ESR studies of X-irradiated strontium mixed calcium tartrate crystals

The electron spin resonance spectra of X-ray irradiated single crystals of strontium doped calcium tartrate tetrahydrate (CST) with molecular formula Ca0.88Sr0.12C4H4O6.4H(2)O grown in gels has been investigated. Only one species of free radical but with two magnetically unequivalent sites was observed at room temperature. The free radical was found to be the result of the splitting of a C-II bond adjacent to both the hydroxyl and carboxyl groups. The a factor was found to be slightly anisotropic. Couplings with two H nuclei, believed to be the proton of the OH group attached directly to the unsaturated asymmetric carbon atom and the proton attached directly to the: other asymmetric carbon atom of the molecule were observed. The principal g-values were found to be 2.0030, 2.0017, 2.0027. The principal elements of the nuclear coupling are 7.45, 6.59, 4.28 and 8.56, 7.22, 18.71 G, respectively. The radical was found to be very stable. (C) 2000 Elsevier Science Ltd. All rights reserved.

An electron paramagnetic resonance study of Pr0.6Ca0.4MnO3 across the charge-ordering transition

We report the first electron paramagnetic resonance studies of single crystals and powders of Pr0.6Ca0.4MnO3 in the 300-4.2 K range, covering the charge-ordering transition (Tco) at ~240 K and antiferromagnetic transition (TN) at ~170 K. The asymmetry parameter for the Dysonian single-crystal spectra shows an anomalous increase at Tco. Below Tco the g-value increases continuously, suggesting a gradual strengthening of the orbital ordering. The linewidth undergoes a sudden increase at Tco and continues to increase down to TN. The intensity increases as the temperature is decreased until Tco is reached, due to the renormalization of the magnetic susceptibility arising from the build-up of ferromagnetic correlations.

Chlorine-35 nuclear quadrupole resonance study of molecular torsional motion in some chloropyridine-N-oxides and chloroquinoline-N-oxides

Chlorine has been substituted at the 2- and 4-positions in the pyridine and quinoline rings of the corresponding N-oxides and 35Cl n.q.r. spectra have been studied in the temperature range 77–300 K. The change in the n.q.r. frequencies in N-oxides as compared to their parent compounds are interpreted in terms of the conjugative effect and the inductive effect of the N+—O– group. The negative temperature coefficients of the resonance frequencies in chloropyridine-N-oxides have been analysed using the Bayer, Kushida and Brown equations. The calculated torsional frequencies, which are in the range 52–78 cm–1, are found to be only slightly temperature dependent.

Neel order, quantum spin liquids, and quantum criticality in two dimensions

This paper is concerned with the possibility of a direct second-order transition out of a collinear Neel phase to a paramagnetic spin liquid in two-dimensional quantum antiferromagnets. Contrary to conventional wisdom, we show that such second-order quantum transitions can potentially occur to certain spin liquid states popular in theories of the cuprates. We provide a theory of this transition and study its universal properties in an epsilon expansion. The existence of such a transition has a number of interesting implications for spin-liquid-based approaches to the underdoped cuprates. In particular it considerably clarifies existing ideas for incorporating antiferromagnetic long range order into such a spin-liquid-based approach.

Geometric quantum computation using fictitious spin-1/2 subspaces of strongly dipolar coupled nuclear spins

Geometric phases have been used in NMR to implement controlled phase shift gates for quantum-information processing, only in weakly coupled systems in which the individual spins can be identified as qubits. In this work, we implement controlled phase shift gates in strongly coupled systems by using nonadiabatic geometric phases, obtained by evolving the magnetization of fictitious spin-1/2 subspaces, over a closed loop on the Bloch sphere. The dynamical phase accumulated during the evolution of the subspaces is refocused by a spin echo pulse sequence and by setting the delay of transition selective pulses such that the evolution under the homonuclear coupling makes a complete 2 pi rotation. A detailed theoretical explanation of nonadiabatic geometric phases in NMR is given by using single transition operators. Controlled phase shift gates, two qubit Deutsch-Jozsa algorithm, and parity algorithm in a qubit-qutrit system have been implemented in various strongly dipolar coupled systems obtained by orienting the molecules in liquid crystal media.

