Low-Temperature Phase Transition Detected by ESR in ����-(BEDT-TTF)2IBr2 Single Crystals

Hysteresis of the electron spin resonance (ESR) spectra of localized charge carriers has been observed in ????-(BEDT-TTF)₂IBr₂ single crystals in the temperature range of 15?C30?K. The first-order phase transition has been detected by ESR components. Two lines observed in the ESR spectrum correspond to low-temperature and high-temperature phases.     

X-ray diffraction and magnetic susceptibility of sodium fullerides NaxC60

The X-ray diffraction (XRD), magnetic susceptibility and electron spin resonance (ESR) measurements have been carried out for Na_xC₆₀. The XRD profiles with x<4 can be assigned to a face-centered cubic (fcc) lattice, while those with 4≦x to a hexagonal one. The temperature dependence of magnetic susceptibility χ for Na_xC₆₀ using SQUID was fitted to the Curie law, and estimated temperature-independent component χ₀. The composition x dependence of the χ₀ for Na_xC₆₀ shows two maxima at around x=3 and x=10, and minimum at x=6. The absence of Pauli contribution at x=6 was confirmed using ESR. A trace of superconducting transition at 14 K has been found for some Na_xC₆₀ specimens with 8<x<9.     

Electron spin-lattice relaxation in noncommon metals YBa2Cu3Ox and Rb1C60

Using an original modulation technique, the electron spin-lattice relaxation have been investigated in two noncommon metals: YBa₂Cu₃O_x, high-T_c material doped with 1% Gd, and Rb₁C₆₀, linear polymer phase fulleride. In the first case, the Korringa-like temperature dependence of the Gd³⁺ longitudinal relaxation time T₁, is found forx = 6.59 in a wide temperature range 4.2 <T < 200 K, both above and below T_c = 56 K. Atx = 6.95 (T_c = 90 K), the T₁ behavior within 50 <T < 200 K is evidently affected by spin gap opening with the gap value of about 240 K. At 200 K, an unexpected acceleration of the relaxation rate takes place, suggesting some change in the relaxation mechanism. The data are discussed in terms of the Barnes-Plefke theory with allowance made for microscopic separation of the normal and superconducting phases. In Rb₁C₆₀, the evolution of the ESR line and relaxation rates have been studied accurately in the range of the metal-insulator transition (below 50 K). Interpretation is suggested which takes into account breaking down the relaxation bottleneck due to opening of the energy gap near the Fermi surface. The gap value of about 100 cm^?1 is estimated from the analysis of relaxation rates, lineshape and spin susceptibilities.     

Electronic properties of ortho-polymer A1C60 (A = K,Rb) and related phases

1 C₆₀. Its singular structural, magnetic and electrical properties greatly enhanced the study of the AC₆₀ phases (A = Rb, K and \chem{Cs}). We report our continuous wave (cw) ESR results on the structural phase transformations, phase instabilities and the related kinetics of K₁C₆₀. We quantitatively explain the origin of the difference between Rb₁C₆₀ and K₁C₆₀ phase diagrams. The stability of OP-Rb₁C₆₀ together with the narrowness of its homogenous cw-ESR linewidth in the temperature range allow to perform pulsed ESR experiments. Pulsed ESR experiments measure precisely both the electronic spin-spin () and spin-lattice () relaxation times. Above , the equality of and , which are the only contributions to the cw-ESR linewidth, constitutes a convincing evidence of the metallic nature of OP-Rb₁C₆₀. The drastic increase (decrease) of the linewidth () below \valunit{40}{K} is interpreted in terms of an \qut{electronic dimerization}. Received: 12 September 1996/Accepted: 14 November 1996     

Investigation of Paramagnetic Centers in Fullerides A2MC60 and AM2C60 (A = K, Rb, M = Mg, Be)

Paramagnetic centers in heterofullerides with the composition A₂MC₆₀ and AM₂C₆₀ (A = K, Rb, M = Mg, Be) were investigated by the electron spin resonance (ESR) method. It was found that the ESR signal can be interpreted as a composition of two lines with different temperature dependence of ESR absorption magnitude. This gives an evidence of the presence of at least two different types of paramagnetic centers. Centers of the first type behave as localized spins, while the rest can be interpreted as conduction electrons. Authors' address: Vladimir A. Kulbachinskii, Physics Faculty, M. V. Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russian Federation     

Study of Stable L-Alanine Radicals by W-Band EPR Spectroscopy

Stable L-alanine radicals, SAR1 and SAR2, induced by γ-irradiation of the L-alanine crystal have been investigated by electron paramagnetic resonance (EPR) technique at W-band (94 GHz) frequency. The study provides assignment of radical centers detected by continuous-wave EPR, saturation transfer mode and echo-detected field-swept EPR at W-band frequencies. The phase memory time, T _m, which was measured simultaneously at X-band (9.5 GHz) and W-band frequencies for different spectral components has been employed to estimate rotation correlation times of CH₃ protons and an effective correlation time related to the local dynamics of the entire SAR1 center at room temperature.     

