Spatial extent of wave functions of gate-induced hole carriers in pentacene field-effect devices as investigated by electron spin resonance

An electron spin resonance (ESR) method is applied to a pentacene field-effect device to investigate gate-induced hole carriers in such devices. Clear field-induced ESR signals are observed, demonstrating that all of the field-injected carriers have S = 1/2 spins. Anisotropic ESR signals due to unpaired pi electrons show the molecular orientation at the interface in the devices. The spatial extent of the spin density distribution (wave function) of the carriers is evaluated from the ESR linewidth, accounting for the hyperfine structure, to be of the order of 10 molecules.     

Why could electron spin resonance be observed in a heavy fermion Kondo lattice?

We develop a theoretical basis for understanding the spin relaxation processes in Kondo lattice systems with heavy fermions as experimentally observed by electron spin resonance (ESR). The Kondo effect leads to a common energy scale that regulates a logarithmic divergence of different spin kinetic coefficients and supports a collective spin motion of the Kondo ions with conduction electrons. We find that the relaxation rate of a collective spin mode is greatly reduced due to a mutual cancellation of all the divergent contributions even in the case of the strongly anisotropic Kondo interaction. The contribution to the ESR linewidth caused by the local magnetic field distribution is subject to motional narrowing supported by ferromagnetic correlations. The developed theoretical model successfully explains the ESR data of YbRh₂Si₂ in terms of their dependence on temperature and magnetic field.     

On the philosophy of optimizing contrast agents. An analysis of 1H NMRD profiles and ESR lineshapes of the Gd(III)complex MS-325+HSA

A generalization of the modified SBM theory is developed in closed analytical form. The theory is applied to describe the paramagnetically enhanced water proton spin-lattice relaxation rates of the aqueous-systems containing a gadolinium(S=7/2) complex(MS-325) in the presence or absence of human serum albumin (HSA). MS-325 binds to HSA: in the absence of the protein the reorientational time, tauR, is short, but when HSA is added tauR becomes much longer. In this way, the effect of reorientational motion, static (Delta s), and transient (Delta t) zero-field splitting (ZFS) interactions on both the water proton relaxivity and the Gd ESR lineshapes are investigated. Two dynamic models of electron spin relaxation are presented, characterized by transient and static ZFS-interactions. X-, Q-, and W-bands ESR spectra of MS-325+HSA are analyzed in order to describe the effect on the electron spin system upon binding to a macromolecule. A computer program based on this theory is developed which calculates solvent water proton T1 NMRD profiles and the corresponding X-, Q-, U-, and W-bands ESR lineshapes.     

Polaron motional narrowing of electron spin resonance in organic field-effect transistors

Polaron states in organic thin-film transistors (TFTs) were investigated by the electron spin resonance (ESR) technique. Gate-field-dependent and temperature-dependent single-Lorentzian ESR spectra were observed for field-induced polarons in pentacene TFTs, demonstrating the effect of motional narrowing due to polaron diffusion. Analyses of the ESR linewidth revealed a considerably long trapping time (tau_(C) approximately 0.7 ns), the variation of which is discussed in terms of the multiple trap-and-release model.     

Kondo lattice with heavy fermions: peculiarities of spin kinetics

A model of spin relaxation of Kondo lattices is proposed to explain the angular dependence of the electron spin resonance (ESR) parameters in the heavy fermion compounds Y bIr(2)Si(2) and Y bRh(2)Si(2). A perturbational scaling approach reveals a collective spin motion of Yb?ions with conduction electrons in the bottleneck regime. A common energy scale due to the Kondo effect regulates the temperature dependence of different kinetic coefficients to result in a mutual cancelation of all divergent parts in a collective spin mode. The angular dependence of the ESR intensity, linewidth and resonant frequency is shown to be in good agreement with experimental data on Y bIr(2)Si(2) and Y bRh(2)Si(2). In particular, the unexpectedly weak dependence of the ESR intensity on the orientation of the microwave magnetic field agrees with the properties of the discussed model.     

