between the probe and the preamplifier as in Ref. . We, in particular, take into account the noise produced by the preamplifier impedance which is fed back to the coil through the transmission line. The combination of the two last terms plays a key role for explaining the experimental observation. After Section II, dedicated to Materials and Methods, in Section III, theoretical models are described, providing evidence of the importance of both electronic elements. After a short description of the classical theory (Section III.A), in Section III.B it is predicted that, depending on the transmission line and preamplifier impedance, the radiation damping contribution can strongly vary altering the nuclearspin resonance line-width and potentially inducing, for a perfectly tuned system (SNTO condition), the appearance of a ''bump'' rather than the usual ''dip'' for the nuclearspin-noise signal superimposed on the average electronic noise level for a thermally equilibrated spin system and a classical probe. In addition in Section III.C the RF excitations affecting the nuclear susceptibility induced by Nyquist noise due to the preamplifier impedance are considered: a model allowing the numerical calculation of the nuclearspin-noise spectra is developed. Conversely to the model of Section III.B, using this model FSTO and SNTO conditions are not always simultaneously fulfilled. Finally, in Section III.D a mathematical framework is introduced which allows us to formally obtain the general shape of the nuclearspin-noise resonance. In this framework, it becomes possible to demonstrate that FSTO and SNTO conditions can differ, such an effect results from magnetization excitations induced by noise fluctuations within the preamplifier impedance combined with dephasing effects due to the transmission line. Section IV is devoted to the careful validation of the model: nuclearspin-noise spectra have been simulated using the measured electronic components and directly confronted to experimental measurements. In Section V, several aspects of the derivation are discussed; in particular an equation which can be used for determining physically relevant parameters is introduced. Finally, conclusions are drawn in Section VI.
A major challenge in analytical chemistry is the detection of small amounts in the presence of an excess of a spectroscopically similar substance. It occurs, for instance, when low-abundance isotopic species are exploited for obtaining a speciﬁc ‘ﬁngerprint’, providing attractive applications, for instance, for characterizing the origin of natural products 1 , for environmental studies 2 , for toxicology 3 or for proteomics 4 . When this challenge is addressed by nuclear magnetic resonance (NMR) three issues appear for distinguishing minor components from the major one: the spectral resolution, the dynamic range and potentially a nonlinear effect, called radiation damping 5,6 , which results from the feed-back ﬁeld induced by the precessing magnetization of the major component. It is particularly manifest when state-of-the-art high sensitivity receiving circuits are used. Radiation damping tends to broaden and shift the main resonances, totally obscuring small signals in the vicinity of large ones. To circumvent these effects, one remedy resides in diluting the mixture until the main compound does not cause these effects anymore. However, this also dilutes the minor species and therefore leads to low signal-to-noise ratios, long measurement times and high analysis costs, since NMR per se is not very sensitive. Moreover, changing the concentration causes a variation of the chemical shifts and therefore may interfere with the ﬁnal aim of the analysis. These issues are encountered in the speciﬁc case of NMR isotopic measurements in 1 H NMR spectra due to low-natural abundance isotopes such as 13 C. Isotope effects on chemical shifts (IECS) 7 induce small differences in chemical shift (in the p.p.b. range), requiring special care for their measurements, if radiation damping affects the main isotopomer component. While detection of one-bond 13 C– 1 H isotopic effects on high-resolution liquid-state NMR spectrometers is facilitated through the appearance of so-called satellite signals split due to the large 1 J scalar coupling, the small 1 H signals caused by two- bond isotopic effects are not easily observable experimentally. Nevertheless long-range IECS have been detected either directly through ultra-high-resolution spectroscopy 8 or indirectly through two-dimensional NMR experiments 9 .
