoIn₅ single crystals were investigated by means of electrical resistivity and magnetization measurements in the temperature range from 300 to 0.5 K under the magnetic field up to 5 T. TmCoIn₅ is an antiferromagnetic metal with a Néel temperature T_N=2.6 K. YbCoIn₅ shows non-magnetic behavior, reflecting of divalent Yb ion.     

Magnetic phase diagrams of Ho1−xDyxNi2B2C

We performed the magnetization measurement on Ho_1−xDy_xNi₂B₂C single crystals (x=0.1, 0.2, 0.3, 0.4, and 0.6) with magnetic field applied perpendicular and parallel to the c-axis. But only for the magnetic field perpendicular to the c-axis, the increase of Dy³⁺ concentration affects the magnetically ordered states of HoNi₂B₂C compound and makes the phase diagram more complicated. The antiferromagnetic ordering state attributed to Dy³⁺ sublattice starts to appear from a case of x=0.2 and finally the magnetic phase diagram becomes analogous to that of DyNi₂B₂C as x is increased which is consistent with the neutron scattering result.     

Quantum phase diagram of granular superconducting quantum dots array

We study the quantum phase diagram of granular superconducting quantum dots (GSQD) array. We implement the physics of granularity by considering site dependent Josephson couplings, on-site charging energies and the intersite interactions. We predict dimer density wave and staggered phases at the insulating state for higher order commensurability. Several parts of the quantum phase diagram of GSQD are in contrast with the clean superconducting quantum dots array. We also obtain the superconducting phase of GSQD. We develop the theory for weak tunneling conductance and the Coulomb energy is smaller than the superconducting gap.     

Non-Fermi liquid behavior in the magnetotransport of quasi two-dimensional heavy Fermion compounds CeMIn5

We observe several non-Fermi liquid behaviors in the normal-state transport properties of CeMIn₅ (M: Rh and Co) under pressure at low temperatures: (1) The dc-resistivity shows T-linear dependence, ρ_xx∝T. (2) The magnitude of Hall coefficient |R_H| increases rapidly with decreasing temperature, and reaches a value much larger than |1/ne| at low temperatures. (3) The magnetoresistance displays T- and H-dependence that strongly violate Kohler's rule, and is well scaled by the tangent of the Hall angle, . These non-Fermi liquid properties in the electron transport are remarkably pronounced when the AF fluctuations are enhanced in the vicinity of the quantum critical point. Since all of these salient features have been also reported for high-T_c cuprates, we infer that the non-Fermi liquid transport properties capture universal features of strongly correlated electron systems in the presence of strong antiferromagnetic fluctuations.     

The influence of hydrogen annealing on magnetism of Co-doped TiO2 films prepared by sol–gel method

Co-doped TiO₂ (Co_xTi_1−xO₂, 0.05?x?0.2) films have been prepared on Si (0 0 1) substrates by sol–gel method. When heat treated in air, Co_xTi_1−xO₂ films are non-ferromagnetic at room temperature. However, after further annealed in a flowing hydrogen atmosphere, Co_xTi_1−xO₂ films show room-temperature ferromagnetism (RTFM). Measurements of magnetization (M) vs. temperature (T), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) fail to detect Co clusters in the hydrogenated Co_0.1Ti_0.9O₂ films, suggesting that RTFM in the hydrogenated Co_0.1Ti_0.9O₂ films may be intrinsic. But, metal Co appears in the hydrogenated Co_0.2Ti_0.8O₂ films, showing that RTFM in the hydrogenated Co_0.2Ti_0.8O₂ films is as least partly due to metal Co. These results indicate that hydrogen annealing can produce room-temperature ferromagnetism in Co_xTi_1−xO₂ films, but it should be carefully designed to avoid the formation of metal Co in the hydrogenated Co_xTi_1−xO₂ films.     

Electric and magnetic signatures of martensitic and intermartensitic transformations in Ni-Mn-Ga crystal

Results are reported on the temperature dependence of resistivity (200≤T≤360 K) and low-field magnetization (5≤T≤350 K) for the off-stoichiometric Ni₄₉Mn₂₉Ga₂₂ single crystal. Measurements are made for both heating and cooling cycles. The resistivity data show two first-order (hysteretic) transformations centered at about 340 and 250 K. The magnetization data show the same two transformations as the resistivity data as well as a third centered at 285 K. The results are consistent with a martensite/austenite transformation near 340 K and two intermartensitic transformations centered at 285 and 250 K (three martensite phases).     

