Spectra Of Spins Research Articles

Utilizing the multifrequency resonances of the electron spins in diamond nitrogen-vacancy centers under strong radio frequency driving for quantum sensing

Multifrequency resonances in the pulsed-optically detected magnetic resonance (ODMR) spectra of electron spins in ensemble nitrogen-vacancy (NV) centers in diamonds are investigated under strong radio frequency (RF) driving at a MHz frequency range and weak microwave driving at a GHz frequency range in a bias static magnetic field for quantum sensing applications. First, we demonstrate that the coherent destruction of tunneling, which leads to the disappearance of the main resonance peaks, can be utilized for precise calibration of the RF amplitude. Next, we clarify the condition for enhancing the sensitivity of a DC magnetic field under strong RF driving at the RF frequency that matches the split frequency due to the hyperfine interaction between 15N nuclear spins and the NV electron spins. Our findings indicate that strong RF driving increases the sensitivity of the DC magnetic field by enhancing the ODMR contrast and reducing the linewidth. The above results contribute to certifying the quantitative accuracy of RF imaging and enhancing the sensitivity of the DC magnetic field imaging using ensemble NV centers in diamonds.

Tunnel current noise spectra of spins in individual dimers of molecular radicals

We report the detection of electron spin resonance (ESR) in individual dimers of the stable free radical 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO). ESR is measured by the current fluctuations in a scanning tunneling microscope (ESR-STM method). The multipeak power spectra, distinct from macroscopic data, are assigned to dimers having exchange and Dzyaloshinskii-Moriya interactions in the presence of spin-orbit coupling. These interactions are generated in our model by interfering electronic tunneling pathways from tip to sample via the dimer's two molecules. This is the first demonstration that tunneling via two spins is a valid mechanism of the ESR-STM method.

Violation and revival of Kramers' degeneracy in open quantum systems

Kramers' theorem ensures double degeneracy in the energy spectrum of a time-reversal symmetric fermionic system with half-integer total spin. Here we are now trying to go beyond the closed system and discuss Kramers' degeneracy in open systems out of equilibrium. In this letter, we prove that the Kramers' degeneracy in interacting fermionic systems is equivalent to the degeneracy in the spectra of different spins together with the vanishing of the inter-spin spectrum. We find the violation of Kramers' degeneracy in time-reversal symmetric open quantum systems is locked with whether the system reaches thermal equilibrium. After a sudden coupling to an environment in a time-reversal symmetry preserving way, the Kramers doublet experiences an energy splitting at a short time and then a recovery process. We verified the violation and revival of Kramers' degeneracy in a concrete model of interacting fermions and we find Kramers' degeneracy is restored after the local thermalization time. By contrast, for time-reversal symmetry $\tilde{\cal T}$ with $\tilde{\cal T}^2=1$, we find although there is a violation and revival of spectral degeneracy for different spins, the inter-spin spectral function is always nonzero. We also prove that the degeneracy in spectral function protected by unitary symmetry can be maintained always.

Multichannel digital heteronuclear magnetic resonance biosensor

Low-field, mobile NMR systems are increasingly used across diverse fields, including medical diagnostics, food quality control, and forensics. The throughput and functionality of these systems, however, are limited due to their conventional single-channel detection: one NMR probe exclusively uses an NMR console at any given time. Under this design, multi-channel detection could only be accomplished by either serially accessing individual probes or stacking up multiple copies of NMR electronics; this approach still retains limitations such as long assay times and increased system complexity. Here we present a new scalable architecture, HERMES (hetero-nuclear resonance multichannel electronic system), for versatile, high-throughput NMR analyses. HERMES exploits the concept of software-defined radio by virtualizing NMR electronics in the digital domain. This strategy i) creates multiple NMR consoles without adding extra hardware; ii) acquires signals from multiple NMR channels in parallel; and iii) operates in wide frequency ranges. All of these functions could be realized on-demand in a single compact device. We interfaced HERMES with an array of NMR probes; the combined system simultaneously measured NMR relaxation from multiple samples and resolved spectra of hetero-nuclear spins (1H, 19F, 13C). For potential diagnostic uses, we applied the system to detect dengue fever and molecularly profile cancer cells through multi-channel protein assays. HERMES holds promise as a powerful analytical tool that enables rapid, reconfigurable, and parallel detection.

