Value Of Spin Gap Research Articles

Thermal properties of rung-disordered two-leg quantum spin ladders: Quantum Monte Carlo study.

A two-leg quenched random bond disordered antiferromagnetic spin-1/2 Heisenberg ladder system is investigated by means of stochastic series expansion quantum Monte Carlo (QMC) method. Thermal properties of the uniform and staggered susceptibilities, the structure factor, the specific heat, and the spin gap are calculated over a large number of random realizations in a wide range of disorder strength. According to our QMC simulation results, the considered system has a special temperature point at which the specific heat takes the same value regardless of the strength of the disorder. Moreover, the uniform susceptibility is shown to display the same character except for a small difference in the location of the special point. Finally, the spin gap values are found to decrease with increasing disorder parameter and the smallest gap value found in this study is well above the weak coupling limit of the clean case.

Spin Gap in β-TeVO4: a Quantum Monte Carlo Study

In this paper, we proposed a two-leg spin ladder model for the description of magnetic properties of the β-TeVO4 compound. Quantum Monte Carlo (QMC) simulation was applied to describe the temperature-dependent magnetic susceptibility data for low temperatures. The two-leg spin ladder model presents a spin gap, and we suggests that β-TeVO4 compound presents such a spin gap also, and therefore, the model proposed here can be experimentally tested by measuring the spin gap of the compound. The susceptibility phase diagram has a rounded peak in the vicinity of T ≈ 12.2 K and obeys Troyer’s law for low temperatures and Curie’s law for high temperatures. We also study the susceptibility diagram in low temperatures and found the spin gap Δ = 8.06 K. The linearization of the equation for susceptibility in low temperatures allows us to obtain the spin gap value, and such a linearization was made with the data from the QMC simulation. In all the results, there is a very good agreement with the experimental data. We also show that the spin gap is null and the susceptibility is proportional to T for low temperatures when relatively high values of the ladders’ coupling is considered. The theoretical results are compared with other studies as well as applied to describe the susceptibility phase diagram of consolidated spin ladder compound, C9H18N2CuBr4.

Magnetism in spin crossover systems: Short-range order and effects beyond the Heisenberg model

To study non-Heisenberg effects in the vicinity of spin crossover in strongly correlated electron systems we derive an effective low-energy Hamiltonian for the two-band Kanamori model. It contains Heisenberg high-spin term proportional to exchange constant as well as low-spin term proportional to spin gap parameter $\varepsilon_s$. Using cluster mean field theory we obtain several non-Heisenberg effects. Near critical value of spin gap $\varepsilon^c_s$ there is a magnetic phase transition of first order. In the vicinity of $\varepsilon^c_s$ in the paramagnetic phase we observe non trivial behavior of the Curie constant in the paramagnetic susceptibility in the wide range of temperature. Reentrant temperature behavior of nearest-neighbor spin-spin correlations is observed at $\varepsilon_s > \varepsilon^c_s$. Finally, pressure-temperature magnetic phase diagram for ferroperriclase is obtained using the effective Hamiltonian.

Antikink dispersions of the J1–J2 sawtooth spin-1/2 anisotropic Heisenberg antiferromagnetic chain

Antikink dispersion of the asymmetric $J_1$-$J_2$ sawtooth spin-1/2 anisotropic Heisenberg antiferromagnetic chain has been derived. Value of spin-gap is obtained by estimating the minimum of dispersion relation variationally. The exact doubly-degenerate ground state energy has been derived at the symmetric point, $J_1=J_2$, for the whole anisotropic regime. Analytic form of dispersion relations of three different antikink states is obtained and their validity in the parameter space is discussed. The value of spin-gap, specific heat and susceptibility are estimated numerically by diagonalizing the Hamiltonian for chains of finite length. The inherent frustration of this antiferromagnetic model leads to the appearance of additional peak at low-temperatures in the specific heat.