Low temperature electron spin resonance of the Kondo ion in a heavy fermion metal: YbRh2Si2

We report an electron spin resonance (ESR) study on single crystals of the heavy fermion metal YbRh2Si2 which shows pronounced non-Fermi liquid behavior related to a close antiferromagnetic quantum critical point. It is shown that the observed ESR spectra can be ascribed to a bulk Yb3+ resonance. This is the first observation of ESR of the Kondo ion itself in a dense Kondo lattice system. The ESR signal occurs below the Kondo temperature (T(K)) which thus indicates the existence of large unscreened Yb3+ moments below T(K). We observe the spin dynamics as well as the static magnetic properties of the Yb3+ spins to be consistent with the results of nuclear magnetic resonance and magnetic susceptibility.     

Electron spin resonance in the spin-1/2 quasi-one-dimensional antiferromagnet with Dzyaloshinskii-Moriya interaction BaCu2Ge2O7

We have investigated the electron spin resonance (ESR) on single crystals of BaCu2Ge2O7 at temperatures between 300 and 2 K and in a large frequency band, 9.6-134 GHz, in order to test the predictions of a recent theory, proposed by Oshikawa and Affleck (OA) [Phys. Rev. Lett. 82, 5136 (1999)]], which describes the ESR in a spin-1/2 Heisenberg chain with the Dzyaloshinskii-Moriya interaction. We find, in particular, that the ESR linewidth, Delta H, displays a rich temperature behavior. As the temperature decreases from T(max)/2 approximately 170 to 50 K, Delta H shows a rapid and linear decrease, Delta H approximately T. At low temperatures, below 50 K, Delta H acquires a strong dependence on the magnetic field orientation and for H axially c it shows a (h/T)(2) behavior which is due to an induced staggered field h, according to OA's prediction.     

ESR and longitudinal response in metals containing localized paramagnetic centers with spin<Emphasis Type="Italic">S</Emphasis>>1/2

The theory describing electron spin resonance (ESR) and the longitudinal magnetization response of coupled spin systems in a metal containing both delocalized conduction electrons (“espins”) and localized paramagnetic centers (“s-spins”) is generalized to the case of arbitrary half-integer spin value,S>1/2, of the s-spins. The consideration is based on the Bloch-Hasegawa equations supplemented by taking into account the coupled evolution of the longitudinal magnetization components and the effect of weak ESR saturation by the microwave field. The ESR transversal susceptibility and longitudinal magnetization response are worked out in terms of normal modes related to the coupled s- and e-spin oscillators taking into account the ESR fine structure (FS) of the s-spins. These modes are characterized by effective (renormalized) frequencies and relaxation rates (decays) which differ from the partial ones. In the specific cases of a well-resolved FS (in the isothermal limit) and of the relaxational collapse of the FS due to strong exchange coupling between the s- and e-spins (in both the isothermal and bottlenecked limits), the analytical expressions are derived which are relevant to the modulation technique of measuring extremely fast spin-lattice relaxation times in metals.     

Continuous-wave far-infrared ESR spectrometer for high-pressure measurements

We present a newly-developed microwave probe for performing sensitive high-field/multi-frequency electron spin resonance (ESR) measurements under high hydrostatic pressures. The system consists of a BeCu-made pressure-resistant vessel, which accommodates the investigated sample and a diamond microwave coupling window. The probe’s interior is completely filled with a pressure-transmitting fluid. The setup operates in reflection mode and can easily be assembled with a standard oversized microwave circuitry. The probe-head withstands hydrostatic pressures up to 1.6 GPa and interfaces with our home-built quasi-optical high-field ESR facility, operating in a millimeter/submillimeter frequency range of 105–420 GHz and in magnetic fields up to 16 T. The overall performance of the probe was tested, while studying the pressure-induced changes in the spin-relaxation mechanisms of a quasi-1D conducting polymer, KC₆₀. The preliminary measurements revealed that the probe yields similar signal-to-noise ratio to that of commercially available low-frequency ESR spectrometers. Moreover, by observing the conduction electron spin resonance (CESR) linewidth broadening for KC₆₀ in an unprecedented microwave frequency range of 210–420 GHz and in the pressure range of up to 1.6 GPa, we demonstrate that a combination of high-pressure ESR probe and high-field/multi-frequency spectrometer allows us to measure the spin relaxation rates in conducting spin systems, like the quasi-1D conductor, KC₆₀.     

