Spin Gap State Research Articles

Renormalization-group approach to the metal-insulator transitions in(DCNQI)2M(DCNQI isN,N′-dicyanoquinonediimine andM=Ag,Cu)

Metal-insulator transitions and different ground-state phases in the quasi-one-dimensional materials, ${(R}_{1}{R}_{2}\ensuremath{-}\mathrm{DCNQI}{)}_{2}M$ ${(R}_{1}{=R}_{2}={\mathrm{CH}}_{3},$ I and $M=\mathrm{Ag},$ Cu), are studied with a renormalization-group method. We use one-dimensional continuum models with backward scatterings, umklapp processes, and couplings with ${2k}_{F}$ and ${4k}_{F}$ phonons (not static lattice distortion). We take a quarter-filled band for $M=\mathrm{Ag}$ and a sixth-filled band coupled with a third-filled band for $M=\mathrm{Cu}.$ Depending on electron-electron and electron-phonon coupling strengths, the ground-state phase becomes a Tomonaga-Luttinger liquid or a state with a gap(s). For $M=\mathrm{Ag},$ there appears a spin-gap state with a dominant ${2k}_{F}$ charge-density-wave correlation, a Mott insulator with a dominant ${4k}_{F}$ charge-density-wave correlation, or a spin-Peierls state with different magnitudes of spin and charge gaps. Three dimensionality is taken into account by cutting off the logarithmic singularity in either the particle-particle channel or the particle-hole channel. The difference between the ground-state phase of the ${R}_{1}{=R}_{2}={\mathrm{CH}}_{3}$ salt (spin-Peierls state) and that of the ${R}_{1}{=R}_{2}=\mathrm{I}$ salt (antiferromagnetic state) is qualitatively explained by a difference in the cutoff energy in the particle-particle channel. For $M=\mathrm{Cu},$ there appears a Mott insulator with a charge-density wave of period 3 and a Peierls insulator with a charge-ensity wave of period 6. The conditions for the experimentally observed, Mott insulator phase are strong correlation in the sixth-filled band, moderate electron-phonon couplings, and finite electron-${4k}_{F}$ phonon coupling. Resistance is calculated as a function of temperature with a memory-function approximation in both cases above. It qualitatively reproduces the differences among the $M=\mathrm{Ag}$ and $M=\mathrm{Cu}$ cases as well as the ${R}_{1}{=R}_{2}={\mathrm{CH}}_{3}$ and ${R}_{1}{=R}_{2}=\mathrm{I}$ cases.

Phase diagram of theS=12frustrated coupled ladder system

We present a theoretical study of the magnetic phase diagram of the frustrated coupled ladder structure realized recently in several materials. This system displays a nondegenerate spin-gap state in the dimer limit and an infinitely degenerate spin-gap state in the regime of weakly coupled zigzag chains. Between these we demonstrate the existence of gapless, magnetically ordered regions whose order is antiferromagnetic close to the honeycomb lattice limit, and incommensurate along the chains when all three magnetic interactions compete.

Effects of impurities in spin gap state of cuprates

The effect of non-magnetic impurities is studied for the cuprates including high-Tc superconductors and two-leg ladder compounds. Two topics are discussed. One is the anomalous Kondo effect in metallic high-Tc cuprates, and the other is the impurity induced antiferromagnetism in the insulators.

Unusual crystal structures and magnetic properties of nitronylnitroxide radical ions

We report unusual crystal structures and magnetic properties of nitronylnitroxide radical ions, (i) A nitronylnitroxide anion, p-(nitronylnitroxide)benzoate ( p-NNBA), is found to crystallize with Mn 2+ and water molecules in the form of Mn( p−, NNBA) 2(H 2O 2. In this crystal, the manganese(II) ion is in an octahedral environment, surrounded by two NO groups, two CO 2 groups and two H 2O. The magnetic properties are well interpreted in terms of the trimer model of [ p-NNBA ( S=l/2)]-[Mn 2+ (S=5/2)]-[ p-NNBA ( S=1/2)]. (ii) A nitronylnitroxide cation, m-N-methylpyridinium nitronylnitroxide ( m-MPYNN), crystallizes with I −, BF 4 −, or ClO 4 − into a trigonal space group, where the spin system is regarded as a spin-1 Kagome antiferromagnet with spin frustration. Low-temperature magnetic measurements reveal a spin-gap state below 0.24 K.

