Magnetic dilution and electronic nature in La0.7Pb0.3Mn1−xZnxO3

We have studied the magnetic dilution and electronic nature of Zn doping on the Mn site in the colossal magnetoresistant material La_0.7Pb_0.3MnO₃ (x≤0.3). Small non-magnetic Zn²⁺ doping tends to separate the system into ferromagnetic clusters to weaken the long-range ferromagnetic order and to reduce the Curie temperature. The spin polarizability of the x=0–0.3 samples is estimated to be 0.97–1.00, indicating that the x=0–0.3 samples are the spin polarized materials in which the conductivity is dominated by single-spin charge carriers. Small doping (x≥0.1) induces the metal–insulator transition and destroys the metallic state with long-range ferromagnetic order.     

Magnetic and transport properties of bilayered manganites La1.2Sr1.8Mn2-xGaxO7（x=0,0.08）

The magnetic and electrical properties of nonmagnetic Ga +3 ion substitution for Mn site are investigated in the bilayer manganite La 1.2 Sr 1.8 Mn 2 O 7.When the Mn is substituted by Ga,the ferromagnetic property obviously weakens,the magnetic transition temperature decreases and a spin-glass behaviour occurs at low temperature.Meanwhile,doping causes the resistivity to dramatically increase,the metal-insulator transition temperature to disappear,and a greater magneto-resistance effect to occur at low temperature.These effects result from the fact that Ga substitution dilutes the magnetic active Mn-O-Mn network and weakens the double exchange interaction,and further suppresses ferromagnetic ordering and metallic conduction.     

Magnetism and magnetocaloric properties of Mn0.95Cr0.05As

In this study the magnetic properties of Mn_0.95Cr_0.05As, prepared by mechanical milling, have been investigated. The results suggest that the presence of strains is very important for the magnetic state of the compound. In the presently studied compound, a combined magnetic and structural transition occurs from paramagnetic MnP phase to ferromagnetic NiAs phase at about 234 K. With further decreasing temperature, at 159 K, a transition from ferromagnetic NiAs phase to helimagnetic (H_a-type) MnP phase is observed, which is accompanied by an inverse magnetocaloric effect. The ferromagnetic phase is recovered when the temperature is increased. At both first-order transitions, at 159 and 234 K, large magnetic-entropy changes are observed.     

Coexistence of magnetic and ferroelectric properties in Y0.1Co1.9MnO4

The magnetic,conductivity,and dielectric properties have been investigated in single-phase polycrystalline Y0.1 Co1.9 MnO4.The temperature-dependent magnetisation reveals the ferromagnetic transition in sample at a low temperature (～186 K).Magnetisation as a function of field H (M-H loop) indicated the weak ferromagnetism of the sample at room temperature.The constant ε and dielectric loss tgδ measurements represent a ferroelectric phase transition at a higher temperature (～650 K),while the conductivity shows an insulator-metallic transition.The ferroelectric hysterisis loops and capacitance-voltage measurements confirm the ferroelectric nature of the sample at room temperature.The observed ferromagnetism and ferroelectric nature in this material suggests a potential multiferroic application.     

On Gossamer metals and insulating behaviour

We extend the Gossamer technique recently proposed to describe superconducting ground states to metallic ground states. The Gossamer metal in a single band model will describe a metallic phase that becomes arbitrarily hard to differentiate from an insulator as one turns the Coulomb correlations up. We were motivated by the phase diagram of V₂O₃ and f-electron systems which have phase diagrams in which a line of first-order metal–insulator transition ends at a critical point above which the two phases are indistinguishable. This means that one can go continuously from the metal to the ‘insulator’, suggesting that they might be the same phase.     

The semiconductor-to-ferromagnetic-metal transition in FeSb2

We propose FeSb₂ to be a nearly ferromagnetic small gap semiconductor, hence a direct analog of FeSi. We find that despite different compositions and crystal structures, in the local density approximation with on-site Coulomb repulsion correction (LDA+U) method magnetic and semiconducting solutions for U=2.6 eV are energetically degenerate similar to the case of FeSi. For both FeSb₂ and FeSi (FeSi_1-xGe_x alloys) the underlying transition mechanism allows one to switch from a small gap semiconductor to a ferromagnetic metal with magnetic moment ≈1 μ_B per Fe ion with external magnetic field.     