Anomalous spin relaxation and quantum criticality in Mn1 − x Fe x Si solid solutions

We report results of the high frequency (60 GHz) electron spin resonance (ESR) study of the quantum critical metallic system Mn_{1 ? x} Fe _x Si. The ESR is observed for the first time in the concentration range 0 < x < 0.24 at temperatures up to 50 K. The application of the original experimental technique allowed carrying out line shape analysis and finding full set of spectroscopic parameters, including oscillating magnetization, line width and g factor. The strongest effect of iron doping consists in influence on the ESR line width and spin relaxation is marked by both violation of the classical Korringa-type relaxation and scaling behavior. Additionally, the non-Fermi-liquid effects in the temperature dependence of the ESR line width, which may be quantitatively described in the theory of Wölfle and Abrahams, are observed at quantum critical points x* ～ 0.11 and x _c ～ 0.24.     

Multifrequency EPR of four triarylmethyl radicals

Continuous-wave spectra at W-band of four triarylmethyl (trityl) radicals at 100 K in 1∶1 water-glycerol exhibit rhombic electron paramagnetic resonance spectra. The rigid-lattice line widths at W-band are only 3 to 5 times larger than at X-band or S-band, and fluid-solution line widths are much narrower than those for rigid lattice, which indicates that unresolved anisotropic nuclear hyperfine couplings make significant contributions to the rigid-lattice line widths. Spin-flip lines are observed in glassy-solution spectra at X-band and S-band, but not at W-band or 250 MHz. At 100 KT _m is dominated by spin diffusion of solvent protons and is independent of microwave frequency. Between about 130 and 170 K, 1/T _m for trityl-CH₃ is enhanced by rotation of the methyl groups at a rate comparable to inequivalences in the hyperfine interaction. Motional averaging of anisotropic interactions enhances spin echo dephasing between about 200 and 300 K. The temperature dependence of 1/T ₁ is similar for the four radicals and is consistent with assignment of the Raman process and a local mode as the dominant relaxation processes. The similarity inT ₁ values at W-band and X-band supports this assignment.     

Magnetism of C60-based molecular complexes: High-field-magnetization and magnetooptical study

Magnetization study of the C₆₀ · TMTSF · 2CS₂ molecular complex in magnetic fields up to 47 T for the temperature range 1.8–300 K and ESR spectroscopy of the molecular complex (ET)₂C₆₀ at T=1.8 K for the frequency range 60–90 GHz in magnetic fields up to 32 T provide experimental evidence that paramagnetic centers with reduced g-factor values g<1 control the magnetic properties of these solids. A model is suggested in which the renormalization of the g factor is due to the dynamic Jahn-Teller effect involving negative C₆₀ ions that appear as defects in the crystalline structure with a weak charge transfer.     

Complete pentagon orientational ordering in C60 fullerite charged with NO

Solid C₆₀ was stored in NO under high pressure, and the gas molecules NO were found to diffuse into the octahedral interstitial sites in its fcc crystal lattice. Its ¹³C NMR MAS spectra are composed of a primary resonance at 143.7 ppm accompanied by two minor peaks shifted 0.4 and 0.8 ppm downfield, respectively. The dopant was found to depress its phase transition temperature at 260 K in pure C₆₀ and to substantially reduce the drop Δ?′ at the phase transition temperature. Furthermore, the spectral features associated with relaxation during glass transition at lower temperature, as observed in impedance spectra, were smeared. The fraction of P-orientation below T _c was calculated to be larger than 11/12. These results show that a completely P-oriented phase occurred in (NO)_0.1C₆₀ and that this phase is favored by a negative pressure on the C₆₀ lattice exerted by NO, as well as by the electrostatic interaction between the two.     

Influence of C60 Aggregation on Pseudorotation in the Excited Triplet State Probed by Multifrequency Time-Resolved EPR

The influence of C₆₀ aggregation on time-resolved (TR) electron paramagnetic resonance (EPR) of C₆₀ in the excited triplet state was investigated by multifrequency EPR techniques. Temperature-independent X-band (9.7 GHz) TR-EPR spectra were observed in a fresh toluene solution, while temperature-dependent ones were reported in literatures. The experimental spectra in this study indicated that the pseudorotation of pristine C₆₀ in frozen toluene solution is not frozen out even at lower temperatures. Careful investigations of TR-EPR and its decay kinetics demonstrated that the pseudorotation can be affected by C₆₀ aggregation. A comparison between X- and W-band (94.9 GHz) results indicated that the aggregation can be accelerated by a capillary effect. Three decay constants were extracted from the analysis of the decay kinetics. The fastest component was ascribed to the pseudorotation, which was independent of temperature in the range of 10–40 K. The temperature dependences of the decay kinetics showed that the pseudorotation is not affected by C₆₀ aggregation at higher temperatures.     