Surface magnetic phase transitions in films

Using a simple Landau model, we discuss the different possibilities of generating magnetic effects at a second-order transition for films. Varying the sample size d$d$ and/or surface coupling γ$\gamma$ one can decrease or increase substantially the surface critical temperature _Ts$T_{\mathrm{s}}$ and the saturation magnetization _Ms$M_{\mathrm{s}}$. In the case of γ>0$\gamma >0$, _Ms$M_{\mathrm{s}}$ and _Ts$T_{\mathrm{s}}$ decrease from the bulk values as the film thickness is reduced. These theoretical results are in nice agreement with the experimental data on superconducting _MgB2${\mathrm{MgB}}_2$ thin films. By contrast, for γ<0$\gamma <0$, an enhancement of both quantities is expected. This extraordinary transition has rarely been observed experimentally and, usually, the situation is far from being clear. We analyze a new experiment on _NiFe2O4${\mathrm{NiFe}}_2{\mathrm{O}}_4$ ultra-thin films, where a very strong enhancement of the saturation magnetization is observed.     

Ground-state phase diagrams of Ising metamagnet in a mixed longitudinal and transverse magnetic field

The ground-state magnetic properties of a two-sublattice Ising metamagnet in a mixed longitudinal and transverse magnetic field are studied within the effective-field theory. A parameter j₂=J₂/J₁ is introduced, which reflects the strength ratio of spin coupling between adjacent planes and in each plane. In addition to the second-order transition lines, the first-order transition lines are also presented, since the ground-state energy can be calculated numerically. The ground-state phase diagrams in h_x–h_z are presented. The results show that when j₂<0 the phase transition of the system is always first-order for h_x<2.751, and when −1000?j₂<0 it is always second-order for h_x>4.36. For the given h_x (0<h_x<14.71), the longitudinal critical magnetic field increases as j₂ decreases. The reentrant phenomenon occurs in the range of j₂<−11.89, h_x>14.71. There is no fourth-order critical point in the phase diagrams given by using EFT as found by using mean field theory (MFT).     

The effect of grain size on electrical transport and magnetic properties of La0.9Te0.1MnO3

The effect of grain size on structural, magnetic and transport properties in electron-doped manganites La_0.9Te_0.1MnO₃ has been investigated. All samples show a rhombohedral structure with the space group at room temperature. The Mn-O-Mn bond angle decreases and the Mn-O bond length increases with the increase of grain size. All samples undergo paramagnetic (PM)-ferromagnetic (FM) phase transitions and the interesting phenomenon that both magnetization and the Curie temperature T_C decrease with increasing grain size is observed, which is suggested to mainly originate from the increase of the Mn-O bond length d_Mn-O. Additionally, ρ obviously increases with decreasing grain size due to the increase of both the height and width of tunneling barriers with decreasing grain size. The results indicate that both the intrinsic colossal magnetoresistance and the extrinsic interfacial magnetoresistance can be effectively tuned in La_0.9Te_0.1MnO₃ by changing grain size.     

The effect of oxygen stoichiometry on electrical transport and magnetic properties of La0.9Te0.1MnOy

The effect of the variation of oxygen content on structural, magnetic and transport properties in the electron-doped manganites La_0.9Te_0.1MnO_y has been investigated. All samples show a rhombohedral structure with the space group The Curie temperature T_C decreases and the paramagnetic-ferromagnetic (PM-FM) transition becomes broader with the reduction of oxygen content. The resistivity of the annealed samples increases slightly with a small reduction of oxygen content. Further reduction in the oxygen content, the resistivity maximum increases by six orders of magnitude compared with that of the as-prepared sample, and the ρ(T) curves of samples with y=2.86 and y=2.83 display the semiconducting behavior (dρ/dT<0) in both high-temperature PM phase and low-temperature FM phase, which is considered to be related to the appearance of superexchange ferromagnetism and the localization of carriers. The results are discussed in terms of the combined effects of the increase in the Mn²⁺/(Mn²⁺+Mn³⁺) ratio, the partial destruction of double exchange interaction, and the localization of carriers due to the introduction of oxygen vacancies in the Mn-O-Mn network.     