Electronic structure and metal-insulator transition in SrTi 1 - x Ru xO 3

We studied the changes in the electronic structure of SrTi_1-xRu_xO₃ across the metal-insulator transition. The parent compound, SrTiO₃, is a well known diamagnetic insulator; whereas the doped compound, SrTi_1-xRu_xO₃, becomes a ferromagnetic metal above x _C = 0.35. The techniques used in the study were photoemission (PES) and O 1 s X-ray absorption (XAS) spectroscopy. The experimental spectra were analyzed in terms of band structure and Hubbard model calculations. The PES and XAS spectra of SrTi_1-xRu_xO₃ show the Ru 4 d bands growing in the band gap of SrTiO₃ . The analysis in terms of the Hubbard model indicates that the Ti 3 d and Ru 4 d bands are mostly decoupled. This suggests that the metal-insulator transition is a percolation transition like that of metals embedded in a rare gas matrix. Electron correlation effects are present in this system, but they do not seem to play a major role in the transition. Received 10 September 2001     

Resistive switching in β-SrV<Subscript>6</Subscript>O<Subscript>15</Subscript>

We investigate the pressure and temperature behavior of current-dependent resistivity of β-SrV₆O₁₅. We observe a switching between states of different resistivities in the insulating state of β-SrV₆O₁₅. In the low pressure phase, the resistive switching appears at temperatures below the semiconductor-insulator transition. In the high pressure phase, under ~1.6 GPa, the switching appears in the temperature range of the phase transition. The existence of switching may imply an important role of strontium off-stoichiometry for the electrical transport in β-SrV₆O₁₅. No electric-field-induced enhancement of the conductivity is observed. However, the conduction is significantly nonlinear under ~1.6 GPa, indicating that the charge order pattern in the high pressure phase is considerably different from that of the low pressure phase.     

Magnetic field and temperature-induced first-order transition in Gd5(Si1.5Ge2.5): a study of the electrical resistance behavior

Magnetic field (0–4 T) and temperature dependencies (4.2–320 K) of the electrical resistance of Gd₅(Si_1.5Ge_2.5), which undergoes a reversible first-order ferromagnetic↔paramagnetic phase transition, have been measured. The electrical resistance of Gd₅(Si_1.5Ge_2.5) indicates that the magnetic phase transition can be induced by both temperature and magnetic field. The temperature dependence of the electrical resistance, R(T), for heating at low temperatures in the zero magnetic field has the usual metallic character, but at a critical temperature of T_cr=216 K the resistance shows a 20% negative discontinuity due to the transition from the low-temperature high-resistance state to the high-temperature low-resistance state. The R(T) dependence for cooling shows a similar but positive 25% discontinuity at 198 K. The isothermal magnetic field dependence of the electrical resistance from 212T224 K indicates the presence of temperature-dependent critical magnetic fields which can reversibly transform the paramagnetic phase into the ferromagnetic phase and vice versa. The critical magnetic fields diagram determined from the isothermal magnetic field dependencies of the electrical resistance of Gd₅(Si_1.5Ge_2.5) shows that the FM↔PM transition in zero magnetic field on cooling and heating occurs at 206 and 213 K, respectively. The full isothermal magnetic filed hysteresis for the FM↔PM transition is 2 T, and the isofield temperature gap between critical magnetic fields is 7 K.     