First-principle study of the electronic, magnetic and structural characteristics of the Mn2CoAs1−xAlx (x = 0,0.25,0.50,0.75) Heusler alloys

First-principles density functional theory approach is adopted to determine the electronic, magnetic and structural characteristics of the Mn2CoAs1−xAlx (x = 0,0.25,0.50,0.75) Heusler alloys. The computations are carried out by WIEN2k code based on full-potential linearized augmented plane wave method (FP-LAPW). Moreover, the exchange-correlation energy functional is treated at the level of the generalized gradient approximation (GGA). Analysis of our computed results of the electronic band structure, as well as the density of states of the Mn2CoAs compound, show it a stable and half-metallic material with an energy band gap value of 0.48 eV. The calculated spin gap values are: 0.627 eV, 0.22 eV and 0.188 eV for Mn2CoAs0.75Al0.25, Mn2CoAs0.50Al0.50 and Mn2CoAs0.25Al0.75 respectively. Furthermore, the calculated total magnetic moment of the Mn2CoAs (4 µB) is found to be in agreement with the Slater–Pauling rule. Thus, our calculations show the Mn2CoAs1−xAlx (x = 0, 0.25, 0.50, 0.75) Heusler alloys potential materials for near future applications in spintronic because of their half-metallic ferromagnetism property.

Exchange enhancement of quasiparticle and ESR spin-gap in symmetric and asymmetric narrow-gap quantum wells

We report a study of electron-electron (e-e) interaction effect on spin-gap in n-type narrow-gap quantum well (QW) heterostructures. By using the Hartree-Fock approximation (HFA) and generalized single-mode approximation (GSMA) based on the 8-band k·p Hamiltonian we demonstrate that spin-orbit interaction and the mixing between |S>- and |P>-states in the conduction and valence bands affect significantly many-body corrections to the spin-gap in symmetric and asymmetric QWs in the integer and fractional Quantum Hall regime. The spin-gap values estimated for 2D electron gas (2DEG) in InAs/AlSb QWs are compared with the experimental results obtained by magnetotransport and by electrically detected electron spin resonance (ESR).

Syntheses, crystal structures, and magnetic properties for two spin-ladder complexes based on bis(2-thioxo-1,3-dithiole-4,5-dithiolato)nickelate monoanion building block

Two isostructural complexes based on bis(2-thioxo-1,3-dithiole-4,5-dithiolato)nickelate monoanion ([Ni(Dmit)2]−) building block, (C19H11NNiF3S10 (I) and C18H10N2F3NiS10 (II)), have been synthesized and characterized structurally and magnetically. Two complexes crystallize in monoclinic space group \(P\bar 1\) with very similar cell parameters and packing structures. The planar [Ni(Dmit)2]− monoanions form face-to-face π-type dimer, and the neighboring dimers are aligned into a ladder-type arrangement via lateral-to-lateral S…S contacts along x axis in the crystals of I and II. Two complexes exhibit similar magnetic behaviors, and their magnetic susceptibility data can be fitted to a Heisenberg antiferromagnetic two-legged spin ladder formula in high-temperature region and reproduced well by the Troyer expression in low-temperature region. The spin-gap values, obtained, respectively, from the fits of magnetic susceptibility data in high- and low-temperature regions, are closed to each other, indicating the chose magnetic exchange model and the results of fits are reasonable.

Magnetic order, magnetic correlations, and spin dynamics in the pyrochlore antiferromagnet Er2Ti2O7

Er${}_{2}$Ti${}_{2}$O${}_{7}$ is believed to be a realization of an $\mathit{XY}$ antiferromagnet on a frustrated lattice of corner-sharing regular tetrahedra. It is presented as an example of the order-by-disorder mechanism in which fluctuations lift the degeneracy of the ground state, leading to an ordered state. Here we report detailed measurements of the low-temperature magnetic properties of Er${}_{2}$Ti${}_{2}$O${}_{7}$, which displays a second-order phase transition at ${T}_{\mathrm{N}}\ensuremath{\simeq}1.2$ K with coexisting short- and long-range orders. Magnetic susceptibility studies show that there is no spin-glass-like irreversible effect. Heat capacity measurements reveal that the paramagnetic critical exponent is typical of a 3-dimensional $\mathit{XY}$ magnet while the low-temperature specific heat sets an upper limit on the possible spin-gap value and provides an estimate for the spin-wave velocity. Muon spin relaxation measurements show the presence of spin dynamics in the nanosecond time scale down to 21 mK. This time range is intermediate between the shorter time characterizing the spin dynamics in Tb${}_{2}$Sn${}_{2}$O${}_{7}$, which also displays long- and short-range magnetic order, and the time scale typical of conventional magnets. Hence the ground state is characterized by exotic spin dynamics. We determine the parameters of a symmetry-dictated Hamiltonian restricted to the spins in a tetrahedron, by fitting the paramagnetic diffuse neutron scattering intensity for two reciprocal lattice planes. These data are recorded in a temperature region where the assumption that the correlations are limited to nearest neighbors is fair.

