We study the energy level structure of the Tavis-Cumming model applied to an ensemble of independent magnetic spins s =1/ 2 coupled to a variable number of photons. Rabi splittings are calculated and their distribution is analyzed as a function of photon number n max and spin system size N. A sharp transition in the distribution of the Rabi frequency is found at n max Ϸ N. The width of the Rabi frequency spectrum diverges as ͱ N at this point. For increased number of photons n max Ͼ N, the Rabi frequencies converge to a value proportional to ͱ n max . This behavior is interpreted as analogous to the classical spin-resonance mechanism where the photon is treated as a classical field and one resonance peak is expected. We also present experimental data demonstrating cooperative, magnetic strong coupling between a spin system and photons, measured at room temperature. This points toward quantum computing implementation with magnetic spins, using cavity quantum-electrodynamics techniques.

Large-grain Nb has become a viable alternative to fine-grain Nb for the fabrication of superconducting radio-frequency cavities. In this contribution we report the results from a heat treatment study of a large-grain 1.5 GHz single-cell cavity made of "medium purity" Nb. The baseline surface preparation prior to heat treatment consisted of standard buffered chemical polishing. The heat treatment in the range 800 -1400 • C was done in a newly designed vacuum induction furnace. Q0 values of the order of 2 × 10 10 at 2.0 K and peak surface magnetic field (Bp) of 90 mT were achieved reproducibly. A Q0-value of (5 ± 1) × 10 10 at 2.0 K and Bp = 90 mT was obtained after heat treatment at 1400 • C. This is the highest value ever reported at this temperature, frequency and field. Samples heat treated with the cavity at 1400 • C were analyzed by secondary ion mass spectrometry, secondary electron microscopy, energy dispersive X-ray, point contact tunneling and X-ray diffraction and revealed a complex surface composition which includes titanium oxide, increased carbon and nitrogen content but reduced hydrogen concentration compared to a non heat-treated sample.

Large-spin Mn2+ ions (S=5/2) diluted in a nonmagnetic MgO matrix of high crystalline symmetry are used to realize a six-level system that can be operated by means of multiphoton coherent Rabi oscillations. This spin system has a very small anisotropy which can be tuned in situ to reversibly transform the system between harmonic and nonharmonic level configurations. Decoherence effects are strongly suppressed as a result of the quasi-isotropic electron interaction with the crystal field and with the 55Mn nuclear spins. These results suggest new ways of manipulating, reading, and resetting spin quantum states which can be applied to encode a qubit across several quantum levels.

We present an analysis of Nb 3 Sn surface layers grown on a bulk Niobium (Nb) coupon prepared at the same time and by the same vapor diffusion process used to make Nb 3 Sn coatings on 1.3 GHz Nb cavities. Tunneling spectroscopy reveals a well-developed, homogeneous superconducting density of states at the surface with a gap value distribution centered around 2.7 ±0.4 meV and superconducting critical temperatures (T c ) up to 16.3K. Scanning transmission electron microscopy (STEM) performed on cross sections of the sample's surface region shows a ∼ 2 microns thick Nb 3 Sn surface layer. The elemental composition map exhibits a Nb:Sn ratio of 3:1 and reveals the presence of buried sub-stoichiometric regions that have a ratio of 5:1. Synchrotron x-ray diffraction experiments indicate a polycrystalline Nb 3 Sn film and confirm the presence of Nb rich regions that occupy about a third of the coating volume. These low T c regions could play an important role in the dissipation mechanisms occurring during RF tests of Nb 3 Sn -coated Nb cavities and open the way for further improving a very promising alternative to pure Nb cavities for particle accelerators.Discovered in 1954 1 , the A-15 compound Nb 3 Sn is a Type II (κ∼20) strong coupling s-wave superconductor 2,3 with a maximum T c of 18 K 4 and superconducting order parameter ∆ of 3.4 meV 5 . Due to its relatively high T c and ability to carry high current densities, Nb 3 Sn is an ideal candidate for replacing NbTi for superconducting wire applications and Nb for superconducting radio frequency (SRF) resonators operating from a few hundred MHz up to several GHz. Early work into developing Nb 3 Sn for SRF applications started in the 1970s 6-9 . In particular, researchers from Wuppertal University optimized a coating recipe 8 based on the diffusion of Sn vapor into elemental Nb at temperatures between 1000 • C to 1200 • C. This approach has the unique advantage of being scalable to applications for which a coating process without a direct line of sight is required. State-of-the-art RF performance tests then 10 showed an extremely high quality factor ∼ 10 11 at 2K and ∼ 10 10 at 4.2K (about 20 times higher than pure Nb) with a strong decrease of the quality factor (Q -slope) above an accelerating field of 5 MV/m. The origin of this Q -slope remains unclear, however it was postulated that the onset of the Q decrease at 5 MV/m (peak surface magnetic field of 22 mT) was due to early vortex penetration above the Nb 3 Sn first critical field B C1 , and therefore was an intrinsic material limitation. A regain of interest was stimulated by recent RF tests done at Cornell University 11 that reproducibly exhibit a similar Q factor ∼ 2 × 10 10 at 4.2K (and 3 × 10 10 at 2K), but a very moderate Q -slope up to a quenching field of 12-17 MV/m, corresponding to a a) Electronic mail: prolier@anl.gov peak surface magnetic field of 50-70 mT, which is significantly higher than the B C1 of 25±7 mT measured on this cavity 11 . Another striking and reproducible feature is the very moderate ...