Dramatically Enhanced Solubility of Halide-Containing Organometallic Species in Diiodomethane: The Role of Solvent⋅⋅⋅Complex Halogen Bonding.

In the current study, we evaluated the solubility of a number of organometallic species and showed that it is noticeably improved in diiodomethane when compared to other haloalkane solvents. The better solvation properties of CH2 I2 were associated with the substantially better σ-hole-donating ability of this solvent, which results in the formation of uniquely strong solvent-(metal complex) halogen bonding. The strength of the halogen bonding is attenuated by the introduction of additional halogen atoms in the organometallic species owing to the competitive formation of more favourable intermolecular complex-complex halogen bonding. The exceptional solvation properties of diiodomethane and its inertness towards organometallic species make this solvent a good candidate for NMR studies, in particular, for the acquisition of spectra of insensitive spins.

Dramatically Enhanced Solubility of Halide‐Containing Organometallic Species in Diiodomethane: The Role of Solvent⋅⋅⋅Complex Halogen Bonding

AbstractIn the current study, we evaluated the solubility of a number of organometallic species and showed that it is noticeably improved in diiodomethane when compared to other haloalkane solvents. The better solvation properties of CH2I2 were associated with the substantially better σ‐hole‐donating ability of this solvent, which results in the formation of uniquely strong solvent–(metal complex) halogen bonding. The strength of the halogen bonding is attenuated by the introduction of additional halogen atoms in the organometallic species owing to the competitive formation of more favourable intermolecular complex–complex halogen bonding. The exceptional solvation properties of diiodomethane and its inertness towards organometallic species make this solvent a good candidate for NMR studies, in particular, for the acquisition of spectra of insensitive spins.

Phonon-driven spin-Floquet magneto-valleytronics in MoS2

Two-dimensional materials equipped with strong spin–orbit coupling can display novel electronic, spintronic, and topological properties originating from the breaking of time or inversion symmetry. A lot of interest has focused on the valley degrees of freedom that can be used to encode binary information. By performing ab initio time-dependent density functional simulation on MoS2, here we show that the spin is not only locked to the valley momenta but strongly coupled to the optical E″ phonon that lifts the lattice mirror symmetry. Once the phonon is pumped so as to break time-reversal symmetry, the resulting Floquet spectra of the phonon-dressed spins carry a net out-of-plane magnetization (≈0.024μB for single-phonon quantum) even though the original system is non-magnetic. This dichroic magnetic response of the valley states is general for all 2H semiconducting transition-metal dichalcogenides and can be probed and controlled by infrared coherent laser excitation.

ChemInform Abstract: Two‐Dimensional Higher Quantum Correlation and Resolved Techniques for the Analyses of Complex 1H NMR Spectra of Scalar‐Coupled Spins

AbstractReview: 94 refs.

Robust and efficient 19F heteronuclear dipolar decoupling using refocused continuous-wave rf irradiation

Refocused continuous wave (rCW) decoupling is presented as an efficient and robust means to obtain well-resolved magic-angle-spinning solid-state NMR spectra of low-γ spins, such as 13C dipolar coupled to fluorine. The rCW decoupling sequences, recently introduced for 1H decoupling, are very robust towards large isotropic and anisotropic shift ranges as often encountered for 19F spins. In rCW decoupling, the so-called refocusing pulses inserted into the CW irradiation eliminate critical residual second- and third-order dipolar coupling and dipolar-coupling against chemical shielding anisotropy cross-terms in the effective Hamiltonian through time-reversal (i.e. refocusing). As important additional assets, the rCW decoupling sequences are robust towards variations in rf amplitudes, operational at low to high spinning speeds, and easy to set-up for optimal performance experimentally. These aspects are demonstrated analytically/numerically and experimentally in comparison to state-of-the-art decoupling sequences such as TPPM, SPINAL-64, and frequency-swept variants of these.