Singlet–Triplet Excitations and High-Field Magnetization in CuTe2O5

By measuring the THz electron spin resonance (ESR) transmission spectra and high-field magnetization on the spin-gapped system CuTe$_2$O$_5$, we identified the singlet-triplet excitations in the dimerized non-magnetic ground state. The determined spin-gap value of $h\nu_0=4.94$ meV at the $\Gamma$ point ($\mathbf{Q}\simeq\mathbf{0}$) is significantly smaller than the strongest antiferromagnetic exchange interaction between the Cu ions predicted by theoretical investigations. We also observed the critical field $H_{c1}^{a^*}=37.6$ T for \textbf{H} $\bot$ \emph{bc}-plane and $H_{c1}^{bc}=40.6$ T for \textbf{H} $\|$ \emph{bc}-plane at the onset of non-zero magnetization, consistent with the gap value and corresponding anisotropic \emph{g}-factors determined previously. The observed singlet-triplet excitations in Faraday and Voigt configurations suggest a mixing of the singlet state with the $S_z=0$ triplet state and the $S_z=\pm 1$ triplet states, respectively, due to the Dzyaloshinskii-Moriya (DM) interaction with a DM vector perpendicular to the crystalline \emph{bc}-plane.

Quantum Hall states under conditions of vanishing Zeeman energy

We report on magneto-transport measurements of a two-dimensional electron gas confined in a Cd$_{0.997}$Mn$_{0.003}$Te quantum well structure under conditions of vanishing Zeeman energy. The electron Zeeman energy has been tuned via the $s-d$ exchange interaction in order to probe different quantum Hall states associated with metallic and insulating phases. We have observed that reducing Zeeman energy to zero does not necessary imply the disappearing of quantum Hall states, i.e. a closing of the spin gap. The spin gap value under vanishing Zeeman energy conditions is shown to be dependent on the filling factor. Numerical simulations support a qualitative description of the experimental data presented in terms of a crossing or an avoided-crossing of spin split Landau levels with same orbital quantum number $N$.

Spin gap behavior inCu2Sc2Ge4O13studied usingSc45nuclear magnetic resonance

We report the results of a $^{45}$Sc nuclear magnetic resonance (NMR) study on the quasi-one-dimensional compound Cu$_2$Sc$_2$Ge$_4$O$_{13}$ at temperatures between 4 and 300 K. This material has been a subject of current interest due to indications of spin gap behavior. The temperature-dependent NMR shift exhibits a character of low-dimensional magnetism with a negative broad maximum at $T_{max}$ $\simeq $ 170 K. Below $% T_{max}$, the NMR shifts and spin lattice relaxation rates clearly indicate activated responses, confirming the existence of a spin gap in Cu$_2$Sc$_2$Ge% $_4$O$_{13}$. The experimental NMR data can be well fitted to the spin dimer model, yielding a spin gap value of about 275 K which is close to the 25 meV peak found in the inelastic neutron scattering measurement. A detailed analysis further points out that the nearly isolated dimer picture is proper for the understanding of spin gap nature in Cu$_2$Sc$_2$Ge$_4$O$_{13}$.

Understanding low-energy magnetic excitations and hydrogen bonding inVOHPO4∙12H2O

We report the variable temperature vibrational properties of single crystals of the $S=1∕2$ Heisenberg antiferromagnet $\mathrm{V}\mathrm{O}\mathrm{H}\mathrm{P}{\mathrm{O}}_{4}∙\frac{1}{2}{\mathrm{H}}_{2}\mathrm{O}$. A pair of peaks in the far infrared spectral response may be due to magnetic excitations. We invoke a dynamic Dzyaloshinskii-Moriya mechanism to explain the activation and polarization dependence of the singlet-to-triplet gap in the far infrared, and we identify the low-energy phonons that likely facilitate this coupling. The spin-gap values are compared to those obtained via magnetic susceptibility, electron spin resonance, and neutron scattering. Vibrational mode splitting in $\mathrm{V}\mathrm{O}\mathrm{H}\mathrm{P}{\mathrm{O}}_{4}∙\frac{1}{2}{\mathrm{H}}_{2}\mathrm{O}$ indicates a weak local symmetry breaking near $180\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and the low-temperature redshift of $\mathrm{V}\mathrm{O}$ and $\mathrm{H}\mathrm{O}$ related modes demonstrates enhanced low-temperature hydrogen bonding. The low lattice symmetry is important for the proposed magnetoelastic interactions.