Chiral Anomaly and Spin Gap in One-Dimensional Interacting Fermions

Semiclassical approach has been developed for the one-dimensional interacting fermion systems. Starting from the incommensurate spin density wave (SDW) mean field state for the repulsive Hubbard model in 1D, the non-Abelian bosonized Lagrangian describing the spin-charge separation is obtained. The Berry phase term is derived from the chiral anomaly, and we obtain the massless Tomonaga-Luttinger liquid in the single chain case while the spin gap opens in the double-chain system. This approach offers a new method to identify the strong-coupling fixed point, and its relation to the Abelian bosonization formalism is discussed on the spin gap state. The generalization to higher dimensions is also discussed.

Nonmagnetic impurity in the spin-gap state.

The effects of nonmagnetic strong scatterers (unitary limit) on magnetic and transport properties are studied for resonating-valence-bond states in both the slave-boson and slave-fermion mean-field theories with the gap for the triplet excitations. In the d-wave pairing state of the slave-boson mean-field theory in two dimensions, there is no true gap for spinons, but the Anderson localization occurs, which leads to the local moment when the repulsive interaction is taken into account. In the slave-fermion mean-field theory, local moments are found bound to nonmagnetic impurities as a result of (staggered) gauge interaction. However, in both theories, localization of spinon does not appear in the resistivity, which shows the classical value for the holon.

Bipolaron picture of spin gap state in double-chain systems

Slightly doped spin gap state in double-chain system is studied in terms of Abelian bosonization method. It is found that (i) The spin gap formation in terms of the interchain exchange coupling J ⊥ persists only when the two chains are doped with equal number of carriers. (ii) Interchain hopping t ⊥ is irrelevant in the spin gap state as long as max( t ⊥, t) δ ⪡ | J ⊥| ( t: intrachain hopping, δ: carrier concentration) is satisfied. (iii) Only the interchain singlet superconductivity and the 4 k F charge density wave show power-law (critical) correlations while all the others decay exponentially. This means that the low energy physics is described in terms of the bipolaron model.

Ground State Energy, Spin Gap and Hole Superconductivity on Square Lattice Heisenberg Antiferromagnet by Majorana Fermion

The mechanism of high Tc superconductivity has not been solved completely yet. It was argued by Anderson that the properties of high Tc superconductivity are normal but the normal state properties are not normal. This seems to be true. We approach the subject of high Tc superconductivity viewing it from the normal state. First the ground state of the S=l/2 square lattice Heisenberg antiferromagnet is investigated using a BCS type variational method and a massless Majorana fermion representation for the spin 1/2 operator. Second when a hole is doped, we investigate how the spin gap appears as an elementary excitation of the spin fluctuation, where we use another Majorana fermion representation for an itinerant hole. Third we investigate the situation in which more holes are doped on the square lattice antifer­ romagnet and spin Majorana fermions are exchanged between itinerant holes. Here, the spin gap becomes unstable, and high Tc superconductivity appears instead of the spin gap, where we describe the superconductive gap by a massive Dirac fermion in the Nambu representation instead of a Majorana fermion. We also discuss the noncoexistence of the spin gap and the superconductive gap formed by the holes and seek to determine the critical concentration of holes, that concentration at which the spin gaps disappear and the superconductive gaps replace them. We also examine the phase diagram of the spin gap state and the supercon­ ductive state as a function of the hole concentration and the temperature. The method we adopt is that introduced by Tsvelik,l) who uses the Majorana fermion representation for spins and holes in the Kondo problem and the heavy fermion problem. A Majorana fermion is massless, and, using this representation, we can avoid the use of both the slave boson and fermion to represent the spin operator and their gauge fields. Then we can use the BCS-type variational method for the interaction of the Majorana fermions without using the Gutzwiller projection together with the restriction of the double occupancy of the opposite spins on the same site, as adopted in the modified variational method by Zhang, Gros, Rice and Shiba. 2 l