Spinnoise spectroscopy is an optical technique which can probe spin resonances non-perturbatively. First applied to atomic vapours, it revealed detailed information about nuclear magnetism and the hyperﬁne interaction. In solids, this approach has been limited to carriers in semiconductor heterostructures. Here we show that atomic-like spin ﬂuctuations of Mn ions diluted in CdTe (bulk and quantum wells) can be detected through the Kerr rotation associated to excitonic transitions. Zeeman transitions within and between hyperﬁne multiplets are clearly observed in zero and small magnetic ﬁelds and reveal the local symmetry because of crystal ﬁeld and strain. The linewidths of these resonances are close to the dipolar limit. The sensitivity is high enough to open the way towards the detection of a few spins in systems where the decoherence due to nuclear spins can be suppressed by isotopic enrichment, and towards spin resonance microscopy with important applications in biology and materials science.
In practice, DNP-MAS is performed in an NMR probe with MAS capabilities that i) can be cooled down to c.a. 100 K, and that ii) is equipped with a waveguide and beam launcher enabling microwave irradiation of the sample under MAS conditions. The continuous wave irradiation is generated by an external gyrotron whose frequency is close to the electron spin resonance frequency in the magnet where the NMR experiment is performed. The microwave irradiation travels through a corrugated waveguide coupled to the bottom of the NMR probe as can be seen on Figure 2a. The sample placed in the low-temperature MAS probe rotates about the magic angle at about 10 to 40 kHz (depending on the probe performances) and the typical operating temperature is 90-110 K 11 . Additional accessories for temperature and microwave control are included in the experimental setting. The in-situ character of this experimental setting makes it compatible with most conventional solid- state NMR experiments. In particular, the approach is fully compatible with 2D NMR spectroscopy. However, the applicability of DNP-MAS may be limited by resolution losses caused by line broadening.
Advanced techniques like shaped pulses, with amplitude and phase ramping, composite pulses [ 30 ], optimal control theory or numerical techniques [ 31 , 32 ] can provide better fidelity. The analytical model of control serves then as an initial guess for the numerical searches. Pulses found in this way correct for the couplings among nuclei and are robust over a wide range of parameters (such as experi- mental errors or the noise associated with static fields). Table I shows the results of simulations in a fictitious NV system with 1–4 nuclear spins and effective frequencies in the m s ¼ 1 manifold ranging from 15 to 2 MHz. We searched numerically via a conjugate gradient algorithm for a control sequence performing a desired unitary evolu- tion, varying amplitude and phase of the w and rf fields. We then simulated the control sequence in the presence of noise, with contributions from a large, static field and a smaller fluctuating one. The projected fidelities in the FIG. 4 (color online). rf and w pulse sequence to implement
Ó 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).
Site directed spin labeling (SDSL) is a useful tool for studying biomolecules by NMR and EPR spectroscopy. 1 e4 InNMR spectros- copy, the dipolar interaction between unpaired electrons, normally nitroxide radicals or paramagnetic metal ions, and nearby nuclei gives rise to the paramagnetic relaxation enhancement (PRE) ef- fect 5 resulting in increased nuclear relaxation rates. The PRE effect has a long range ( >25 A), a strong distance-dependence, and can be quanti ﬁed making it well suited to providing additional accurate distance restraints for the structure determination of protein e- protein and protein eoligonucleotide complexes.
Our results fundamentally differ from previous NMR work of B¨ uttgen et al. [ 27 ]. First, below saturation they observe an inconsistency between the bulk magnetization and the NMR data. They conclude that the majority of the sample is already in the saturated phase as moni- tored by NMR even though the magnetization still ex- hibits a linear slope due to the presence of defects. This is in contrast to our result, where the bulk magnetization measured on the same sample batch approximately co- incides with the local magnetization measured by NMR, and also shows the signature of the nematic phase [ 28 ]. Second, they attribute a possible spin-nematic phase to a narrow field range between 40.5 T and 41.4 T, where the (local) magnetization exhibits a very steep slope and the NMR spectra present a continuous change of ∆H int
2 gas used to cool and spin the sample through pressurized heat exchangers that are submerged in liquid N 2 .
