Ultralong 100 ns spin relaxation time in graphite at room temperature.

Graphite has been intensively studied, yet its electron spins dynamics remains an unresolved problem even 70 years after the first experiments. The central quantities, the longitudinal (T1) and transverse (T2) relaxation times were postulated to be equal, mirroring standard metals, but T1 has never been measured for graphite. Here, based on a detailed band structure calculation including spin-orbit coupling, we predict an unexpected behavior of the relaxation times. We find, based on saturation ESR measurements, that T1 is markedly different from T2. Spins injected with perpendicular polarization with respect to the graphene plane have an extraordinarily long lifetime of 100 ns at room temperature. This is ten times more than in the best graphene samples. The spin diffusion length across graphite planes is thus expected to be ultralong, on the scale of ~ 70 µm, suggesting that thin films of graphite - or multilayer AB graphene stacks - can be excellent platforms for spintronics applications compatible with 2D van der Waals technologies. Finally, we provide a qualitative account of the observed spin relaxation based on the anisotropic spin admixture of the Bloch states in graphite obtained from density functional theory calculations.

Ultralong Spin Lifetime in Light Alkali Atom Doped Graphene.

Today's great challenges of energy and informational technologies are addressed with a singular compound, Li- and Na-doped few-layer graphene. All that is impossible for graphite (homogeneous and high-level Na doping) and unstable for single-layer graphene works very well for this structure. The transformation of the Raman G line to a Fano line shape and the emergence of strong, metallic-like electron spin resonance (ESR) modes attest the high level of graphene doping in liquid ammonia for both kinds of alkali atoms. The spin-relaxation time in our materials, deduced from the ESR line width, is 6-8 ns, which is comparable to the longest values found in spin-transport experiments on ultrahigh-mobility graphene flakes. This could qualify our material as a promising candidate in spintronics devices. On the other hand, the successful sodium doping, this being a highly abundant metal, could be an encouraging alternative to lithium batteries.

A highly accurate measurement of resonator Q-factor and resonance frequency.

The microwave cavity perturbation method is often used to determine material parameters (electric permittivity and magnetic permeability) at high frequencies, and it relies on the measurement of the resonator parameters. We present a method to determine the Q-factor and resonance frequency of microwave resonators which is conceptually simple but provides a sensitivity for these parameters which overcomes those of existing methods by an order of magnitude. The microwave resonator is placed in a feedback resonator setup, where the output of an amplifier is connected to its own input with the resonator as a bandpass filter. After reaching steady-state oscillation, the feedback circuit is disrupted by a fast microwave switch, and the transient signal, which emanates from the resonator, is detected using down-conversion. The Fourier transform of the resulting time-dependent signal yields directly the resonance profile of the resonator. Albeit the method being highly accurate, this comes with a conceptual simplicity, ease of implementation, and lower circuit cost. We compare existing methods for this type of measurement to explain the sensitivity of the present technique, and we also make a prediction for the ultimate accuracy for the resonator Q and f 0 determination.

Giant microwave absorption in fine powders of superconductors.

Enhanced microwave absorption, larger than that in the normal state, is observed in fine grains of type-II superconductors (MgB2 and K3C60) for magnetic fields as small as a few % of the upper critical field. The effect is predicted by the theory of vortex motion in type-II superconductors, however its direct observation has been elusive due to skin-depth limitations; conventional microwave absorption studies employ larger samples where the microwave magnetic field exclusion significantly lowers the absorption. We show that the enhancement is observable in grains smaller than the penetration depth. A quantitative analysis on K3C60 in the framework of the Coffey-Clem (CC) theory explains well the temperature dependence of the microwave absorption and also allows to determine the vortex pinning force constant.

Non-calorimetric determination of absorbed power during magnetic nanoparticle based hyperthermia.

Nanomagnetic hyperthermia (NMH) is intensively studied with the prospect of cancer therapy. A major challenge is to determine the dissipated power during in vivo conditions and conventional methods are either invasive or inaccurate. We present a non-calorimetric method which yields the heat absorbed during hyperthermia: it is based on accurately measuring the quality factor change of a resonant radio frequency circuit which is employed for the irradiation. The approach provides the absorbed power in real-time, without the need to monitor the sample temperature as a function of time. As such, it is free from the problems caused by the non-adiabatic heating conditions of the usual calorimetry. We validate the method by comparing the dissipated power with a conventional calorimetric measurement. We present the validation for two types of resonators with very different filling factors: a solenoid and a so-called birdcage coil. The latter is a volume coil, which is generally used in magnetic resonance imaging (MRI) under in vivo condition. The presented method therefore allows to effectively combine MRI and thermotherapy and is thus readily adaptable to existing imaging hardware.

A time domain based method for the accurate measurement of Q-factor and resonance frequency of microwave resonators.

We present a novel method to determine the resonant frequency and quality factor of microwave resonators which is faster, more stable, and conceptually simpler than the yet existing techniques. The microwave resonator is pumped with the microwave radiation at a frequency away from its resonance. It then emits an exponentially decaying radiation at its eigen-frequency when the excitation is rapidly switched off. The emitted microwave signal is down-converted with a microwave mixer, digitized, and its Fourier transformation (FT) directly yields the resonance curve in a single shot. Being a FT based method, this technique possesses the Fellgett (multiplex) and Connes (accuracy) advantages and it conceptually mimics that of pulsed nuclear magnetic resonance. We also establish a novel benchmark to compare accuracy of the different approaches of microwave resonator measurements. This shows that the present method has similar accuracy to the existing ones, which are based on sweeping or modulating the frequency of the microwave radiation.