arXiv subjects

C. P. Landee

Publications and source records attributed to C. P. Landee.

29 records · Page 2Linked to original sources

arXiv2009

Quantum critical dynamics of a S = 1/2 antiferromagnetic Heisenberg chain studied by 13C-NMR spectroscopy

We present a 13C-NMR study of the magnetic field driven transition to complete polarization of the S=1/2 antiferromagnetic Heisenberg chain system copper pyrazine dinitrate Cu(C_4H_4N_2)(NO_3)_2 (CuPzN). The static local magnetization as well as the low-frequency spin dynamics, probed via the nuclear spin-lattice relaxation rate 1/T_1, were explored from the low to the high field limit and at temperatures from the quantum regime (k_B T << J) up to the classical regime (k_B T >> J). The experimental data show very good agreement with quantum Monte Carlo calculations over the complete range of parameters investigated. Close to the critical field, as derived from static experiments, a pronounced maximum in 1/T_1 is found which we interpret as the finite-temperature manifestation of a diverging density of zero-energy magnetic excitations at the field-driven quantum critical point.

cond-mat.str-el↗

arXiv2009

Quantum effects in a weakly-frustrated S=1/2 two-dimensional Heisenberg antiferromagnet in an applied magnetic field

We have studied the two-dimensional S=1/2 square-lattice antiferromagnet $Cu(pz)_2(ClO_4)_2$ using inelastic neutron scattering and series expansion calculations. We show that the presence of antiferromagnetic next-nearest neighbor interactions enhances quantum fluctuations associated with resonating valence bonds. Intermediate magnetic fields lead to a selective tuning of resonating valence bonds and a spectacular inversion of the zone-boundary dispersion, providing novel insight into 2D antiferromagnetism in the quantum limit.

cond-mat.str-el↗

arXiv2009

2D XY Behavior observed in quasi-2D quantum Heisenberg antiferromagnets

The magnetic properties of a new family of molecular-based quasi-two dimension $S=1/2$ Heisenberg antiferromagnets are reported. Three compounds, ($Cu(pz)_2(ClO_4)_2$, $Cu(pz)_2(BF_4)_2$, and $[Cu(pz)_2(NO_3)](PF_6)$) contain similar planes of Cu$^{2+}$ ions linked into magnetically square lattices by bridging pyrazine molecules (pz = $C_4H_4N_2$). The anions provide charge balance as well as isolation between the layers. Single crystal measurements of susceptibility and magnetization, as well as muon spin relaxation studies, reveal low ratios of Néel temperatures to exchange strengths ($4.25 / 17.5 = 0.243$, $3.80/15.3=0.248$, and $3.05/10.8=0.282$, respectively) while the ratio of the anisotropy fields $H_A$ (kOe) to the saturation field $H_\mathrm{SAT}$ (kOe) are small ($2.6/490 = 5.3\times10^{-3}$, $2.4/430=5.5\times10^{-3}$, and $0.07/300=2.3\times10^{-4}$, respectively), demonstrating close approximations to a 2D Heisenberg model. The susceptibilities of ClO$_4$ and BF$_4$ show evidence of an exchange anisotropy crossover (Heisenberg to $XY$) at low temperatures; their ordering transitions are primarily driven by the $XY$ behavior with the ultimate 3D transition appearing parasitically. The PF$_6$ compound remains Heisenberg-like at all temperatures, with its transition to the Néel state due to the interlayer interactions. Effects of field-induced anisotropy have been observed.