Submillimeter and infrared spectroscopy on La0.85Sr0.15MnO3

The optical conductivity of La_0.85Sr_0.15MnO₃ single crystals was studied by means of submillimeter and infrared spectroscopy for frequencies cm^-1 and temperatures 10 K < T <300 K. The submillimeter conductivity follows the temperature dependence of the dc-data. The phonon spectrum of La_0.85Sr_0.15MnO₃ changes considerably below K revealing a structural phase transition induced by charge or orbital order. At T =10 K a number of phonon modes can be identified in addition to the room-temperature spectrum. The optical conductivity () in the mid-infrared reveals the characteristics of small polaron absorption. Below the magnetic ordering temperature the polaron binding energy is highly reduced, but the onset of charge order interrupts the formation of free charge carriers with a Drude-like behavior. The frequency and temperature dependence of in this regime qualitatively resembles the small polaron predictions by Millis et al. (Phys. Rev. B 54, 5405 (1996)). Received 5 November 1999     

Magnetic collapse and electronic phase transitions at high pressure in transition metal oxides

A review of electronic and magnetic phase transition in metal oxides with strong electron correlations (SEC) is given. The bandwidth control of the insulator gap is expected in the Hubbard model when the decreasing of the interatomic distance results in the bandwidth W(P) increase and at some critical value P_c, W(P_c)∼U and the Mott–Hubbard gap disappears. The other situation takes place in transition metal boroxides FeBO₃ and GdFe₃(BO₃)₄, where the increase of crystal field parameter Δ(P) results in the high spin–low spin crossover.     

Canted antiferromagnetism and magnetoelastic coupling in metallic Ho0.1Ca0.9MnO3

Ho0.1Ca0.9MnO3 is a canted antiferromagnet with the magnetic space group Pn ' ma '. The magnetic structure is a superposition C x F y A z of the three types of order allowed in Pn ' ma '. In the Ca-rich corner of the system Ho _1-x Ca x MnO ₃ the title compound has a strong magnetoelastic distortion , the highest metallic conductivity and a ferromagnetic component F y close to the maximum in the series. Among the areas ab, bc, ca calculated from the lattice constants only ca shows a strong magnetoelastic effect below T N = 106 K. The x-, y-, z-spin components depend differently on the temperature. This gives rise to spin rotation which is particularly strong close to T N. MnO ₆ octahedra have short bond lengths with a temperature independent average . They are practically regular at room temperature and show a Jahn-Teller distortion of 3.5% in the magnetically ordered state. Above T N we find small polaron conductivity. The presence of the Jahn-Teller distortion due to the only small abundance (10%) of Mn ³⁺ in the t _2g ³ e g configuration is attributed to delocalised e g electrons. In the magnetically ordered state the averaged magnetic moment of Mn is reduced appreciably from the paramagnetic value due to spin disorder. Received 21 January 1999     

Metal-insulator transition in the low-dimensional organic conductor (TMTSF)<Subscript>2</Subscript>FSO<Subscript>3</Subscript> probed by infrared microspectroscopy

We present measurements of the infrared response of the quasi-one-dimensional organic conductor (TMTSF)₂FSO₃ along (E ) and perpendicular (E ) to the stacking axis as a function of temperature. Above the metal-insulator transition related to the anion ordering the optical conductivity spectra show a Drude-like response. Below the transition an energy gap of about 1500 cm^-1 (185 meV) opens, leading to the corresponding charge transfer band in the optical conductivity spectra. The analysis of the infrared-active vibrations gives evidence for the long-range crystal structure modulation below the transition temperature and for the short-range order fluctuations of the lattice modulation above the transition temperature. We also report about a new infrared mode at around 710 cm-1 with a peculiar temperature behavior, which has so far not been observed in any other (TMTSF)₂X salt showing a metal-insulator transition. A qualitative model based on the coupling between the TMTSF molecule vibration and the reorientation of electrical dipole moment of the FSO₃ anion is proposed, in order to explain the anomalous behavior of this new mode.     

Sub-picosecond photo-induced melting of a charge-ordered state in a perovskite manganite

Ultrafast melting of a charge-ordered state has been observed in the photo-irradiated colossal magnetoresistive compound Pr_0.7Ca_0.3MnO₃. Pump-and-probe spectroscopy experiments reveal the formation of a conducting phase with typical features of an insulator–metal transition (IMT) after less than 1 ps. This phase is metastable and can be maintained for about 1 μs unless it is stabilized persistently into a pathlike metallic region by an electric field. Although laser-induced lattice heating may play a role in the initial excitation, electronic correlations are the dominant effect which leads to the formation of the metallic state upon the breakdown of the charge-ordered state. Received: 26 January 2000 / Published online: 16 June 2000     