We report observation of the nonlinear Meissner effect (NLME) in Nb films by measuring the resonance frequency of a planar superconducting cavity as a function of the magnitude and the orientation of a parallel magnetic field. Use of low power rf probing in films thinner than the London penetration depth, significantly increases the field for the vortex penetration onset and enables NLME detection under true equilibrium conditions. The data agree very well with calculations based on the Usadel equations. We propose to use NLME angular spectroscopy to probe unconventional pairing symmetries in superconductors. 74.25.Ha, 74.25.Op, 74.78.Na Meissner effect is one of the fundamental manifestations of the macroscopic phase coherence of a superconducting state. Meissner screening current density J = −en s v s induced by a weak magnetic field is proportional to the velocity v s of the condensate. At higher fields, the superfluid density n s becomes dependent on v s due to pairbreaking effects, resulting in the nonlinear Meissner effect (NLME) [1][2][3][4][5]. NLME has attracted much attention since it probes unconventional pairing symmetries of moving condensates, for example, the dwave pairing in cuprates or multiband superconductivity in pnictides. For a single band isotropic type-II superconductor, the NLME at v s ≪h/mξ 0 is described by,where λ 0 is the London penetration depth, ξ 0 is the coherence length, Q = v s /mh = ∇θ + 2πA/φ 0 , m is the quasiparticle mass, θ is the phase of the order parameter, A is the vector potential, φ 0 is the flux quantum. NLME can manifest itself in a variety of different behaviors, which reveal the underlying pairing symmetry. For instance, Eq. (1) describes a clean d-wave superconductor at high temperatures k B T > p F v s or a d-wave superconductor with impurities, where p F is the Fermi momentum [2-4], while in the clean limit at k B T < p F v s , the nonlinear term in Eq. (1) takes the singular form ≃ a d ξ|Q| [2]. In the s-wave clean limit, the NLME is absent at T ≪ T c where a ∝ exp(−∆/T ) [6], but in the dirty limit a ∼ 1 even for T → 0. In multiband superconductors NLME can probe the onset of nonlinearity due to the appearance of interband phase textures suggested for MgB 2 [7] or the line nodes and interband sign change in the order parameter or mixed s-d pairing symmetries, which have been discussed recently for iron pnictides [8].To date, experiments aiming to observe the NLME in high-T c cuprates and other extreme type-II superconductors have been inconclusive [9-11] mostly because of a very small field region of the Meissner state. Since NLME becomes essential in fields H of the order of the thermodynamic critical field H c = φ 0 /2 √ 2πµ 0 λ 0 ξ 0 [1], penetration of vortices above the lower critical field µ 0 H c1 = (φ 0 /4πλ 2 0 )(ln κ + 0.5) ≪ µ 0 H c imposes the strong restriction H < H c1 , which reduces the nonlinear correction in Eq. (1) Yet even small NLME terms in Eq. (1) result in intermodulation effects [3] under strong ac fields, as observed in YBa 2 Cu ...

We report on multi-photon Rabi oscillations and controlled tuning of a multi-level system at room temperature (S = 5/2 for Mn 2+ :MgO) in and out of a quasi-harmonic level configuration. The anisotropy is much smaller than the Zeeman splittings, such as the six level scheme shows only a small deviation from an equidistant diagram. This allows us to tune the spin dynamics by either compensating the cubic anisotropy with a precise static field orientation, or by microwave field intensity. Using the rotating frame approximation, the experiments are very well explained by both an analytical model and a generalized numerical model. The calculated multi-photon Rabi frequencies are in excellent agreement with the experimental data.

We describe the design and testing of a point contact tunneling spectroscopy device that can measure material surface superconducting properties (i.e., the superconducting gap Δ and the critical temperature T(C)) and density of states over large surface areas with size up to mm(2). The tip lateral (X,Y) motion, mounted on a (X,Y,Z) piezo-stage, was calibrated on a patterned substrate consisting of Nb lines sputtered on a gold film using both normal (Al) and superconducting (PbSn) tips at 1.5 K. The tip vertical (Z) motion control enables some adjustment of the tip-sample junction resistance that can be measured over 7 orders of magnitudes from a quasi-ohmic regime (few hundred Ω) to the tunnel regime (from tens of kΩ up to few GΩ). The low noise electronic and LabVIEW program interface are also presented. The point contact regime and the large-scale motion capabilities are of particular interest for mapping and testing the superconducting properties of macroscopic scale superconductor-based devices.

Articles you may be interested inLow sheet resistance titanium nitride films by low-temperature plasma-enhanced atomic layer deposition using design of experiments methodology J. Vac. Sci. Technol. A 32, 031506 (2014); 10.1116/1.4868215Electron/phonon coupling in group-IV transition-metal and rare-earth nitridesIn situ spectroscopic ellipsometry study on the growth of ultrathin TiN films by plasma-assisted atomic layer deposition Heteroepitaxial growth of selected group IV-VI nitrides on various orientations of sapphire (a-Al 2 O 3 ) is demonstrated using atomic layer deposition. High quality, epitaxial films are produced at significantly lower temperatures than required by conventional deposition methods. Characterization of electrical and superconducting properties of epitaxial films reveals a reduced room temperature resistivity and increased residual resistance ratio for films deposited on sapphire compared to polycrystalline samples deposited concurrently on fused quartz substrates. V C 2013 AIP Publishing LLC. [http://dx.