Influence of lattice distortion on the Curie temperature and spin-phonon coupling in LaMn0.5Co0.5O3

Two distinct ferromagnetic phases of LaMn0.5Co0.5O3 having monoclinic structure with distinct physical properties have been studied. The ferromagnetic ordering temperature T-c is found to be different for both the phases. The origin of such contrasting characteristics is assigned to the changes in the distance(s) and angle(s) between Mn-O-Co resulting from distortions observed from neutron diffraction studies. Investigations on the temperature dependent Raman spectroscopy provide evidence for such structural characteristics, which affects the exchange interaction. The difference in B-site ordering which is evident from the neutron diffraction is also responsible for the difference in T-c. Raman scattering suggests the presence of spin-phonon coupling for both the phases around the T-c. Electrical transport properties of both the phases have been investigated based on the lattice distortion.

A pulsed field gradient spin echo NMR spectrometer for diffusion coefficient measurements

A pulsed field gradient spin echo NMR spectrometer has been assembled by interfacing a programmable pulse generator and a data acquisition system designed and fabricated in our laboratory with other imported units. Calibration results of the magnetic field gradients are presented.

Contactless conductivity of nanoparticles from electron magnetic resonance lineshape analysis

A contactless method to determine the electrical conductivity of nanoparticles is presented. It is based on the lineshape analysis of electron magnetic resonance signals which are `Dysonian' for conducting samples of sizes larger than the skin depth. The method is validated bymeasurements on a bulk sample of La0.67Sr0.33MnO3 where it gives values close to those obtained from direct measurement of conductivity and is then used to determine the conductivity of nanoparticles of La0.67Sr0.33MnO3 dispersed in polyvinyl alcohol as a function of temperature. (C) 2010 Elsevier Ltd. All rights reserved.

Field induced spin reorientation transition in epitaxial La0.5Sr0.5CoO3 films

Strained epitaxial La0.5Sr0.5CoO3 films are grown on LaAlO3 substrate. Structural, electrical,and magnetic measurements were carried out. Out of plane lattice parameter of the film undergoes compressive strain and the coercivity is enhanced. The zero field cooled (ZFC) magnetization curve for a field applied parallel to the film plane shows a jump, which suggests a spin reorientation transition (SRT), while ZFC magnetization for a field applied perpendicular to the film plane is featureless. This jump in magnetization is shifted to higher temperatures when the magnetic field is reduced. The SRT is attributed to the strain in the film. (C) 2010 Elsevier B.V. All rights reserved.

Enantiodiscrimination and extraction of short and long range homo- and hetero-nuclear residual dipolar couplings by a spin selective correlation experiment

A two dimensional correlation experiment for the measurement of short and long range homo- and hetero- nuclear residual dipolar couplings (RDCs) from the broad and featureless proton NMR spectra including C-13 satellites is proposed. The method employs a single natural abundant C-13 spin as a spy nucleus to probe all the coupled protons and permits the determination of RDCs of negligible strengths. The technique has been demonstrated for the study of organic chiral molecules aligned in chiral liquid crystal, where additional challenge is to unravel the overlapped spectrum of enantiomers. The significant advantage of the method is demonstrated in better chiral discrimination using homonuclear RDCs as additional parameters. (C) 2010 Elsevier B.V. All rights reserved.

Electron paramagnetic resonance evidence for Jahn-Teller glasses

Single crystal E.P.R. studies of copper as a dopant in lithium potassium sulphate, lithium ammonium sulphate and lithium sodium sulphate have been carried out from room temperature down to 77K. The three Jahn-Teller (JT) systems behave very similarly to one another. The room temperature dynamic JT spectra with giso = 2·19 ± 0·01 and Aiso = ±(33 ± 4) times 10-4 cm-1 transform around 247 K to spectra characterized by randomly frozen-in axial strains with g‖ = 2·4307 ± 0·0005, g⊥ = 2·083 ± 0·001, A‖ = ±(116 ± 2) times 10-4 cm-1 and A⊥ = ∓(14 ± 4) times 10-4 cm-1. We proposed that the low temperature phase (below 247 K) of each of these systems provides an example of a Jahn-Teller glass.

