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.     

X-Band ESR and51V NMR study of the Haldane system PbNi2−xMgxV2O8

The temperature and angular dependence of the X-band electron spin resonance (ESR) and⁵¹V nuclear magnetic resonance (NMR) spectra have been measured in a recently discovered Haldenegap system, PbNi_2-xMg_xV₂O₈ (0≤x≤0.24). The angular dependence of the ESR signal suggests that both the spin diffusion as well as the magnetic anisotropy determine the electronic spin correlation functions. However, in doped samples the magnetic anisotropy increasingly dominates the spin dynamics on cooling. The huge broadening of the⁵¹V NMR spectra in doped samples at low temperatures provides evidence for localized magnetic moments in the vicinity of the Mg impurities. Locally distorted structure around each Mg impurity may slightly modify the magnetic interactions and be potentially responsible for the antiferromagnetic ordering (belowT _N≈ 3.5K) in doped compositions.     

Multiple-pulse coherence enhancement of solid state spin qubits

We describe how the spin coherence time of a localized electron spin in solids, i.e., a solid state spin qubit, can be prolonged by applying designed electron spin resonance pulse sequences. In particular, the spin echo decay due to the spectral diffusion of the electron spin resonance frequency induced by the non-Markovian temporal fluctuations of the nuclear spin flip-flop dynamics can be strongly suppressed using multiple-pulse sequences akin to the Carr-Purcell-Meiboom-Gill pulse sequence in nuclear magnetic resonance. Spin coherence time can be enhanced by factors of 4-10 in GaAs quantum-dot and Si:P quantum computer architectures using composite sequences with an even number of pulses.     

Nuclear tuning and detuning of the electron spin resonance in a quantum dot: theoretical consideration

We study nuclear spin dynamics in a quantum dot close to the conditions of electron spin resonance. We show that at a small frequency mismatch, the nuclear field detunes the resonance. Remarkably, at larger frequency mismatch, its effect is opposite: The nuclear system is bistable, and in one of the stable states, the field accurately tunes the electron spin splitting to resonance. In this state, the nuclear field fluctuations are strongly suppressed, and nuclear spin relaxation is accelerated.     

Low-Frequency and d.c. Phenomena in Magnetic Resonance

A review is given of some low-frequency and d.c. magnetic resonance phenomena studied since 1970s up to date. The content includes: the enhanced longitudinal susceptibility effect (ELSE) based on the concept of the dipole–dipole reservoir in EPR; direct registration of NMR in rotating frames; modulation method of measuring extremely fast electron spin longitudinal relaxation; resonance magnetoresistance and resonance spin rectification (“spin dynamo”) in conducting ferromagnetic films. Physical mechanism of these effects, as well as applications in studying spin dynamics and relaxation in solids, including dynamic nuclear polarization, high-temperature superconductivity and properties of rare-earth manganites are considered.     

Entanglement between an electron and a nuclear spin 1/2

We report on the preparation and detection of entangled states between an electron spin 1/2 and a nuclear spin 1/2 in a molecular single crystal. These were created by applying pulses at ESR (9.5 GHz) and NMR (21 MHz, 46 MHz) frequencies. Entanglement was detected by using a special entanglement detector sequence based on a unitary back transformation including phase rotation.     

Evolution of the 4f electron localization from YbRh2Si2 to YbRh2Pb studied by electron spin resonance

We report on electron spin resonance (ESR) experiments on the Heusler alloy YbRh₂Pb and compare its spin dynamics with that of several other Yb-based intermetallics. A detailed analysis of the derived ESR parameters indicates the extremely weak hybridization, more localized distribution of the 4f states, and a smaller RKKY interaction in YbRh₂Pb. These findings reveal the important interplay between hybridization effects, chemical substitution, and crystalline electric field interactions that determines the ground state properties of strongly correlated electron systems.     

Dynamic nuclear spin resonance in n-GaAs

The dynamics of optically detected nuclear magnetic resonance is studied in n-GaAs via time-resolved Kerr rotation using an on-chip microcoil for rf field generation. Both optically allowed and optically forbidden NMR are observed with a dynamics controlled by the interplay between dynamic nuclear polarization via hyperfine interaction with optically generated spin-polarized electrons and nuclear spin depolarization due to magnetic resonance absorption. Comparing the characteristic nuclear spin relaxation rate obtained in experiment with master equation simulations, the underlying nuclear spin depolarization mechanism for each resonance is extracted.     