3 – MAS-DNP-assisted NMR crystallography
Structural studies of organic solids which are not suitable for single crystal X-ray diffraction can greatly profit from ssNMR. “NMR crystallography” studies have relied strongly on 1 H NMR (e.g. 1 H-X HETCOR spectra) and the comparison of experimental chemical shifts to those calculated by density functional theory (DFT) or other computational methods. 47–49 Through this comparison chemical structures can be validated from a given set of models. In general, 1 H-X HETCOR experiments are used because of the high sensitivity of 1 H compared to the lower-γ X nucleus. From these experiments the chemical shifts of X-, and if resolution permits, 1 H-nuclei are measured for comparison to those calculated from candidate structures. In cases where greater spectral dispersion is required for the assignment of resonances 13 C- 13 C correlation spectra can be acquired using INADEQUATE-type experiments. 50,51 These methods have been successfully used for structure determination on numerous systems and this type of approach has been expedited when used in combination with MAS-DNP techniques. 47,48,52,53
were computed with the gauge-including projector-augmented wave (GIPAW) 72 ap-
proach, using its implementation in the Quantum ESPRESSO 115 software package. The crystal structure of cyclo-FF 98 served as the initial input for the DFT calcula- tions, and hydrogen atom positions were optimized before the GIPAW calculations. The expected polarization buildups were then simulated with SPINEVOLU- TION, based on the CSA tensor parameters from the DFT computations, atomic positions from the crystal structure, and isotropic chemical shifts from experiment. The polarization buildups for all relevant spin pairs were simulated separately, and the individual curves were then co-added for each peak. It is important to stress again that, although several different distances contribute to each buildup curve, we consider these contributions to be coming from isolated spin pairs. Deviations from this approximation by the presence of a third spin leading to dipolar truncation at NA will be discussed in section 3.3.4.
Our novel strategy combines coherent control of the NV sensor with an intrinsic quantum memory to enhance the sensor spectral resolution. This control strategy not only creates a sharp dynamic filter by alternating periods of a spin-lock Hamiltonian with evolution under a gradient field, but it also provides other advantages. The sequence is compatible with homonuclear decoupling, thus allowing sensing beyond the natural biomolecule NMR linewidth. In addition, our technique allows mapping the couplings among the spins themselves, using them as local probes of their environment. The resulting multidimensional NMR spectra highlight spatial correlations in the sample, lift spectral overlaps due to symmetries, and aid the structure reconstruction algorithms. This would allow us to resolve the contributions in the NV signal arising from different nuclear spins in a dense sample and to use the acquired information to determine the nuclearspin positions. Reconstructing a protein local 3D structure under its natural conditions would allow researchers to work backwards and design compounds that interact with specific sites.
To address the challenges imposed by low sensitivity, dynamic nuclear polarization (DNP) may be applied. In DNP, the large electron polarization of a paramagnetic species is transferred to neighbouring nuclei via microwave irradiation. This efficiency of this process is optimal at cryogenic temperatures where electron spin and nuclear relaxation times increase and allow for efficient polarization transfer. Bis-nitroxide biradicals are the most successful polarizing agents due to the increased electron-electron coupling (20–35 MHz) that is required for cross effect (CE) efficiency.  To date solid-state DNP research faces two main issues: a) determining the kind of samples that are amenable for DNP and b) understanding and preventing the loss of resolution at cryogenic temperatures. Although several studies have demonstrated the possibility of polarizing the surface of silica-based materials by DNP,  it is not trivial to deduce that the method is also suited for increasing the sensitivity in the study of proteins encapsulated within a biosilica matrix. In terms of sample preparation, the protein and the radical may reside in different phases, separated by the silica matrix, as opposed to the case of a homogenous glassy preparation,  and as such it could give no enhancement at all, as in the case of frozen solution without cryoprotectant. On the contrary, in this work we could demonstrate that substantial enhancements are achieved inNMR sensitivity of catMMP12 and AS-SOD proteins entrapped in biosilica by employing high field DNP (5 T and 16.4 T), providing the proof of principle that high-field DNP enhancement can be obtained for these systems.