Magnetic resonance imaging and ultrasonography in brachial plexus birth injury

Brachial plexus birth injury (BPBI) is caused by stretching, tearing or avulsion of the C5-C8 or Th1 nerve roots during delivery. Foetal-maternal disproportion is the main reason for BPBI. The goal of this study was to find out the incidence of posterior subluxation of the humeral head during first year of life in BPBI and optimal timing of the ultrasonographic screening of the glenohumeral joint. The glenohumeral congruity and posterior subluxation of the humeral head associated to muscle atrophy were assessed and surgical treatment of the shoulder girdle as well as muscle changes in elbow flexion contracture were evaluated. The prospective, population based part of the study included all neonates born in Helsinki area during years 2003-2006. Patients with BPBI sent to the Hospital for Children and Adolescents because of decreased external rotation, internal rotation contracture or deformation of the glenohumeral joint as well as patients with elbow flexion contracture were also included in this prospective study. The incidence of BPBI was calculated to be 3.1/1000 newborns in Helsinki area. About 80% of the patients with BPBI recover totally during the follow-up within the first year of life. Permanent plexus injury at the age of one year was noted in 20% of the patients (0.64/1000 newborns). Muscle imbalance resulted in sonographically detected posterior subluxation in one third of the patients with permanent BPBI. If muscle imbalance and posterior subluxation are left untreated bony deformities will develop. All patients with internal rotation contracture of the glenohumeral joint presented muscle atrophy of the rotator cuff muscles. Especially subscapular and infraspinous muscles were affected. A correlation was found particularly between greatest thickness of subscapular muscle and subluxation of the humeral head, degree of glenoid retroversion, as well as amount of internal rotation contracture. Supinator muscle atrophy was evident among all the studied patients with elbow flexion contracture. Brachial muscle pathology seemed to be an important factor for elbow flexion contracture in BPBI. Residual dysfunction of the upper extremity may require operative treatment such as tendon lengthening, tendon transfers, relocation of the humeral head or osteotomy of the humerus. Relocation of the humeral head improved the glenohumeral congruency among patients under 5 years of age. Functional improvement without remodeling of the glenohumeral joint was achieved by other reconstructive procedures. In conclusion: Shoulder screening by US should be done to all patients with permanent BPBI at the age of 3 and 6 months. Especially atrophy of the subscapular muscle correlates with glenohumeral deformity and posterior subluxation of the humeral head, which has not been reported in previous studies. Permanent muscle changes are the main reason for diminished range of motion of the elbow and forearm. Relocation of the humeral head, when needed, should be performed under the age of 5 years.

First-principles analysis of electron correlation, spin ordering and phonons in the normal state of FeSe1-x

We present first-principles density-functional-theory-based calculations to determine the effects of the strength of on-site electron correlation, magnetic ordering, pressure and Se vacancies on phonon frequencies and electronic structure of FeSe1-x. The theoretical equilibrium structure (lattice parameters) of FeSe depends sensitively on the value of the Hubbard parameter U of on-site correlation and magnetic ordering. Our results suggest that there is a competition between different antiferromagnetic states due to comparable magnetic exchange couplings between first- and second-neighbor Fe sites. As a result, a short range order of stripe antiferromagnetic type is shown to be relevant to the normal state of FeSe at low temperature. We show that there is a strong spin-phonon coupling in FeSe (comparable to its superconducting transition temperature) as reflected in large changes in the frequencies of certain phonons with different magnetic ordering, which is used to explain the observed hardening of a Raman-active phonon at temperatures (similar to 100 K) where magnetic ordering sets in. The symmetry of the stripe antiferromagnetic phase permits an induced stress with orthorhombic symmetry, leading to orthorhombic strain as a secondary order parameter at the temperature of magnetic ordering. The presence of Se vacancies in FeSe gives rise to a large peak in the density of states near the Fermi energy, which could enhance the superconducting transition temperature within the BCS-like picture.