One-dimensional Spin Dynamics Research Articles

Role of generic scale invariance in a Mott transition from a U(1) spin-liquid insulator to a Landau Fermi-liquid metal

We investigate the role of generic scale invariance in a Mott transition from a U(1) spin-liquid insulator to a Landau Fermi-liquid metal, where there exist massless degrees of freedom in addition to quantum critical fluctuations. Here, the Mott quantum criticality is described by critical charge fluctuations, and additional gapless excitations are U(1) gauge-field fluctuations coupled to a spinon Fermi surface in the spin-liquid state, which turn out to play a central role in the Mott transition. An interesting feature of this problem is that the scaling dimension of effective leading local interactions between critical charge fluctuations differs from that of the coupling constant between U(1) gauge fields and matter-field fluctuations in the presence of a Fermi surface. As a result, there appear dangerously irrelevant operators, which can cause conceptual difficulty in the implementation of renormalization group (RG) transformations. Indeed, we find that the curvature term along the angular direction of the spinon Fermi surface is dangerously irrelevant at this spin-liquid Mott quantum criticality, responsible for divergence of the self-energy correction term in U(1) gauge-field fluctuations. Performing the RG analysis in the one-loop level based on the dimensional regularization method, we reveal that such extremely overdamped dynamics of U(1) gauge-field fluctuations, which originates from the emergent one-dimensional dynamics of spinons, does not cause any renormalization effects to the effective dynamics of both critical charge fluctuations and spinon excitations. However, it turns out that the coupling between U(1) gauge-field fluctuations and both matter-field excitations still persists at this Mott transition, which results in novel mean-field dynamics to explain the nature of the spin-liquid Mott quantum criticality. We discuss physical implications of effective one-dimensional spin dynamics and extremely overdamped gauge dynamics at the Mott quantum criticality.

Spin-liquid Mott quantum criticality in two dimensions: Destabilization of a spinon Fermi surface and emergence of one-dimensional spin dynamics

Resorting to a recently developed theoretical device called dimensional regularization for quantum criticality with a Fermi surface, we examine a metal-insulator quantum phase transition from a Landau's Fermi-liquid state to a U(1) spin-liquid phase with a spinon Fermi surface in two dimensions. Unfortunately, we fail to approach the spin-liquid Mott quantum critical point from the U(1) spin-liquid state within the dimensional regularization technique. Self-interactions between charge fluctuations called holons are not screened, which shows a run-away renormalization group flow, interpreted as holons remain gapped. This leads us to consider another fixed point, where the spinon Fermi surface can be destabilized across the Mott transition. Based on this conjecture, we reveal the nature of the spin-liquid Mott quantum critical point: Dimensional reduction to one dimension occurs for spin dynamics described by spinons. As a result, Landau damping for both spin and charge dynamics disappear in the vicinity of the Mott quantum critical point. When the flavor number of holons is over its critical value, an interacting fixed point appears to be identified with an inverted XY universality class, controlled within the dimensional regularization technique. On the other hand, a fluctuation-driven first order metal-insulator transition results when it is below the critical number. We propose that the destabilization of a spinon Fermi surface and the emergence of one-dimensional spin dynamics near the spin-liquid Mott quantum critical point can be checked out by spin susceptibility with a $2 k_{F}$ transfer momentum, where $k_{F}$ is a Fermi momentum in the U(1) spin-liquid state: The absence of Landau damping in U(1) gauge fluctuations gives rise to a divergent behavior at zero temperature while it vanishes in the presence of a spinon Fermi surface.

NMR evidence for antiferromagnetic transition in the single-component molecular system [Cu(tmdt)2

The magnetic state of the single-component molecular compound [Cu(tmdt)${}_{2}$], where tmdt indicates tetramethylene tetrathiafulvalene dithiolate, is investigated by means of ${}^{1}$H NMR. An abrupt spectral broadening below 13 K and a sharp peak in the nuclear spin-lattice relaxation rate ${T}_{1}^{\ensuremath{-}1}$ at 13 K are observed as clear manifestations of a second-order antiferromagnetic transition, which is consistent with the previously reported magnetic susceptibility and electron paramagnetic resonance measurements. The ordered moment is estimated at $0.22{\ensuremath{\mu}}_{B}$--$0.45{\ensuremath{\mu}}_{\mathrm{B}}$ per molecule. The temperature dependence of ${T}_{1}^{\ensuremath{-}1}$ above the transition temperature indicates one-dimensional spin dynamics and supports the theory that the spins are on the central part of this molecule, unlike other isostructural compounds.