Detection of single spin decoherence in a quantum dot via charge currents

We consider a quantum dot attached to leads in the Coulomb blockade regime that has a spin 1 / 2 ground state. We show that, by applying an ESR field to the dot spin, the stationary current in the sequential tunneling regime exhibits a new resonance peak whose linewidth is determined by the single spin decoherence time T2. The Rabi oscillations of the dot spin are shown to induce coherent current oscillations from which T2 can be deduced in the time domain. We describe a spin inverter which can be used to pump current through a double dot via spin flips generated by ESR.     

Electron spin resonance in polyphenylacetylene

By polymerizing phenylacetylene in the absence of initiator and in vacuo while observing electron spin resonance, it was established that the paramagnetism in the thermal polymer (PPA-T) cannot be due to interaction with oxygen but must be a property of the chemically uncontaminated macromolecule. The ESR behavior of a concentrated solution of PPA(T) in monomer and of solid (amorphous) PPA(T) displays a Curie dependence, whereas the ESR absorption of crystalline polymer made with coordination catalysts increases by one to two orders of magnitude over the temperature of about 60 to 140°C. The major portion of the XRD crystallinity disappears more sharply, at about 120°C. The paramagnetism in PPA must therefore be associated with “conformational defects” in the individual macromolecules.     

Simultaneous ESR and optical absorption measurements of K in solid Ar

An apparatus was developed for simulataneous electron spin resonance (ESR) and optical absorption experiments on atoms imbedded in rare gas matrices. In a first application the temperature dependence of ESR signals and of optical absorption spectra of K atoms in Ar matrices were studied. One of the numerous ESR lines could be unambiguously assigned to the so-called red triplet absorption due to their identical annealing behaviour at 12–14 K. The correspondence of other ESR lines to optical absorptions is discussed.     

Magnetic susceptibility, heat capacity, and optical conductivity of LiPc and LiPcI

The magnetic susceptibility, using dc and electron spin resonance (ESR) methods, the specific heat, and the infrared properties of the one-dimensional molecular semiconductors lithium phthalocyanine (LiPc) and the iodinated compound LiPcI have been investigated for temperatures K. LiPc has a half-filled conduction band and is expected to be an organic metal. However, due to the strong Coulomb repulsion the system is a one-dimensional Mott-Hubbard insulator with a Hubbard gap of 0.75 eV as inferred from optical measurements. The localized electrons along the molecular stacks behave like a S = 1/2 antiferromagnetic spin chain. The spin susceptibility, as determined by ESR experiments, and the magnetic contribution to the heat capacity show a Bonner-Fisher type of behavior with an exchange constant K. LiPcI is an intrinsic narrow-gap semiconductor with an optical gap of 0.43 eV. In ESR experiments it is silent, indicating that all the unpaired electrons have been removed from the macrocycle via doping with iodine. Received: 16 June 1998 / Accepted: 14 July 1998     

Electron spin resonance determination of proton locations in a hydride

The low temperature electron spin resonance (ESR) spectrum of dilute Er in ScH_x powder contains distinct signals which are due to the presence of octahedral as well as tetrahedral protons in the vicinity of the Er ions. The occupation probability for the octahedral protons versus the bulk hydrogen-to-metal ratio is determined from the ESR signal intensities and compared with a lattice-gas calculation.     

Microwave frequency modulation in continuous-wave far-infrared ESR utilizing a quasi-optical reflection bridge

We report the development of the frequency-modulation (FM) method for measuring electron spin resonance (ESR) absorption in the 210- to 420GHz frequency range. We demonstrate that using a high-frequency ESR spectrometer without resonating microwave components enables us to overcome technical difficulties associated with the FM method due to nonlinear microwave-elements, without sacrificing spectrometer performance. FM was achieved by modulating the reference oscillator of a 13GHz Phase-Locked Dielectric Resonator Oscillator, and amplifying and frequency-multiplying the resulting millimeter-wave radiation up to 210, 315 and 420GHz. ESR spectra were obtained in reflection mode by a lock-in detection at the fundamental modulation frequency, and also at the second and third harmonic. Sensitivity of the setup was verified by conduction electron spin resonance measurement in KC60.     

