Density-of-state oscillation of quasiparticle excitation in the spin density wave phase of (TMTSF)2ClO4

Systematic measurements of the magnetocaloric effect, heat capacity, and magnetic torque under a high magnetic field up to 35 T are performed in the spin density wave (SDW) phase of a quasi-one-dimensional organic conductor (TMTSF)2ClO4. In the SDW phase above 26 T, where the quantum Hall effect is broken, rapid oscillations (ROs) in these thermodynamic quantities are observed, which provides clear evidence of the density-of-state (DOS) oscillation near the Fermi level. The resistance is semiconducting and the heat capacity divided by temperature is extrapolated to zero at 0 K in the SDW phase, showing that all the energy bands are gapped, and there is no DOS at the Fermi level. The results show that the ROs are ascribed to the DOS oscillation of the quasiparticle excitation.     

Short-range magnetic order in a spin density wave in Cr-based multilayers

A mechanism of the formation of the short antiferromagnetic order with a spin density wave (SDW) in the vicinity of the interfaces in the Fe/Cr type multilayers is proposed. The main reason behind the emergence of magnetic ordering with SDWs is the redistribution of charge (and, hence, spin) density in the vicinity of Fe/Cr interfaces, which leads to the paramagnetic phase instability at a temperature considerably higher than the Néel temperature in chromium. The Ginzburg-Landau expansion for the free energy of the system is used for determining the inhomogeneous collinear structures of CDWs and for constructing the phase diagram (the dependence of the transition temperature on the thickness of the antiferromagnetic interlayer). The obtained results are used for discussing the experimental data on neutron scattering and tunnel microscopy.     

Coexistence of superconductivity and antiferromagnetism probed by simultaneous nuclear magnetic resonance and electrical transport in (TMTSF)2PF6 system

We report simultaneous NMR and electrical transport experiments in the pressure range near the boundary of the antiferromagnetic spin density wave (SDW) insulator and the metallic/superconducting (SC) phase in (TMTSF)2PF6. Measurements indicate a tricritical point separating a line of second-order SDW/metal transitions from a line of first-order SDW/metal(SC) transitions with coexistence of macroscopic regions of SDW and metal(SC) order, with little mutual interaction but strong hysteretic effects. NMR results quantify the fraction of each phase.     

Dephasing of Andreev pairs entering a charge density wave

A Cooper pair from a s-wave superconductor (S) entering a conventional charge density wave (CDW) below the Peierls gap dephases on the Fermi wavelength while one particle states are localized on the CDW coherence length ξ_CDW. It is thus practically impossible to observe a Josephson current through a CDW. The paths following different sequences of impurities interfere destructively, due to the different electron and hole densities in the CDW. The same conclusion holds for averaging over the conduction channels in the ballistic system. We apply two microscopic approaches to this phenomenon: (i) a Blonder, Tinkham, Klapwijk (BTK) approach for a single highly transparent S-CDW interface; and (ii) the Hamiltonian approach for the Josephson effect in a clean CDW and a CDW with non magnetic disorder. The Josephson effect through a spin density wave (SDW) is limited by the coherence length ξ_SDW, not by the Fermi wave-length. A Josephson current through a SDW might be observed in a structure with contacts on a SDW separated by a distance ξ_SDW.     

Absence of modulated phase transition and spin density wave phase transition in one- and two-dimensional systems

We prove that the modulated phase transition with any wave vector and the spin density wave (SDW) phase transition cannot exist in one- and two-dimensional systems at finite temperatures.     

Possibility of a spin-density-wave or a valley-density-wave in the ground state of a two-dimensional electron fluid

It is suggested that the two-dimensional electron fluid in the Si[100] inversion layers may spontaneously distort to form a spin density wave (SDW) or valley density wave (VDW) at densities ? 8 × 10¹¹ cm^-2. The relative stability of the SDW to VDW states is determined by the sign of the total intervalley interactions.     

Coexistence of CDW phase and SDW phase

The coexistence of charge density wave (CDW) and spin density wave (SDW) phases is studied by introducing a singlet coupling in the Rice model as well as the Shibatani-Motizuki-Nagamiya (SMN) model. The conditions of the coexistence, and the other parameters are derived for T = 0 K. It is found that the coexistent phase is unrealizable in pure chromium.     

Static magnetic order in metallic K0.49CoO2

By means of muon-spin spectroscopy, we have found that K0.49CoO2 crystals undergo successive magnetic transitions from a high-T paramagnetic state to a magnetic ordered state below 60 K and then to a second ordered state below 16 K, even though K0.49CoO2 is metallic at least down to 4 K. An isotropic magnetic behavior and wide internal-field distributions suggest the formation of a commensurate helical spin density wave (SDW) state below 16 K, while a linear SDW state is likely to exist above 16 K. It was also found that exhibits a further transition at 150 K presumably due to a change in the spin state of the Co ions. Since the dependence of the internal-field below 60 K was similar to that for Na0.5CoO2, this suggests that magnetic order is more strongly affected by the Co valence than by the interlayer distance or interaction and/or the charge ordering.     