Quantum spherical model in a random field: coherent state path integral approach

By introducing boson operators, a quantum spherical XY model in the presence of a random field has been studied by the coherent state path integral approach. The phase diagram is obtained, and the effects of the random-field fluctuations on the possibilities of the existence of a ferromagnetic phase are discussed. At the critical point, , the order parameter M describing the ordered ferromagnetic phase disappears as .Since the model is equivalent to a Bose system, we also show that the phase transition at zero temperature between the superfluid and the disordered Mott insulator phases occurs at the chemical potential , where J₀ is the strength of the exchange interaction. As the temperature T goes to zero, the asymptotic behavior of the entropy and the specific heat are and , respectively. Received: 20 May 1997 / Accepted: 20 October 1997     

Behavior of the atomic and magnetic structure of La0.35Pr0.35Ca0.30MnO3 at a metal-insulator phase transition

The evolution of the structural and magnetic properties of the CMR (colossal-magnetoresistance) compound La_0.35Pr_0.35Ca_0.30MnO₃ as the temperature changes from 10 to 293 K is investigated by means of neutron diffraction. It is shown that the changes in the transport and magnetic properties are directly related with the rearrangement of the atomic structure. A phase transition to the metallic state occurs together with simultaneous ferromagnetic ordering of the manganese moments and is accompanied by a jump in volume. The static distortions of the oxygen octahedra which are observed to occur prior to the magnetic phase transition and which are practically absent at room temperature and in the FM phase attest to the orbital ordering of oxygen atoms on the bonds, with freezing-in of the Jahn-Teller modes. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 672–677 (10 May 1998)     

Band effects on the conductivity for a two-band Hubbard model

The band effects on the conductivity of a one-dimensional two-band Hubbard model is studied based on the ground state energy analysis. It is found that the system with filling factor one is a metal at zero temperature if the on-site interaction U is smaller than a critical value U_c, and is an insulator if U is larger than U_c. The value of metal-insulator transition point U_c is obtained. This result is different from that of 1D single-band Hubbard model where the quantum phase transition point U_c=0. Therefore, the orbital degree of freedom plays an essential role in the states of matter.     

Effects of divalent alkaline earth ions on the magnetic and transport features of La0.65A0.35Mn0.95Fe0.05O3 (A = Ca, Sr, Pb, Ba) compounds

We have studied the magnetic and transport properties of Fe doped La_0.65A_0.35Mn_0.95Fe_0.05O₃ (A = Ca, Sr, Pb, Ba) manganites. All the compositions show ferromagnetic/metal to paramagnetic/insulator transition (T_C) except the Pb doped sample which is insulating and ferromagnetic (FM) in the entire temperature range. The magnetization and T_C are decreased by decreasing the cation size on La site. The transition temperature and magnetic moment at 77 K is a maximum for Sr doped sample and is decreasing if we increase or decrease the cation size from Sr size. The maximum value of T_C and magnetic moment for Sr based sample is most likely due to the closer ionic sizes of La and Sr as compared to the other dopants (Ca, Pb, and Ba). We observed a spin freezing type effect in the Pb doped sample below 120 K in resistivity, ac susceptibility and in magnetization. This suggests that the AFM interactions introduced by the Fe are most effective in the Pb doped composition leading to increased competition between the FM and AFM interactions. This FM and AFM interaction generates some degree of frustration leading to the appearance of spin glass like phase whose typical magnetic behavior is studied for small ion when the metallic like behavior is lost.     

Effects of electron localization in V2 − y O3

The effect of nonstoichiometry on the metal-insulator phase transition in V₂O₃ is studied. It is established that an increase in the vanadium deficiency in V2 ? yO₃ brings about a shift in the phase transition temperature toward lower temperatures and an increase in the width of the temperature hysteresis loop of the electrical conductivity. As the vanadium deficiency increases to a level corresponding to the composition ～V_1.974O₃, the phase transition completely disappears and the sample remains metallic down to T = 1.6 K. The magnetoresistance is measured for samples of this composition in longitudinal and transverse magnetic fields at T = 4.2 K.