Magnetic properties of Ising metamagnet in both external longitudinal and transverse fields

The phase diagrams of a two-sublattice Ising metamagnet at finite temperature in a mixed longitudinal field and a transverse magnetic field are investigated by the use of an effective-field theory (EFT) with correlations. In addition to the second-order transition lines, the first-order transition lines are also presented in the phase diagrams, since the Gibbs free energy can be calculated numerically. The results show that there is no fourth-order critical line in the phase diagrams given by using EFT as found by using mean-field theory (MFT). The tricritical lines and their projection in the t−h_x plane obtained by using EFT are also quite different from those by using MFT. Only one type of phase diagram is obtained by using EFT while three kinds of phase diagrams are obtained by using MFT, which indicates that only the first kind of phase diagrams obtained by using MFT is reliable. Furthermore, it is shown that the region of first-order transitions increases as the transverse magnetic field h_x decreases.     

Size of the interpolated cation and hole carrier density: two key parameters for the optimisation of colossal magnetoresistive properties of Pr-based manganites

The exploration of the magnetic and transport properties of four series of manganese perovskites, Pr_0.7Ca_0.34−xA_xMnO_3−δ (A=Sr, Ba), Pr_0.7−xLa_xCa_0.3 MnO_3−δ and Pr_0.66Ca_0.34−x Sr_xMnO_3−δ has allowed four phases with colossal magnetoresistive (CMR) properties to be isolated: Pr_0.7Ca_0.25Sr_0.025MnO_3−δ and Pr_0.66Ca_0.26Sr_0.08MnO_3−δ that exhibit a variation of resistance of 2.5. 10⁷% and 10⁹% at μ₀ H=5 T for T=88 K and 50 K respectively, Pr_0.58La_0.12Ca_0.3 MnO_3−δ that exhibits a variation of 6.10⁶% for μ₀ H=5 T at T=80 K and Pr_0.7Ba_0.025Ca_0.275MnO_3−δ for which a resistance variation of 5.10⁹%, at T=50 K, for μ₀ H=5 T is evidenced. for each compound of this series except the barium phase, one observes that the temperature T_max, which corresponds to the resistance maximum on the R(T) curves in zero magnetic field, increases dramatically as the mean size of the interpolated cations increases, and that the CMR effect correlatively decreases dramatically. The comparison of the two series Pr_0.7Ca_0.3−xSr_xMnO_3−δ and Pr_0.66Ca_0.34−xSr_xMnO_3−δ shows also the crucial role of the hole carrier density: for a same mean ionic radius of the interpolated cation T_max is decreased of about 50 K by introducing 0.034 hole per Mn mole.     

Epitaxial MnP thin films: epitaxial growth,magnetic and electrical properties

We have grown 500 Å MnP on undoped GaAs(1 0 0) substrate using solid-source molecular beam epitaxy. In order to characterize the crystal structure of MnP, we performed in-situ reflection high energy electron diffraction and θ–2θ XRD X-ray diffraction studies. From the measurements of superconducting quantum interference device, Quantum Design, MnP thin film shows ferromagnetic ordering at around 291.5 K. It shows a metallic resistivity in MnP thin film.     

Electronic state on the surface in the strongly correlated electron systems

In order to clarify the tunneling spectroscopy in high-T_c cuprates, we study electronic state of the surface in the strongly correlated electron systems. First, we obtain Green's function of strongly correlated normal bulk system using the fluctuation exchange (FLEX) approximation. Next, we insert infinite potential into the bulk system and obtain Green's function of surface. We find that the density of states (DOS) in strongly correlated bulk systems are different from that on the surface, and the difference decreases as the magnitude of Coulomb interaction (U) increases.     

Unusual magnetic and transport properties above the Curie temperature in La2/3Ca1/3MnO3

Measurements of susceptibility, electron paramagnetic resonance and resistivity for La_2/3Ca_1/3MnO₃ reveal a common temperature range between ∼266 and ∼300 K, where unusual aspects of physical properties are observed. The experimental results are discussed from the Griffiths theory that predicts the formation of ferromagnetic clusters before the Curie temperature is reached. It is emphasized that this theory incorporating the double-exchange mechanism could provide a good physical basis for the understanding of unusual transport properties.