Josephson and quasiparticle currents in tunneling junctions between partially dielectrized (partially gapped) superconductors with spin density waves

The current-voltage characteristics (CVC) are calculated for the Josephson, interference, and quasiparticle components of the current through a tunneling junction formed by two superconductors with spin density waves (SDW). The treatment is based on the model of partial dielectrization (gapping) of the Fermi surface and the assumption of pinning of the spin density waves. The following particular cases are studied in detail: asymmetric SDW superconductor-ordinary superconductor junctions and symmetric junctions between two identical SDW superconductors. The positions and nature of the singularities in the CVC are determined. For a symmetric contact the possibility of the existence of asymmetric CVC’s is predicted. The calculations are in qualitative agreement with the experimentally observed behavior of the CVC’s of tunneling junctions and microcontacts containing the SDW superconductor with heavy fermions URu₂Si₂. Fiz. Tverd. Tela (St. Petersburg) 41, 1743–1749 (October 1999)     

Spin density wave and ferromagnetism in a quasi-one-dimensional organic polymer

The spin density wave(SDW) — charge density wave(CDW) phase transition and the magnetic properties in a half-filled quasi-one-dimensional organic polymer are investigated by the world line Monte Carlo simulations. The itinerant π electrons moving along the polymer chain are coupled radically to localized unpaired d electrons, which are situated at every other site of the polymer chain. The results show that both ferromagnetic and anti-ferromagnetic radical couplings enhance the SDW phase and the ferromagnet order of the radical spins, but suppress the CDW phase. By finite size scaling, we are able to obtain the phase transition line in the parameter space. The ferromagnetic order of the radical spins are observed to coexist with the SDW phase. As compared to the system being free of the radical coupling, the phase transition line is shifted upward in the U-V parameter space in favor of larger V, where U is the on-site repulsion and V is the nearest-neighbor interaction between the π electrons. All of these findings can be understood qualitatively by a second-order perturbation theory starting from the classical state at zero temperature in the strong coupling limit. We also address the consequences of the radical coupling for the persistent current if the polymer chain is fabricated as a mesoscopic ring.     

Growth and transport properties of CaFeAsF_(1-x) single crystals

Using self-flux method,we have successfully grown the parent phase of the single crystals of CaFeAsF1-x.The X-ray di?raction indicates good crystallinity.In-plane resistivity shows a bad metallic behavior with a sharp drop of resistivity at about T SDW=119K.This anomaly is associated with the possible spin density wave(SDW)order.Interestingly near T SDW,the resistivity exhibits a cusp-like feature,which may be understood as the strong coupling effect between the electrons and the antiferromagnetic(AF)spin fluctuations.A reduction of fluorine or application of a high pressure will suppress the SDW feature and induce superconductivity.Hall effect measurements reveal a positive Hall coefficient below T SDW indicating a dominant role of the hole-like charge carriers in the parent phase.Strong magnetoresistance has been observed below T SDW suggesting multiple conduction channels of the charge carriers.     

The competition between CDW and SDW in quasi-one-dimensional organic magnetic polymer with interchain coupling interaction

Based on a theoretical model proposed for interchain-coupled quasi-one-dimensional organic magnetic polymer, the effects of the interchain couplings and electron–electron interactions on the charge density wave (CDW) and spin density wave (SDW) that exist in the system are studied. It is found that the amplitude of the SDW along the main chain will decrease with increasing of the oscillatory term of the interchain couplings in the system, which is unfavorable to the ferromagnetic ground state of the system. Moreover, with different interchain couplings, there will all exist a critical value of the inter-site electron–electron Coulomb repulsion, and at this value, the system will experience a transformation from strong SDW state to strong CDW one, which will weaken the mediating function of the antiferromagnetic SDW along the main chain. As a result, the ferromagnetic correlation intensity between the spins of the side radicals will be affected and consequently the stability of the ferromagnetic state in the system will be weakened.     