cond-mat.mtrl-sci↗

arXiv2007

Magnetic properties of the Haldane-gap material NENB

Results of magnetization and high-field ESR studies of the new spin-1 Haldane-chain material [Ni(C2H8N2)2NO2](BF4) (NENB) are reported. A definite signature of the Haldane state in NENB was obtained. From the analysis of the frequency-field dependence of magnetic excitations in NENB, the spin-Hamiltonian parameters were calculated, yielding Delta = 17.4 K, g_parallel = 2.14, D = 7.5 K, and |E| = 0.7 K for the Haldane gap, g factor and the crystal-field anisotropy constants, respectively. The presence of fractional S = 1/2 chain-end states, revealed by ESR and magnetization measurements, is found to be responsible for spin-glass freezing effects. In addition, extra states in the excitation spectrum of NENB have been observed in the vicinity of the Haldane gap, which origin is discussed.

cond-mat.other↗

arXiv2006

Magnetic order in the S=1/2 two-dimensional molecular antiferromagnet, copper pyrazine perchlorate Cu(Pz)_2(ClO_4)_2

We present an investigation of magnetic ordering in the two-dimensional S=1/2 quantum magnet Cu(Pz)_2(ClO_4)_2 using specific heat and zero field muon-spin relaxation (μ^+SR). The magnetic contribution to the specific heat is consistent with an exchange strength of 17.7(3) K. We find unambiguous evidence for a transition to a state of three-dimensional long range order below a critical temperature T_N=4.21(1) K using μ^+SR even though there is no feature in the specific heat at that temperature. The absence of a specific heat anomaly at T_N is consistent with recent theoretical predictions. The ratio of T_N/J=0.24 corresponds to a ratio of intralayer to interlayer exchange constants of |J'/J|=6.8x10^-4, indicative of excellent two-dimensional isolation. The scaled magnetic specific heat of [Cu(Pz)_2(HF_2)]BF_4, a compound with an analogous structure, is essentially identical to that of Cu(Pz)_2(ClO_4)_2 although both differ slightly from the predicted value for an ideal 2D S=1/2 Heisenberg antiferromagnet.

cond-mat.str-el↗

arXiv2006

Neutron scattering from a coordination polymer quantum paramagnet

Inelastic neutron scattering measurements are reported for a powder sample of the spin-1/2 quantum paramagnet $\rm Cu(Quinoxaline)Br_2$. Magnetic neutron scattering is identified above an energy gap of 1.9 meV. Analysis of the sharp spectral maximum at the onset indicates that the material is magnetically quasi-one-dimensional. Consideration of the wave vector dependence of the scattering and polymeric structure further identifies the material as a two-legged spin-1/2 ladder. Detailed comparison of the data to various models of magnetism in this material based on the single mode approximation and the continuous unitary transformation are presented. The latter theory provides an excellent account of the data with leg exchange $J_{\parallel}=2.0$ meV and rung exchange $J_{\perp}=3.3$ meV.

cond-mat.str-el↗

arXiv2005

Magnetic order in the quasi-one-dimensional spin 1/2 chain, copper pyrazine dinitrate

We present the first evidence of magnetic order in the quasi-one-dimensional spin 1/2 molecular chain compound, copper pyrazine dinitrate Cu(C4H4N2)(NO3)2}. Zero field muon-spin relaxation measurements made at dilution refrigerator temperatures show oscillations in the measured asymmetry, characteristic of a quasistatic magnetic field at the muon sites. Our measurements provide convincing evidence for long range magnetic order below a temperature T_N=107(1) mK. This leads to an estimate of the interchain coupling constant of |J'|/k_B=0.046 K and to a ratio |J'/J| = 4.4 x 10^-3.

cond-mat.str-el↗

arXiv2003

Extended quantum critical phase in a magnetized spin-1/2 antiferromagnetic chain

Measurements are reported of the magnetic field dependence of excitations in the quantum critical state of the spin S=1/2 linear chain Heisenberg antiferromagnet copper pyrazine dinitrate (CuPzN). The complete spectrum was measured at k_B T/J <= 0.025 for H=0 and H=8.7 Tesla where the system is ~30% magnetized. At H=0, the results are in quantitative agreement with exact calculations of the dynamic spin correlation function for a two-spinon continuum. At high magnetic field, there are multiple overlapping continua with incommensurate soft modes. The boundaries of these continua confirm long-standing predictions, and the intensities are consistent with exact diagonalization and Bethe Ansatz calculations.