Faraday effect and aggregation of paramagnetic ions in borate glasses

The magneto-optical Faraday effect and electron spin resonance (ESR) in a potassium-aluminum borate glass containing small amounts of impurities of Fe and Mn oxides have been investigated. The Faraday effect measurements and ESR data made it possible to reveal the formation of clusters of paramagnetic ions interacting via oxygen even in the stage of glass synthesis. Heat treatment leads to cluster coarsening and formation of nanoparticles.     

Application of field-cycling NMR relaxometry to the study of ultrasound-induced effects in the molecular dynamics and order of mesomorphic materials

A salient characteristic of nuclear magnetic resonance (NMR) techniques is the possibility to scan nuclear spin evolutions within a broad Larmor frequency range. Special instrumentation was developed to extend nuclear spin relaxation studies up to proton Larmor frequencies in the sub-kilohertz regime, a technique known as field-cycling NMR relaxometry. This article refers to an experimental version where the sample under study is selectively subjected to ultrasonic irradiation. The fact that ultrasound couples selectively to the collective dynamics of liquid crystals, offers new insights for the study of the molecular dynamics in these materials using NMR relaxation.     

High-Field Phenomena of Qubits

Electron and nuclear spins are very promising candidates to serve as quantum bits (qubits) for proposed quantum computers, as the spin degrees of freedom are relatively isolated from their surroundings and can be coherently manipulated, e.g., through pulsed electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR). For solid-state spin systems, impurities in crystals based on carbon and silicon in various forms have been suggested as qubits, and very long relaxation rates have been observed in such systems. We have investigated a variety of these systems at high magnetic fields in our multifrequency pulsed EPR/ENDOR (electron nuclear double resonance) spectrometer. A high magnetic field leads to large electron spin polarizations at helium temperatures, giving rise to various phenomena that are of interest with respect to quantum computing. For example, it allows the initialization of both the electron spin as well as hyperfine-coupled nuclear spins in a well-defined state by combining millimeter and radio-frequency radiation. It can increase the T ₂ relaxation times by eliminating decoherence due to dipolar interaction and lead to new mechanisms for the coherent electrical readout of electron spins. We will show some examples of these and other effects in Si:P, SiC:N and nitrogen-related centers in diamond.     

Generating entanglement and squeezed states of nuclear spins in quantum dots

We present a scheme for achieving coherent spin squeezing of nuclear spin states in semiconductor quantum dots. The nuclear polarization dependence of the electron spin resonance generates a unitary evolution that drives nuclear spins into a collective entangled state. The polarization dependence of the resonance generates an area-preserving, twisting dynamics that squeezes and stretches the nuclear spin Wigner distribution without the need for nuclear spin flips. Our estimates of squeezing times indicate that the entanglement threshold can be reached in current experiments.     

An NMR Investigation of the Conformational Effect of Nitroxide Spin Labels on Ala-Rich Helical Peptides

Nitroxide spin labels, in conjunction with electron spin resonance (ESR) experiments, are extensively employed to probe the structure and dynamics of biomolecules. One of the most ubiquitous spin labeling reagents is the methanethiosulfonate spin label which attaches a spin label selectively to Cys residues via a disulfide bond (Cys-SL). However, the actual effect of the nitroxide spin label upon the conformation of the peptide or protein cannot be unambiguously determined by ESR. In this study, a series of 16-residue Ala-rich helical peptides was characterized by nuclear magnetic resonance techniques. The C_αH chemical shift analysis, NOEs, and³J_NHαcoupling constants for peptides with no Cys, free Cys, and Cys-SL (with the N–O group reduced) were compared. These results indicate that while replacement of an Ala with a Cys residue causes a loss of overall helical structure, the Cys-SL residue is helix supporting, as would be expected for a non-β-branched aliphatic amino acid. Thus, the Cys-SL residue does not perturb helical structure and, instead, exhibits helix-stabilizing characteristics similar to that found for Ala, Met, and Leu.     