μ+SR study of heavy-fermion magnetism in Ce(Cu1-xNix)2Ge2

We report muon spin relaxation (μ⁺SR) studies on the magnetic phase diagram of Ce(Cu_1-xNi_x)₂Ge₂ polycrystals for 0.5≤ x ≤ 0.8. A sharp magnetic transition, evidenced by the appearance of a fast Gaussian relaxation component σ, has been observed in the x = 0.5 alloy at 4.0 K in zero applied field. The average local field < B_μ> at the stopping sites of the muons, extracted from σ, exhibits a linear temperature dependence. We associate these features with an incommensurate spin density wave (SDW) ordering. Magnetic ordering, either long range or short range, and signatures of non-Fermi liquid behaviour have not been observed down to 2.0 K at x = 0.8. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dipole-dipole spin anisotropy energy for spin density waves in highly anisotropic crystals: (TMTSF) 2X

The dipole-dipole spin anisotropy energy for a spin density wave (SDW) is compatible with experimental easy, intermediate, and hard spin polarization axes in the (TMTSF) ₂X class of organic conductors only when electron-hole correlations of the SDW are atomic in size. Magnitudes of the spin anisotropy energy and the spin-flip magnetic field in this case agree with experiment for a SDW amplitude comparable to 10^-2 Bohr magnetons/TMTSF molecule.     

Electronic properties of quasi-one-dimensional compounds with a charge/spin-density wave ground state

First, a review of the general properties of the collective transport induced by the charge (CDW)/spin (SDW) density wave motion in quasi-one-dimensional conductors is presented. Then the three recent developments in this field are emphasized, namely: high spatial resolution x-ray study of the field-induced CDW deformations; quantum interference effects in magnetotransport of a sliding CDW through columnar defects; manifestation of disorder in the CDW/SDW ground state in thermodynamic properties at very low temperatures. Fiz. Tverd. Tela (St. Petersburg) 41, 759–763 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Strain wave in pure chromium

New satellites were observed in a single crystal of pure chromium below the Néel point by X-ray diffraction at the positions apart by 2δ from the Bragg points. The observed anisotropy of the satellite intensity around the Bragg point and the temperature dependence of 2δ yield the conclusion that the lattice spacing in the direction of the wave vector of the spin density wave are modulated periodically and that the period is a half of that of the SDW. The experimental results coincide with the prediction made by a theory of the exchange striction in transition metal alloys.     

Magnetic Phase Transitions to an Incommensurate Structure in LiMn 2 O 4 Compound

Possible second-order magnetic phase transitions to the incommensurate magnetic structure in the orthorhombic phase of LiMn2O4 compound have been investigated. It is shown that “weak Lifshitz condition” holds for this compound (i.e., Lifshits invariants are immaterial in all transitions being considered), and only the incommensurate phase, in which the wavevector near the transition point varies continuously with temperature and pressure, can be formed. A transition is considered both in the exchange approximation occurring in three irreducible representations forming an exchange multiplet as well as in one and two irreducible representations. Expressions have been derived for the mean density of the magnetic moment emerging because of such transitions. It is found that structures of the types of a longitudinal spin wave, a transverse spin wave with polarization along the crystallographic axes perpendicular to the wavevector of the structure, as well as certain superpositions of these waves can be formed in the system.     

Reconstruction of the Fermi surface in the pseudogap state of cuprates

Reconstruction of the Fermi surface of high-temperature superconducting cuprates in the pseudogap state is analyzed within a nearly exactly solvable model of the pseudogap state, induced by short-range order fluctuations of the antiferromagnetic (AFM), spin-density wave (SDW), or a similar charge-density wave (CDW) order parameter, competing with the superconductivity. We explicitly demonstrate the evolution from “Fermi arcs” (on the “large” Fermi surface) observed in the ARPES experiments at relatively high temperatures (when both the amplitude and phase of the density waves fluctuate randomly) towards the formation of typical “small” electron and hole “pockets,” which are apparently observed in the de Haas-van Alphen and Hall resistance oscillation experiments at low temperatures (when only the phase of the density waves fluctuate and the correlation length of the short-range order is large enough). A qualitative criterion for the quantum oscillations in high magnetic fields to be observable in the pseudogap state is formulated in terms of the cyclotron frequency, the correlation length of fluctuations, and the Fermi velocity. The text was submitted by the authors in English.     

Divergency of SDW and structure transition in Fe1−xNixSe0.82 superconductors

The effect of the Ni-substitution on the superconductivity of α-FeSe_0.82 system has been investigated. It is found that the superconductivity of α-FeSe_0.82 can be quickly suppressed when a small quantity of Fe is substituted by Ni element. The resistivity and moment of the sample at normal state increase with increasing Ni-substitution content. Moreover, it is found that the Ni-substitution does not change the structure transition temperature, but increases the spin density wave (SDW) order temperature. It is suggested that the quick suppression of superconductivity of α-FeSe_0.82 caused by the Ni-substitution is related to the increase of SDW order temperature. Therefore, Fe atom plays a key role for the appearance of the superconductivity in FeSe system.