cond-mat.str-el↗

arXiv2001

New two dimensional S=1/2 Heisenberg antiferromagnets: synthesis, structure and magnetic properties

The magnetic susceptibility and magnetization of two new layered S=1/2 Heisenberg antiferromagnets with moderate exchange are reported. The two isostructural compounds, (2-amino-5-chloropyridinium)2CuBr4 ((5CAP)2CuBr4) and (2-amino-5-methylpyridinium)2CuBr4 ((5MAP)2CuBr4), contain S=1/2, Cu(II) ions related by C-centering, yielding four equivalent nearest neighbors. The crystal structure of newly synthesized compound, (5CAP)2CuBr4, shows the existence of layers of distorted copper(II)-bromide tetrahedra parallel to the ab plane, separated by the organic cations along the c axis. Magnetic pathways are available through the bromide-bromide contacts within the layers and provide for moderate antiferromagnetic exchange. Susceptibility measurements indicate interaction strengths to be 8.5(1) K, and 6.5(1) K and ordering temperatures of 5.1(2) K and 3.8(2) K for (5CAP)2CuBr4 and (5MAP)2CuBr4 respectively. High field magnetization experiments on both compounds show upward curvature of M(H,T). Magnetization measurements made at T = 1.3 K show saturation occurs in (5MAP)2CuBr4 at 18.8 T and in (5CAP)2CuBr4 at 24.1 T. The magnetization curves are consistent with recent theoretical predictions. Single crystal magnetization measured at 2.0 K indicate a spin flop transition at 0.38 T and 0.63 T for (5CAP)2CuBr4 and (5MAP)2CuBr4 respectively.

cond-mat.mtrl-sci↗

arXiv2000

Magnetic properties of a new molecular-based spin-ladder system: (5IAP)2CuBr4*2H2O

We have synthesized and characterized a new spin-1/2 Heisenberg antiferromagnetic ladder: bis 5-iodo-2-aminopyridinium tetrabromocuprate(II) dihydrate. X-ray diffraction studies show the structure of the compound to consist of well isolated stacked ladders and the interaction between the Cu(2+) atoms to be due to direct Br...Br contacts. Magnetic susceptibility and magnetization studies show the compound to be in the strong-coupling limit, with the interaction along the rungs (J' ~ 13 K) much greater than the interaction along the rails (J ~ 1 K). Magnetic critical fields are observed near 8.3 T and 10.4 T, respectively, establishing the existence of the energy gap.

cond-mat↗

arXiv1998

Characterization of a Quasi-One Dimensional Spin-1/2 Magnet which is Gapless and Paramagnetic for g mu_B H <= J and k_B T << J

High field mangetization, field-dependent specific heat measurements, and zero field inelastic magnetic neutron scattering have been used to explore the magnetic properties of copper pyrazine dinitrate (Cu(C4H4N2)(NO3)2). The material is an ideal one-dimensional spin-1/2 Heisenberg antiferromagnet with nearest neighbor exchange constant J=0.90(1) meV and chains extending along the orthorhombic a-direction. As opposed to previosly studied molecular-based spin-1/2 magnetic systems, coppyer pyrazine dinitrate remains gapless and paramagnetic for g mu_B H/J at least up to 1.4 and for k_B T/J at least down to 0.03 this makes the material an excellent model system for exploring the T=0 critical line which is expected in the H - T phase diagram on the one-dimensional spin-1/2 Heisenberg antiferromagnet. As a first example of such a study we present accurate measurements of the Sommerfeld constant of the spinon gas versus g mu_B H/J < 1.4 which reveal a decrease of the averages spinon velocity by 32% in that field range. The results are in excellent agreement with numerical calculations based on the Bethe ansatz with no adjustable parameters.

cond-mat.str-el↗