Metal-insulator and structural phase transition observed by ESR spectroscopy and x-ray diffraction in KC60

We have performed electron spin resonance (ESR) spectroscopy and x-ray diffraction experiments at low temperature on KC60 single crystals. ESR data reveal the occurrence of a metal-insulator phase transition at about 50 K. In the same temperature range, we observe the stabilization of a superstructure which doubles the volume of the unit cell. We suggest that displacements of the K atoms and a modulation of the C60 charge may be involved in the mechanism of this phase transition. These results shed new light on the subtle interplay of structure, dimensionality, and electronic properties in the AC60 fullerides.     

ESR study of the undoped heavy-fermion compound YbRh2Si2

The electron spin resonance (ESR) of the heavy-fermion metal YbRh₂Si₂ has been studied. The angular variation and the temperature dependence of the ESR line width have been measured in YbRh₂Si₂ single crystals in the temperature range of 4–25 K. The characteristic spin-fluctuation temperatureT ^* ～ 17 K estimated from these studies coincides very well with other experimental data. A well-behaved ESR signal due to local Yb³⁺ moments strongly supports the localized moment scenario for heavy-fermion quantum critical points.     

Submillimeter-wave ESR measurements of Ca1−xCuO2 (x = 0.164) with edge-sharing CuO2 chains

Quasi-one-dimensional (1-D) cupric oxide Ca_1?xCuO₂ (x = 0.164) is the system with 25–40% hole-doped edge-sharing CuO₂ chains. However, the holes are almost localized in Ca_1?xCuO₂ and its magnetic susceptibility with a peak at 30 K was explained by the model considering both 1-D antiferromagnetic chains and spin dimers (Z. Hiroi, M. Okumura, Y. Nabeshima, T. Yamada, M. Takano: J. Phys. Soc. Jpn.69, 1824, 2000). To clarify the magnetic nature of Ca_1?xCuO₂, we performed submillimeter-wave electron spin resonance (ESR) measurements on a powder sample of Ca_0.83?6CuO₂. The resonance above 12 K showed typical powder ESR of Cu²⁺ and theg-values were determined to be g∥= 2.33 and g_⊥ = 2.06 from the analysis. The resonance below 12 K changed completely from ESR. The frequency-field relation of ESR at 1.8 K clearly showed the easy-axis type antiferromagnetic resonance.     

Multifrequency electron spin resonance study of the charge-ordered manganite Nd<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript>

We performed multifrequency electron spin resonance (ESR) on the antiferromagnetic (T_N = 160 K) and charge-ordered (T _co = 250 K) insulating manganite Nd_0.5Ca_0.5MnO₃. Temperature (4–300 K) and frequency (9.4–285 GHz) dependence of the linewidth, intensity and position of the ESR line were studied. In the paramagnetic state we observe a single Lorentzian absorption line. For a given frequency, the ESR line position is temperature independent (close tog = 1.99). A strong linewidth broadening is observed below T_co. This indicates that there is no magnetic order in the temperature rangeT _cos>T >T _N but strong antiferromagnetic fluctuations are present. Below T_N, due to high-frequency and high-field ESR (up to 12 T) measurements, we were able to observe unexpected lines within the antiferromagnetic gap revealing the presence of a phase separation.     

A Variable Temperature X- and W-Band EPR Study of Fe-Doped SiCN Ceramics Annealed at 1000, 1100, and 1285 °C: Dangling Bonds,Ferromagnetism and Superparamagnetism

Polymer-derived SiCN ceramics, annealed (also referred to as pyrolyzed) at 1000, 1100, and 1285 °C, and doped with Fe(III) acetylacetonate, are investigated by electron paramagnetic resonance (EPR) from 4 to 120 K at X-band (9.425 GHz). In addition, the SiCN ceramic, annealed at 1100 °C, was studied by EPR at 300 K at W-band (93.96 GHz). There was observed a significant increase in EPR linewidth due to dangling bonds (g = 2.001) below 20 K at X-band. The low-field X-band FMR line (g ≈ 12) indicated the presence of ferromagnetic Fe₅Si₃ crystallites. There were found two EPR lines due to carbon-related dangling bonds: (1) those present as defects on the surface of the free-carbon phase (as sp² carbon-related dangling bonds with g = 2.0011) and (2) those present within the bulk of carbon phase (as sp³ carbon-related dangling bonds with g = 2.0033). On the other hand, the intense low-field EPR signal observed at X-band was not observed at W-band. As well, there was observed splitting of the single broad EPR signal observed at g = 2.05 at X-band into two signals at W-band at g = 1.99 and g = 2.06, due to two different Fe-containing superparamagnetic nanocrystallites. Two new EPR signals, not observed at X-band, were observed at W-band, namely at g = 2.28 and g = 3.00, which are also due to g_∥ of these superparamagnetic nanocrystallites.     