NMR spectra are usually obtained by exciting, through a rf field, the nuclear magnetization and then by monitoring the induction, it creates. An alternative approach, named spinnoise, exists: it consists in searching for correlations in the noise signal at the probe detection output, a concept up to now used for only a single spin species . Here, we report its extension for looking to small signals in the presence of a major one and show that this technique allows sensitivity enhancement for their detection, in particular when the temperature of detection coil is lower than that of the sample. Signals resulting from small species appear as bumps, superimposed on the dip which results from the main component contribution.
located below each QD layer.
To probe the resident hole spin polarization, we measured the photo-induced circular dichroism (PCD) in the QD sample. A picosecond Ti:sapphire laser is split into pump and probe beams (the repetition frequency is 76 MHz). The pump beam polarization is σ+/σ- modulated at 42 kHz with a photo-elastic modulator; the probe beam is linearly polarized. After transmission through the sample, the probe beam is decomposed into its two circular components, and the difference in their intensities is measured with a balanced optical bridge. To improve the signal-to-noise ratio, a double lock-in amplifier analysis of the signal is performed, the pump and probe beams being modulated with a mechanical chopper at two different frequencies. In the same sample, the electron spin dynamics has been measured by time-resolved photoluminescence (PL) experiments; 1.5 ps pulses generated by a Ti-Sapphire laser at a repetition frequency of 82 MHz were used as excitation light, and the PL signal was recorded by using a S1 photocathode streak camera with an overall time resolution of 20 ps.
1 measurements were performed by first
rotating the nuclear magnetization out of equilibrium by a short saturation chain of rf-pulses, then waiting a variable recovery time, t, and finally measuring the in- tegrated spin-echo intensity, m(t). As discussed below, the quadrupolar splitting of the NMR line is very small, about 15 kHz, so that a single exponential form is ex- pected for m(t) as long as all parts of the sample have the same value of T 1 −1 . To monitor any deviation from the single exponential behavior, we used a stretched ex- ponential to fit our data:
4.2.2. Parahydrogen-based hyperpolarisation
Hyperpolarised spin order can be prepared efficiently and with comparatively simple instrumentation for dihydrogen gas. Dihydrogen exists as two spin isomers, ortho and para, corresponding to the three symmetric (triplet) states and the antisymmetric (singlet) state of the spin pair. The existence of spin isomers is due to the Pauli prin- ciple and is only observed for small and highly symmetric molecules. For dihydrogen, the ortho and para forms interconvert efficiently when the gas is in contact with a paramagnetic solid, but an out-of-equilibrium ortho/para distribution can otherwise be retained for several days. At room temperature, the four energy levels are nearly equally populated, resulting in a 75:25 ortho:para distribution. ‘Para-hydrogen gas’ (dihydrogen gas with a non equilibrium ortho:para distribution) is prepared by flow- ing dihydrogen gas at temperatures of 20 to 80 K, in the presence of a paramagnetic solid particles. Parahydrogen may be seen as a reservoir of long-lived singlet order. While parahydrogen is NMR silent and is unaffected by pulse sequences, its spin order can be released by a chemical addition. The addition is either directly onto a substrate of interest  (the PHIP mechanism, parahydrogen induced polarisation), in which case the enhanced spin order on the substrate is not renewable, or onto a metal centre on which a substrate is bound reversibly, in which case through-bond spin order trans- fer to the substrate spins can occur multiple times over several complex formations  (the SABRE mechanism, signal amplification by reversible exchange).