Magnetic Excitations of Spin-Gap System Na3Cu2SbO6 with Distorted Honeycomb Structure

Magnetic excitation spectra of quantum spins of Na3Cu2SbO6 with the distorted honeycomb structure have been measured by neutron inelastic scattering. The intensity distribution and the dispersion curves of the excitations indicate that the spin system can be well understood by considering one-dimensional spin chains formed of Cu2+ ions on a lattice of the alternation of the shorter- and longer-spacings along the b axis. The exchange interactions between the neighboring spins with the shorter- and longer-spacings correspond to ferromagnetic and antiferromagnetic ones, respectively. These findings are consistent with those we reported previously on the basis of macroscopic measurements. The observed spin gap is 8.9 meV and the antiferromagnetic- and ferromagnetic-interactions have been found to be 13.9 and -12.5 meV, respectively.

On the orientational dependence of resolution in1H solid-state NMR, and its role in MAS, CRAMPS and delayed-acquisition experiments

Numerical simulations and experiments are used to show that the spin dynamics of the dipolar-coupled networks in solids is often strongly dependent on crystallite orientation. In particular, different rates of dephasing of the magnetisation mean that NMR signals obtained at longer dephasing times are dominated by orientations in which the local dipolar coupling strength is relatively weak. This often leads to a distinct improvement in spectral resolution as the dephasing time is increased. The effects are particularly noticeable under magic-angle spinning (MAS), but are also observed when homonuclear decoupling is used to reduce the rate of dipolar dephasing. Numerical simulation is seen to be a powerful and easily used tool for understanding the behaviour of solid-state NMR experiments involving dipolar-coupled networks. The implications for solid-state NMR spectra of abundant spins acquired under MAS and homonuclear decoupling are discussed, as well as insights provided into the performance of 'delayed-acquisition' and 'constant-time' experiments.

Spin State Selective Detection of Single Quantum Transitions Using Multiple Quantum Coherence: Simplifying the Analyses of Complex NMR Spectra

NMR spectra of molecules oriented in liquid-crystalline matrix provide information on the structure and orientation of the molecules. Thermotropic liquid crystals used as an orienting media result in the spectra of spins that are generally strongly coupled. The number of allowed transitions increases rapidly with the increase in the number of interacting spins. Furthermore, the number of single quantum transitions required for analysis is highly redundant. In the present study, we have demonstrated that it is possible to separate the subspectra of a homonuclear dipolar coupled spin system on the basis of the spin states of the coupled heteronuclei by multiple quantum (MQ)-single quantum (SQ) correlation experiments. This significantly reduces the number of redundant transitions, thereby simplifying the analysis of the complex spectrum. The methodology has been demonstrated on the doubly 13C labeled acetonitrile aligned in the liquid-crystal matrix and has been applied to analyze the complex spectrum of an oriented six spin system.

75As, 113,115In, and 123Sb NMR Studies of Indirect Nuclear Spin–Spin Coupling in InX (X=As, Sb)

Indirect nuclear spin–spin coupling in InX (X=As, Sb) semiconductors has been investigated by magic-angle spinning (MAS) NMR for 75As, 113,115In and 123Sb spins. The coupling constants, J, of the isotropic indirect interaction for neighboring 113,115In–X (X=75As, 123Sb) spins were obtained as 478±10 and 730±40 Hz, respectively, from triple-resonance NMR experiments employing cross polarization from rare 113In to X and 115In decoupling during acquisition. The J values above and those for the 113,115In–121Sb spins were also estimated by a second moment analysis for the MAS NMR spectra of 75As, 113,115In and 123Sb spins, but the moment analysis provided values that deviated from the correct ones by a factor of up to 1.7. The reduced indirect coupling constant K was calculated from the above J values for InX (X=As, Sb) and the previously obtained value of J=224±5 Hz for InP [T. Iijima et al.: J. Phys. Soc. Jpn. 73 (2004) 1045]. The increase in K with decreasing band gap energy of the semiconductors of InX (X=P, As, Sb) was explained by considering that indirect coupling is dominated by the second-order perturbation energy of the nuclear–electron interaction with occupied and unoccupied states being in the valence and conduction bands, respectively.