Synthesis of spin-labelled poly(acrylic acid)s and their segmental motion study

Understanding the segmental dynamics of polymer chains is cardinal to decipher the microscopic behaviour in order to modulate the bulk properties of polymers. The study of electron spin resonance (ESR) spectroscopy of spin-labelled polymers is useful to understand the segmental dynamics of polymer chains in solution. In this paper, poly(acrylic acid)s (PAAs) were spin labelled with 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl radicals. Spin-labelled PAAs (SL-PAAs) were characterised by Fourier-transform infrared spectroscopy, ultraviolet–visible spectroscopy, thermogravimetric analysis, differential scanning calorimetry, cyclic voltammetry (CV), and ESR analyses. The polyelectrolyte complexes of SL-PAAs were prepared by employing poly(diallyldimethylammonium chloride) (PDADMAC) as the polycation and analysed by transmission electron microscopy, dynamic light scattering (DLS), and ESR spectroscopies. The effect of molar mass on the segmental dynamics of SL-PAAs in pristine as well as in the form of polyelectrolyte complexes (PECs) was studied. The results indicated that SL-PAAs show a differential complexation behaviour with PDADMAC in the PECs depending on their molar mass.     

Lattice distortion (Peierls Transition) caused by spin interaction in the chaotic impurity system of a semiconductor

The effect of an elastic spontaneous distortion of the crystal lattice of a doped semiconductor Ge:As near the insulator–metal (IM) phase transition has been discovered. The effect is manifested in the electron spin resonance (ESR) of neutral As atoms as a splitting of the single resonance absorption line. It observed at electron concentrations in the range 0.8 < n/n_C < 1 at low temperatures T < 100 K (n_C = 3.7 × 10¹⁷ cm^‐3 is the critical electron concentration for the IM phase transition). The splitting is the strongest along each of the six [110] directions, which indicates that the local lattice distortion occurs just in these directions. As a result, a sample is possibly divided into separate domains differing in the directions of compressive or tensile deformations. A study of concentration, temperature, and angular dependences of the effect has shown that the phenomenon discovered can be understood in terms of the Peierls spin transition model.     

Thermal effect in metamorphic rock around an intrusion zone with ESR studies

Hornfels, and intrusion rocks were studied with electron spin resonance (ESR) as a function of the distance from an intrusion zone in Mino Natural Park, Osaka, Japan. The temperature distribution in metamorphic rocks around an intrusion zone was calculated based on a simplified model of one-dimensional thermal conduction. The age as well as the thermal effect due to an intrusion rock were assessed using ESR signal intensities of paramagnetic defect centers (E’, oxygen-hole, and Al) in quartz grains. Geothermal heating effects are observable more than ten million years later for stable detects such as the E’ center.     

二茂铁基化合物的ESR研究进展

二茂铁基化合物因具有独特的氧化还原、电、光、磁和催化性能,而被广泛研究与应用.电子自旋共振(ESR)技术是观察物质顺磁性最直接的手段,在研究二茂铁基化合物的电子结构、电子转移和分子间相互作用等方面具有独特的优势.ESR技术还具有灵敏度高、样品处理简单、直接检测不破坏样品等优点.该文综述了二茂铁基化合物的ESR研究进展.     

Shot noise in a quantum dot coupled to non-magnetic leads: effects of Coulomb interaction

We study electron transport through a quantum dot, connected to non-magnetic leads, in a magnetic field. A super-Poissonian electron noise due to the effects of both interacting localized states and dynamic channel blockade is found when the Coulomb blockade is partially lifted. This is sharp contrast to the sub-Poissonian shot noise found in the previous studies for a large bias voltage, where the Coulomb blockade is completely lifted. Moreover, we show that the super-Poissonian shot noise can be suppressed by applying an electron spin resonance (ESR) driving field. For a sufficiently strong ESR driving field strength, the super-Poissonian shot noise will change to be sub-Poissonian.