Stripes and dimensional crossovers in high- Tc cuprates

The key problem in high T c physics is whether the doping is inhomogeneous and holes are expelled into the 1D domains (“stripes”). Direct comparison of the resistivity vs T curves of the 1D spin-ladder cuprates and underdoped cuprates below the pseudogap temperature T o shows a remarkable similarity which implies that the main scattering mechanism is the same for these two classes of cuprates and therefore in the pseudogap regime the transport in underdoped cuprates is essentially 1D. All the resistivity curves in underdoped cuprates (including high field transport data in fields up to 60 T) can be fitted by the model of the 1D quantum transport with the inelastic length in charge stripes being fully controlled by the magnetic spin correlation length in even-chain spin-ladders formed in the Cu-O plane as a result of inhomogeneous doping. Close to the insulator - metal transition disorder strongly influences the formation of stripes. Knight shift data are also successfully interpreted by taking the same spin-gap value as the one found from resistivity data. The possible dimensional crossovers observed with decreasing temperature at T o(x) (2D-1D) and T c(x) (1D–2D) are discussed and the main unsolved problems of the evolution of the normal state properties with doping are outlined.

Bilayer exchange coupling and neel temperature of YBa2Cu3O6.2

The present paper attempts to study the Neel temperature of bilayer antiferromagnetic cuprate YBa2Cu3O6.2 within anisotropic Heisenberg model. The double time Green’s function formalism within random phase approximation (RPA) has been used to obtain various correlation functions. The magnetization and the Neel temperature (TN) are evaluated. It is observed that the ratio of intrabilayer to inplane exchange coupling (r=J⊥/J‖) plays an important role in the magnetic dynamics of bilayer systems. The recent experimental data of bilayer system YBa2Cu3O6.2 have been used to estimate the ratio r from the expression for Neel temperature. The estimated values of spin gap and the ratio of hopping matrix elements t⊥/t‖ are found to be in fairly good agreement with the existing experimental results.

Spin gap state in α'-NaV2O5 studied by far infrared spectroscopy

We present far infrared transmission measurements of α'-NaV 2 O 5 single crystals. This material is known to undergo a spin-Peierls like transition at Tsp=35K. Below the transition temperature we observed together with expected new sharp phonon lines a strong gap-like depression of absorption occurring in the low energy part of a continuum likely connected with two-magnon excitations. The optical gap temperature dependence and its value closely match the spin gap values measured by neutron scattering experiments. Taking the optical gap as a fingerprint of the spin gap, we found that the relationship between the spin gap value and the dimerisation lattice parameter do not follow the scaling relation expected for the SP systems.

Coexistence of the spin liquid state and the magnetic ordering in the spin ladder compound; Sr 14− xCa xCu 24O 41

The magnetic ground state in the hole-doped two-leg ladder system Sr 2.5Ca 11.5Cu 24O 41 both at ambient pressure and under high pressure up to 3 GPa was investigated by a neutron scattering experiment. The spin gap value for this system at ambient pressure is found to be 32.1±0.3 meV, being almost the same as that of the parent material Sr 14Cu 24O 41. With increasing pressure, the spin gap energy is almost unchanged, however, the inelastic neutron scattering intensity at the zone center of the ladder is decreased progressively. We also observed antiferromagnetic long range ordering below T N(∼2.1 K), which was increased with increasing pressure. A possible magnetic structure of this system is also discussed. These results show the coexistence of the spin liquid state and antiferromagnetic ordering in the hole-doped two-leg ladder system.

Doped CuO 2 Planes in High T c Cuprates: 2D Or Not 2D ?

The key problem in physics of underdoped cuprates is whether the doping is inhomogeneous and holes are expelled into the 1D domains (“stripes”). Direct comparison of the resistivity vs T curves of the recently discovered 1D spin-ladder cuprates and underdoped cuprates below the pseudogap temperature T* shows a remarkable similarity which implies that the main scattering mechanism is the same for these two classes of cuprates and therefore in the pseudogap regime the transport in underdoped cuprates is essentially 1D. All the resistivity curves in underdoped cuprates can then be fitted by the model of the 1D quantum transport with the inelastic length in charge stripes being fully controlled by the magnetic spin correlation length in even-chain spin-ladders formed in the Cu-O plane as a result of inhomogeneous doping. Knight shift data are also successfully interpreted by taking the same spin-gap value as the one found from resistivity data.