Modelling the effects of dynamic exchange on the central transition lineshape
An examination of the possible orientations for a single water molecule in ice-Ih or a clathrate hydrate shows that if the protons are distinguished, there are 12 different orienta- tions of equal probability (Fig. 4). Assuming a single-step jump about an O-H axis only (not about O–H hydrogen bond axes) is required for the reorientational change, then each of the 12 orientations can have 4 possible single-step jumps to the other orientations. With the simplifying approx- imation that the coordination is perfectly tetrahedral, then all of these single-step reorientations are equivalent in terms of their kinetic parameters, as are forward and reverse ex- changes. In fact this is a very reasonable assumption based on the structure of ice-Ih, but a little less so for the THF clathrate hydrate with three crystallographically distinct O atoms and some angles with larger deviations from Td. While this model provides a more complete picture of water reorientation (compared with the 4-site tetrahedral jump model used to simulate the dynamic deuterium NMR spectra of D 2 O ice-Ih), 28 as far as the 17 O tensor is concerned, pairs
We provide in this section our numerical results [ 61 ] about the relaxation rate 1/T 1 using models and tech-
niques presented in the previous sections II and III . First of all we will focus on the XX model (equivalent to free fermions) for which we can compute exactly the dynam- ical correlations for all temperatures and that will serve as a benchmark for our simulations. Next, we will turn to the interacting XXZ case for S = 1/2, and then to a S = 1 chain model relevant to the DTN material.
The spin of a carrier confined in a semiconductor quan- tum dot 共QD兲 is considered as a good candidate for realizing quantum bits in the solid state. The hyperfine interaction 共HI兲 of an electron spin with the nuclei spins has been identified as the most efficient mechanism of electron spin relaxation or decoherence in QDs at low temperature. 1 – 4 The electron HI has a Fermi-contact character, due to the s-type Bloch function of an electron, and is then expected to be much more efficient than the HI of a hole, which has a p-type Bloch function. Recent theoretical studies have shown that, for a hole spin, the dipole-dipole HI term is the dominant one and leads to an anisotropic HI, in contrast to the isotropic electron HI. 5 , 6 Moreover, the hole-nuclei interaction is far from being negligible and is predicted to be only one order of magnitude weaker than the electron-nuclei interaction in InAs QDs. Very recently, the first experimental evidences of the hole HI have been found, 7 , 8 but its strength relative to the electron HI is a real question from an experimental point of view, whereas it is of prime importance for estimating not only decoherence times but also spin cooling rates. In this letter, we measure this relative strength by studying the magnetic-field dependence of the hole- and electron-spin po- larizations obtained in p-doped and n-doped InAs QDs, re- spectively. Indeed, for both electron and hole, the HI can be modelized by a frozen effective nuclear field whose strength and direction vary from QD to QD. 2 , 5 , 6 In the absence of an external magnetic field, each spin optically prepared or- thogonally to the plane of QDs precesses coherently around the local nuclear field. The average carrier spin polarization in the QD ensemble partially decays with a characteristic initial time T ⌬ e 共T ⌬ h 兲 for electrons 共holes兲, as a consequence of the random distribution of the local nuclear effective fields. 2 , 6 When an external field is applied in the out-of-the-plane di- rection, the effect of the HI on the carrier spin polarization can be strongly reduced if this external field is larger than the dispersion of the in-plane fluctuations of the effective nuclear
Quantum Hall Ferromagnet. Sci. Rep. 7, 43553; doi: 10.1038/srep43553 (2017).
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Cholesterol is known to regulate a diverse range of membrane protein functions. For example, it modulates the conformational equilibria, stability, and ligand-binding affinities of G-protein coupled receptors to affect signal transduction 7–11 . It is implicated in
Alzheimer’s disease and binds the amyloid precursor protein 12,13 . Cholesterol also changes the conformation and localization of viral fusion proteins 14–16 . Cholesterol can modulate membrane protein function by specific binding to proteins or by non-specific effects on the membrane physical properties. However, the precise mechanism of cholesterol’s influence on membrane proteins is so far poorly understood, and likely varies from protein to protein. The lack of information is largely due to the small size and complex dynamics of cholesterol in lipid bilayers, and the incompatibility of cholesterol with the detergent micelles and small isotropic bicelles that are commonly used for membrane protein reconstitution before solution NMR studies 17 . In comparison, cholesterol can be advantageous for protein crystallization when used in concert with curvature lipids such as monoolein 18 . Of the approximately dozen high-resolution (< 3 Å) cholesterol-bound membrane protein structures that have been deposited in the Protein Databank so far, most were solved using X-ray crystallography. However, in most these cases, the bound cholesterol in these structures results from co-crystallization from monoolein-cholesterol lipid cubic phases, and has no