Quantification of homonuclear dipolar coupling networks from magic-angle spinning 1H NMR

Numerical simulations of magic-angle spinning (MAS) spectra of dipolar-coupled nuclear spins have been used to assess different approaches to the quantification of dipolar couplings from 1H solid-state NMR. Exploiting the translational symmetry of periodic spin systems allows extended networks with ‘realistic’ numbers of spins to be considered. The experimentally accessible parameter is shown to be the root-sum-square of the dipolar couplings to a given spin. The effectiveness of either fitting the resulting spinning sideband spectra to small spin system models, or using analyses based on moment expansions, has been examined. Fitting of the spinning sideband pattern is found to be considerably more robust with respect to experimental noise than frequency domain moment analysis. The influence of the MAS rate and system geometry on robustness of the quantification is analysed and discussed.

Determination of molecular structure by NMR using low order orientation in bicelles

NMR spectra of molecules oriented in liquid crystalline phase provide information on the structure and orientation of the molecules. Thermotropic liquid crystals used for such a purpose as orienting media result in the spectra of spins that are generally strongly coupled unless heteronuclei are involved. The analyses of such spectra are carried out by diagonalising the Hamiltonian numerically, adopting the least square techniques. Phospholipid bicelles on the other hand are weak orienting systems which provides well-resolved proton and carbon spectra of spins that are, in general, weakly coupled. A spin system which is of the type AA′BB′C in thermotropic liquid crystal behaves as AA′MM′X type in bicelles. Though the resultant spectrum is not always first order, nevertheless, the estimate of the proton–proton and proton–carbon dipolar couplings and the chemical shifts can be made from the spectrum directly. The use of such a media as an alternative to thermotropic liquid crystalline phase for the determination of the structure of small molecules is demonstrated in this paper. The molecule pyridine is chosen for such a purpose. The structure of pyridine, including 13C spins, in both bicelle and thermotropic nematic phase have been determined.

Double-quantum filtered STMAS

Double-quantum and double-quantum-filtered satellite-transition magic-angle spinning (STMAS) experiments are proposed. The experiments efficiently convert satellite-transition coherence from single- to double-quantum with a central-transition selective π-pulse. The conversion allows the selection of double-quantum coherence transfer pathways with phase cycling that completely filters out unwanted diagonal and outer satellite-transition peaks. Both experiments are demonstrated with RbNO 3 and AlPO 4-berlinite as model compounds for obtaining clean STMAS spectra of spins 3/2 and 5/2, respectively.

Scaling and labeling the high-resolution isotropic axis of two-dimensional multiple-quantum magic-angle-spinning spectra of half-integer quadrupole spins

(obs) ! of the center of gravity of an MQ-filtered central-transition peak in the two dimensions allows us to determine the true isotropic chemical shift of an absorption line, which is related to the mean bond angle in a compound. Only the isotropic chemical shift and the second-order quadrupole interaction for a sample rotating at the magic angle at a high spinning rate are considered during the free precession of the spin system. On the other hand, only the first-order quadrupole interaction for a static sample is considered during the pulses. The hypercomplex detection method is used to obtain a pure 2D absorption spectrum. The pulse program and the successive stages of data processing are described. For simplicity, only the density matrix for a spin I5 3 at the end of the first pulse of phase w is calculated in detail, which allows us to deduce the phase cycling of the pulse sequence that selectively detects the 63-quantum coherences generated by the first pulse. The positions of the echo and antiecho relative to the second pulse, and that of the MQ-filtered central-transition peak relative to the carrier frequency (v 0) along the F1 dimension are derived for the four half-integer quadrupole spins. The frequency offset of v 0 relative to an external aqueous solution in the F1 dimension is linearly related to that in the F2 dimension. The shearing transformation, whose main interest is to shift the beginning of the acquisition period from the end of the second pulse to the echo position and to yield a high-resolution spectrum along the F1 dimension, is presented in great detail. In the literature, two conventions are used to define the evolution period. Convention Cz takes the experimental evolution period as the evolution period, whereas convention Ck considers the echo position relative to the first pulse as the evolution period. The true isotropic chemical shift, the second-order quadrupole shift of the center of gravity of a peak, and the parameter CQh relating the quadrupole coupling constant with the asymmetry parameter are functions of d G1 (obs) and d G2 (obs) ; their relationships depend on the conventions. In fact, convention Ck introduces a scaling factor ~depending on the spin and the coherence order! that modifies not only the spectral width and the linewidth, but also the positions of the spinning sidebands in the F1 dimension. Convention Cz is recommended for linewidth comparison. The analytical expressions are checked using 87 Rb (I5 3 2 ) in the extensively studied RbNO3 powder with two carrier frequencies: 98.2 and 163.6 MHz. Our data are in agreement with those of the literature.