Pyrophosphate-Bridged CuII Chain Magnet: {[Na3Cu(P2O7)(NO3)]·3H2O}n

A Cu(II)...Cu(II) pyrophosphate-bridged compound of formula {[Na(3)Cu(P(2)O(7))(NO(3))].3H(2)O}(n) (1) has been characterized. X-ray diffraction measurements show that it crystallizes in the monoclinic space group P2(1)/m, with unit cell dimensions a = 7.2492(5) A, b = 8.2446(6) A, c = 9.9050(7) A, beta = 107.123(1) degrees, and Z = 2. The structure consists of chains of Cu(II) cations at inversion symmetry sites bound to four equatorial oxygen atoms provided by two pyrophosphate anions halved by a symmetry plane and two axial oxygen atoms of nitrate anions. The molar magnetic susceptibility chi(0) of a powdered sample was measured in the temperature range 2 K < T < 273 K, and an isothermal magnetization curve, M(B(0),T), was obtained at T = 30 K, with the magnetic field B(0) between 0 and 5 T. Fitting a spin-chain model to the susceptibility data, we evaluate an antiferromagnetic exchange coupling 2J = -24.3(1) cm(-1) (defined as H(ex) = -2JS(i)S(j)) between Cu(II) neighbors. For any orientation of B(0), single-crystal electron paramagnetic resonance (EPR) spectra obtained at 9.8 and 33.9 GHz at 300 K display a single signal having a g matrix with orthorhombic symmetry, arising from the merger produced by the exchange interaction of the resonances corresponding to the two rotated Cu(II) sites. The g matrices of the individual molecules calculated assuming axial symmetry yielded principal values g(parallel) = 2.367(1) and g(perpendicular) = 2.074(1) at both frequencies, indicating a d(x(2)-y(2)) ground-state orbital for the Cu(II) ions. The angular variation of the EPR line width suggests exchange narrowing in a system with one-dimensional spin dynamics, as expected from the structure and susceptibility data. The results, discussed in terms of the crystal and electronic structures and of the spin dynamics of the compound, are compared with those obtained in other materials.

One-dimensional spin dynamics in a halogen-bridged mixed-valence palladium (II), (IV) complex studied by 1H NMR

The 1H NMR spin—lattice relaxation time T 1 of [Pd(chxn) 2][PdBr 2(chxn) 2]Br 4 (chxn = 1R,2R-cyclohexanediamine) in the solid state was measured between 90 K and room temperature at Larmor frequencies of 10–60 MHz. The presence of thermally activated diffusive spins formed on paramagnetic Pd III sites along the halogen-bridged 1D chain was revealed from the Larmor frequency dependence of T 1. The diffusion rates were estimated to be 2 × 10 13, 9 × 10 11, and 6 × 10 10 rad s −1 at 303, 198 and 125 K, respectively. The rate observed around room temperature showed an Arrhenius-type temperature dependence with an activation energy of ≈1700 K. It is shown that the diffusive spin is delocalized over several Pd sites. The observed spin motion is attributable to the neutral soliton by referring to the electrical conductivity data.

Effect of delocalization on one-dimensional spin dynamics

We discuss the combined effect of spin delocalization and one-dimensional (1D) diffusion on the hyperfine-controlled nuclear relaxation and EPR linewidth. We calculate the local-spin-correlation functions in the 1D random-walk model for a particle delocalized over several sites. At low frequency the divergent diffusive contribution is independent of the delocalization; however, the delocalization introduces a negative frequency-independent contribution which is proportional to the delocalization length.

Effect of interchain coupling on ultrasonic attenuation in nearly one-dimensional antiferromagnets

A quantitative theory is presented for the frequency dependence of the ultrasonic attenuation in the nearly one-dimensional antiferromagnet which has small, but not negligible, interchain couplings. At temperatures well above the Neel temperature TN, the ultrasonic attenuation alpha is described by a scaling function, alpha = omega 3G( omega t0), where t0 indicates the characteristic time after which the three-dimensional diffusion takes over. The theory is compared with the frequency dependence of the ultrasonic attenuation in CsNiCl3 (a one-dimensional Heisenberg antiferromagnet) and the effect of the interchain coupling on the one-dimensional spin dynamics is discussed.

Sidebands on EPR Lines in the Impure Heisenberg Linear Chain (CH3)4NMnCl3: Cu

The intensity and linewidth of the sidebands on EPR lines have been measured in the impure Heisenberg linear chain (CH 3 ) 4 NMn 1- x Cu x Cl 3 ( x <0.092). The intensity, compared with the pure chain ( x =0), is related to { Δ H p p m [ x ]/ Δ H p p m [0]} 3/2 , where Δ H p p m [ x ] is the peak-to-peak linewidth of the main Zeeman-resonance at concentration x for the applied field along the chain axis. This result is consistent with predictions of one-dimensional spin dynamics.

Concentration dependence of the electron-spin-resonance linewidth in(CH3)4NMn1−xCuxCl3

The electron-spin-resonance linewidth $\ensuremath{\Delta}H$ has been measured in ${(\mathrm{C}{\mathrm{H}}_{3})}_{4}$NMn${\mathrm{Cl}}_{3}$ (TMMC) samples having a fraction $x$ of ${\mathrm{Cu}}^{++}$ substituted for ${\mathrm{Mn}}^{++}$, with $x$ up to 0.22. For the applied field along the chain axis, $\ensuremath{\Delta}H$ increases with $x$ in a manner consistent with predictions of one-dimensional spin dynamics.

Comment on "Exchange Narrowing in One-Dimensional Systems"

Attention is drawn to the similarity in one-dimensional spin dynamics between the recently investigated inorganic salt ${(\mathrm{C}{\mathrm{H}}_{3})}_{4}$NMn${\mathrm{Cl}}_{3}$ (TMMC) [R. E. Dietz et al., Phys. Rev. Letters 26, 1186 (1971)] and a variety of organic charge-transfer and free-radical crystals previously studied by electron paramagnetic resonance. The theoretical description developed for the organic crystals based on the linear Heisenberg antiferromagnet is applicable also to the inorganic systems.