Phase Diagram ofS=1/2 Quasi-One-Dimensional Heisenberg Model with Dimerized Antiferromagnetic Exchange

The quantum phase transition between a spin gap state and an antiferromagnetic phase is investigated. We study S =1/2 antiferromagnetic Heisenberg chains coupled by antiferromagnetic interchain interaction. The intrachain exchanges have alternating strength. The phase boundary between the antiferromagnetically ordered phase and a spin gap phase is also obtained in a parameter space of the amplitude of the interchain coupling and the dimerization. The spin-wave approximation substantially overestimates the antiferromagnetic phase. The competition between the long range order and the spin gap is examined in detail. We estimate a variety of critical exponents at the transition, namely, exponents ν, θ and z defined as the exponent of the correlation length, the magnetization curve and the dynamical exponent, respectively. From the quantum Monte Carlo simulation, the exponents ν, θ and z are estimated to be unity. The exponents ν and θ are different from the estimated values in one dimension. It suggests that the universality changes due to the dimensionality change. In our estimates, the exponent ν does not agree with the prediction from three dimensional classical Heisenberg model. We also discuss the relevance of our result to spin-Peierls systems with lattice distortion.

Mott Transition and Transition to Incompressible States –Variety and Universality–

Critical properties of transitions between a variety of quantum fluids and the Mott insulator are examined in terms of the transition to an incompressible state. In one dimension, they show universally common properties each other irrespective of the statistics, components and the interaction of particles. In two dimensions, they show qualitative differences originated from the differences in the nature of the quantum fluid. It is useful to categorize the singularity at the Mott transition into two types, namely, one characterized by the charge mass divergence and the other by the vanishment of the carrier number. This difference originates from the number of components. Its variety is further explored by examining various cases such as strongly correlated fermions, hard core bosons and boson t - J models. Critical properties to other incompressible states such as spin gap states and the fractional quantum Hall state are also discussed from the same viewpoint. Relevance of our analyses to the understandin...

Spin-gap state and superconducting correlations in a one-dimensional dimerized t-J model.

The phase diagram of one-dimensional dimerized t-J model is investigated by exact-diagonalization, quantum tranfer-matrix, and quantum Monte Carlo methods. The spin gap persists near but away from half-filling as well as in a strongly dimerized region, while the charge can be described as a Tomonaga-Luttinger liquid in the entire region. The spin-gap region extends to the weak-dimerization region as well as to the Haldane-gap region. Critical exponents of various correlation functions are discussed. The pairing correlation shows remarkable enhancement and by far dominates over other correlations near half-filling. At quarter filling, charge-gap formation is observed.

Localized states and conductivity in silicon amorphized by ion implantation

Taking into account the connection between spin density and gap state density in amorphous silicon (a-Si), the EPR data are evisaged aimed at controlling the density of localized states with different radiation and heat treatment of Si amorphized by ion implantation. Results are considered on thermal annealing of ion-implanted Si layers, the interaction between hydrogen and dangling bonds which are responsible for localized states in a-Si, as well as the interaction between transition elements (Cr, Mn, Cu, and Zn) and dangling bonds. The data on combined irradiation are considered too: firstly the group III or V ion implantation is performed to Si (or a-Si) and then the electron beam irradiation conducted. Some ways of doping a-Si are discussed. One of the ways consists in the group III or V ion implantation to high doses (> 1016 cm−2). The other way is the combined irradiation of a-Si. [Russian Text Ignored]