High-field electron paramagnetic resonance study of the polymerization in Na2Rb1−x Cs x C60

We report on the study of the magnetic properties of the low-temperature polymer phases in N₂Rb_1?xCs_xC₆₀ at 110 GHz microwave frequency withx varying between 0 and 1. The magnetic and structural properties of the polymer phase strongly depend on the Cs content and its electronic structure progressively becomes quasi-one-dimensional asx is increased. While the electronic properties of the polymeric Na₂RbC₆₀ appear to be close to three-dimensional metal, Na₂Rb_0.3Cs_0.7C₆₀ shows characteristics of quasi-one-dimensional metal where instability in the electronic structure was found as detected by the sudden disappearance of the ESR intensity due to the opening of the gap at the Fermi surface. The observation of an additional resonance line below 15 K, which could be attributed to antiferromagnetic resonance, suggests that the low-temperature polymeric phase in Na₂Rb_0.3Cs_0.7C₆₀ has a well-defined magnetic ground state.     

Simulation analysis of ESR spectrum of polymer alkyl-C60 radicals formed by photoinitiated reactions of low-density polyethylene

Polymer alkyl-fullerene (P-C₆₀) radical adducts produced by ultraviolet (UV) photoinitiated reactions between low-density polyethylene (LDPE) and C₆₀ in the presence of benzophenone (BP) as a photoinitiator have been detected and identified for the first time by electron spin resonance (ESR) and confirmed by simulation analysis of the spectrum. A well-resolved ESR spectrum was recorded in situ upon UV irradiation of the LDPE/BP/C₆₀ sample in the molten state (413 K). Detailed analysis of hyperfine structures shows that the observed spectrum is composed of three components: a broad singlet atg = 2.0025 from the C₆₀ radical anion; an innermost pair of¹³C satellites; and a 12-line spectrum superposed on the broad singlet. The simulation analysis of the spectrum shows that the 12-line spectrum is due to the overlapping of two kinds of radical adducts of tertiary carbon-C₆₀ (A) and secondary carbon-C₆₀ (B), which have slightly differentg-values and almost the same integral intensity I_A/I_B (48.4/51.6). The spectrum simulated on the basis of the¹H and¹³C hyperfine interaction parameters is in good agreement with the observed spectrum. These results provide positive evidence that the C₆₀-bound LDPE materials can be obtained directly by a simple method of BP-photoinitiated reaction of the LDPE/C₆₀ system.     

Partition coefficient of small spin probes: An improved ESR determination method

We report on the improved determination of the partition coefficient,P _e, of the small hydrophobic spin probe di-tert-butyl nitroxide (DTBN) between the aqueous and the fluid hydrophobic phase of dipalmitoylphosphatidyl choline (DPPC) multilayers, made in terms of the second integral evaluation of the isotropic ESR resonance lines. The calculation of the second integral was carried out taking into account both the width and amplitude of the resonance lines and a shape factor related to the degree of inhomogeneous line broadening. The expression ofP _c obtained in this way has been tested by computer simulation using Voightian lineshapes for the ESR absorption resonance lines. The method has been applied to the determination of the degree of cooperativity of the gel → liquid-crystal phase transition of DPPC multilamellae, and the results were compared with those obtained using the traditional expression ofP _e. Finally, the calculation of the cooperativity parameter of the main phase transition of DPPC multilayers results improved of about 70% by using the new expression ofP _c.     

Dielectric properties of C60 molecular complexes

The permittivity ? of the molecular complexes (BTX)C₆₀CS₂, (DAN)C₆₀(C₆H₆)₃, and (S₄N₄)_1.2C₆₀(C₆H₆)_0.8 has been studied at 100 MHz. A maximum has been observed to appear in the temperature dependence ?(T) at T≈90 K, which can be assigned to freezing of the orientational disorder. A region of anomalous growth of ? with decreasing temperature has been found in the low-temperature range T≤25 K, this feature being apparently sensitive to the actual packing pattern of C₆₀ spheres in the molecular complex.