Dynamic 2H nuclear magnetic resonance of rotating solids

The sensitivity of one-dimensional dynamic magic-angle spinning (MAS) and off-MAS 2H nuclear magnetic resonance spectra to changes in the parameters of jump-type molecular motions is studied. The Floquet theory approach is used to simulate spectra of spins with I = 1, which are involved in exchange processes in rotating solids. The solution of the Bloch-McConnell equations for rotating samples are derived and some simulated frequency spectra are shown. The dependence of the lineshapes of the center and sidebands of the MAS and off-MAS spectra on the exchange parameters are discussed. Experimental results of 2H spectra of perdeuterated dimethyl sulfone, obtained in the temperature range 20–55 °C, are demonstrated. The methyl groups in this molecule undergo π flips at rates that can be detected by MAS and off-MAS NMR. The shapes of the experimental sidebands are compared with simulated results.

Applications of multiple-pulse sequences to the simplification of liquid-crystal NMR spectra

Abstract It is shown that by changing their pulse timing, several multiple-pulse sequences normally used to obtain high-resolution NMR spectra of abundant spins in solids can afford spectra in which the effective dipolar couplings appear scaled by a factor ranging between 1 and −1 2 . This was used to design a family of par tial de coupling sequences (PARDES) which may be useful for recording simplified spectra of solute molecules oriented in liquid-crystal solvents. Upon exploring the experimental performance of these pulse sequences, it was found that their instrumental requirements are considerably less demanding than those necessary to perform dipolar averaging in the solid state. Moreover, it was observed that when proper experimental conditions are chosen, these PARDES perform a selective averaging of the dipolar couplings within the solute molecules without affecting the broad coupling patterns that characterize the nematic solvent, preserving thereby one of the main properties of liquid-crystal 1 H NMR experiments.

The use of composite pulses in the TOSS experiment

In order to obtain high-resolution spectra of dilute spins in powdered or amorphous solids, magic-angle spinning (MAS) has become almost indispensable ( 1-4). Spinning a sample with a frequency of vR results in a set of rotational echoes, spaced apart by a time interval of 1 /Q after the original free induction decay signal. Fourier transformation results in a characteristic pattern from which the centerband gives the isotropic frequency shift. The sideband intensities can be used to obtain the principal values of the chemical shielding tensor. However, for some applications it may be useful to eliminate the rotational sidebands, e.g., for analytical purposes, where the isotropic chemical shifts are used to distinguish nuclei with different chemical environments. In order to separate both the isotropic and the anisotropic shifts of powders containing chemically distinctive nuclei, two-dimensional techniques must be employed, for which it is also necessary at some stage to suppress the rotational sidebands (5). A useful method for sideband suppression has been devised by Dixon (6, 7) and has been named TOSS (total suppression of sidebands). The original sequence consists of four rotor-synchronized r pulses applied within one rotor period. The timing of the pulses is such that it leads to an in-phase alignment of the centerbands of each crystallite, while the sideband phases are such that they cancel when averaged over a powder (8, 9). Crucial for complete suppression of the sidebands are exact timing of the pulses, resonance offset, and pulse lengths. The timing of the pulses is prescribed by vR. The time interval between pulses is measured from the center of the pulses, implying that the x pulses must be kept as short as possible for the system to evolve freely between pulses. Pulse length errors and offset effects lead to incomplete cancellation of sidebands, especially when the sidebands are very narrow. This is the case in systems exhibiting a very small shift dispersion. As is well known, composite pulses can be used to compensate for these effects ( 10, 11). This, however, reduces the free evolution time of the system. Thus, the use of composite pulses in the TOSS experiment sets a limit to the spinning speed. Furthermore, the duration of the composite pulses must be kept as short as possible. It is the purpose of this Note to demonstrate that the application of composite pulses together with phase cycling in the TOSS sequence drastically reduces the artifacts of applying this sequence. A system with very narrow sidebands suitable for demonstrating