ESR and ENDOR studies of solitons and polarons in conjugated polymers

Nonlinear excitations such as solitons and polarons in conjugated polymers carry spins. In this case electron spin resonance (ESR) and electron-nuclear double resonance (ENDOR) provide unique methods to determine their wave functions. In this review article, the case of solitons in polyacetylene, CH_x, and polarons in an electroluminescent polymer, poly(paraphenylene vinylene) (PPV) are discussed as typical examples. High-resolution proton ENDOR spectra, obtained with stretch-oriented samples, yield the half extension of the excitations of 18 carbon atoms and 4 phenyl rings for CH_x and PPV, respectively. These extensions are well described by the theories in the case of finite electron correlation. In addition, light-induced ESR technique is shown to be useful in obtaining site-selective information of spin distribution in the case of PPV derivatives, as well as the excitation spectra of polarons.     

Electron Spin Resonance Measurements of a Demagnetizing Field on the Surface of Metal Samples

By comparing the signals of electron spin resonance (ESR) from two crystals of a diamond (spin–labels) the demagnetizing field of the Co, Fe, and Ni samples in the shape of strongly elongated ellipsoids of revolution (disks) has been measured. The magnetic permeabilities of the metals in the external magnetic field corresponding to the ESR of the broken chemical bonds in a natural diamond irradiated with fast reactor neutrons have been determined.     

Poly(ethylene glycol) and phospholipid packing in the structure of reverse micelles

The influence of water substitution by a substance with a different polarity on the structure of phospholipid monolayer interface in water-in-oil microemulsion has been studied by the Fourier-transform pulsed-gradient spin-echo (FT PGSE)¹H nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spin-label methods. For this purpose the soybean phosphatidylcholine-based microemulsion and water soluble poly(ethylene glycol) with molecular weight 400 (PEG400) were used. Self-diffusion coefficients of all microemulsion components obtained by the FT PGSE NMR technique provided information about both the size of reverse micelles and distribution of components between different microemulsion compartments. The maximum hyperfine splitting, 2A _max, in the ESR spectra was used to characterize the degree of the phospholipid hydrocarbon chain mobility. It was shown that PEG400 alters significantly the size of the reverse micelles and the motion of the labeled segments of the lipid tails. A mechanism of PEG400 acting in solution of the phospholipid-based reverse micelles on the basis of the rough decrease of the micelle core polarity was suggested.     

Interaction of two magnetic resonance modes in the polar phase of superfluid <Superscript>3</Superscript>He

We report results of low frequency nuclear magnetic resonance (NMR) experiments in the superfluid polar phase of ³He, which is stabilized by a new type of “nematic” aerogel—nafen. We have found that an interaction between transverse and longitudinal NMR modes may essentially influence the spin dynamics. Theoretical formulas for NMR resonant frequencies are derived and applied for interpretation of the experimental results.     

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.     

Spin-labeled gel for the production of radical-free dynamic nuclear polarization enhanced molecules for NMR spectroscopy and imaging

Dynamic nuclear polarization (DNP) has recently received much attention as a viable approach to enhance the sensitivity of nuclear magnetic resonance (NMR) spectroscopy and the contrast of magnetic resonance imaging (MRI), where the significantly higher electron spin polarization of stable radicals is transferred to nuclear spins. In order to apply DNP-enhanced NMR and MRI signal to biological and in vivo systems, it is crucial to obtain highly polarized solution samples at ambient temperatures. As stable radicals are employed as the source for the DNP polarization transfer, it is also crucial that the highly polarized sample lacks residual radical concentration because the polarized molecules will be introduced to a biological system that will be sensitive to the presence of radicals. We developed an agarose-based porous media that is covalently spin-labeled with stable radicals. The loading of solvent accessible radical is sufficiently high and their mobility approximates that in solution, which ensures high efficiency for Overhauser mechanism induced DNP without physically releasing any measurable radical into the solution. Under ambient conditions at 0.35 T magnetic field, we measure the DNP enhancement efficiency of (1)H signal of stagnant and continuously flowing water utilizing immobilized stable nitroxide radicals that contain two or three ESR hyperfine splitting lines and compare them to the performance of freely dissolved radicals.     

Erratum to: Hydrodynamic dispersion in β-lactoglobulin gels measured by PGSE NMR

The displacement scale dependent molecular dynamics of solvent water molecules flowing through β-lactoglobulin gels are measured by pulse gradient spin echo (PGSE) nuclear magnetic resonance (NMR). Gels formed under different p H conditions generate structures which are characterized by magnetic resonance imaging (MRI) and PGSE NMR measured dynamics as homogeneous and heterogeneous. The data presented clearly demonstrate the applicability of the theoretical framework for modeling hydrodynamic dispersion to the analysis of protein gels.