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We use a bulk acoustic wave resonator to demonstrate coherent control of the excited orbital states in a diamond nitrogen-vacancy (NV) center at cryogenic temperature. Coherent quantum control is an essential tool for understanding and mitigating decoherence. Moreover, characterizing and controlling orbital states is a central challenge for quantum networking, where optical coherence is tied to orbital coherence. We study resonant multi-phonon orbital Rabi oscillations in both the frequency and time domain, extracting the strength of the orbital-phonon interactions and the coherence of the acoustically driven orbital states. We reach the strong-driving limit, where the physics is dominated by the coupling induced by the acoustic waves. We find agreement between our measurements, quantum master equation simulations, and a Landau-Zener transition model in the strong-driving limit. Using perturbation theory, we derive an expression for the orbital Rabi frequency versus acoustic drive strength that is non-perturbative in the drive strength and agrees well with our measurements for all acoustic powers. Motivated by continuous wave spin resonance-based decoherence protection schemes, we model the orbital decoherence and find good agreement between our model and our measured few-to-several nanoseconds orbital decoherence times. We discuss the outlook for orbital decoherence protection., Comment: 19 pages, 11 figures including supplement

The spectrally narrow, spin-dependent optical transitions of nitrogen vacancy (NV) center defects in diamond can be harnessed for quantum networking applications. Key to such networking schemes is the generation of indistinguishable photons. Two challenges limit scalability in such systems: defect-to-defect variations of the optical transition frequencies caused by local strain variation, and spectral diffusion of the optical frequencies on repeated measurement caused by photoexcitation of nearby charge traps. In this experimental study we undertake a group theoretic approach to quantifying spectral diffusion and strain, decomposing each into components corresponding to Jahn-Teller symmetries of the NV center. We investigate correlations between the components of strain, spectral diffusion, and depth from surface, finding that strain and spectral diffusion are each dominated by longitudinal perturbations. We also find a weak negative correlation between transverse static strain and total spectral diffusion suggesting that transverse strain provides some degree of protection from spectral diffusion. Additionally, we find that spectral diffusion becomes more pronounced with increasing depth in the diamond bulk. Our symmetry-decomposed technique for quantifying spectral diffusion can be valuable for understanding how a given nanoscale charge trap environment influences spectral diffusion and for developing strategies of mitigation., Comment: 29 pages in preprint format including figures and appendix

Vanadium tetracyanoethylene (V[TCNE]$_{x}$, $x\approx 2$) is an organic-based ferrimagnet with a high magnetic ordering temperature $\mathrm{T_C>600 ~K}$, low magnetic damping, and growth compatibility with a wide variety of substrates. However, similar to other organic-based materials, it is sensitive to air. Although encapsulation of V[TCNE]$_{x}$ with glass and epoxy extends the film lifetime from an hour to a few weeks, what is limiting its lifetime remains poorly understood. Here we characterize encapsulated V[TCNE]$_{x}$ films using confocal microscopy, Raman spectroscopy, ferromagnetic resonance and SQUID magnetometry. We identify the relevant features in the Raman spectra in agreement with \textit{ab initio} theory, reproducing $\mathrm{C=C,C\equiv N}$ vibrational modes. We correlate changes in the effective dynamic magnetization with changes in Raman intensity and in photoluminescence. Based on changes in Raman spectra, we hypothesize possible structural changes and aging mechanisms in V[TCNE]$_x$. These findings enable a local optical probe of V[TCNE]$_{x}$ film quality, which is invaluable in experiments where assessing film quality with local magnetic characterization is not possible., Comment: 14 pages including supplement

Context: In the laboratory, hydrogen peroxide (HOOH) was proven to be an intermediate product in the solid-state reaction scheme that leads to the formation of water on icy dust grains. When HOOH desorbs from the icy grains, it can be detected in the gas phase. In combination with water detections, it may provide additional information on the water reaction network. Hydrogen peroxide has previously been found toward $\rho$ Oph A. However, further searches for this molecule in other sources failed. Hydrogen peroxide plays a fundamental role in the understanding of solid-state water formation and the overall water reservoir in young stellar objects (YSOs). Without further HOOH detections, it is difficult to assess and develop suitable chemical models that properly take into account the formation of water on icy surfaces. Aims: The objective of this work is to identify HOOH in YSOs and thereby constrain the grain surface water formation hypothesis. Methods: Using an astrochemical model based on previous work in combination with a physical model of YSOs, the sources R CrA-IRS\,5A, NGCC1333-IRAS\,2A, L1551-IRS\,5, and L1544 were identified as suitable candidates for an HOOH detection. Long integration times on the APEX 12m and IRAM 30m telescopes were applied to search for HOOH signatures in these sources. Results: None of the four sources under investigation showed convincing spectral signatures of HOOH. The upper limit for HOOH abundance based on the noise level at the frequency positions of this molecule for the source R CrA-IRS\,5A was close to the predicted value. For NGC1333-IRAS 2A, L1544, and L1551-IRS\,5, the model overestimated the hydrogen peroxide abundances., Comment: 13 pages, 4 figures

Using a high quality factor 3 GHz bulk acoustic wave resonator device, we demonstrate the acoustically driven single quantum spin transition ($\left|m_{s}=0\right>\leftrightarrow\left|\pm1\right>$) for diamond NV centers and characterize the corresponding stress susceptibility. A key challenge is to disentangle the unintentional magnetic driving field generated by device current from the intentional stress driving within the device. We quantify these driving fields independently using Rabi spectroscopy before studying the more complicated case in which both are resonant with the single quantum spin transition. By building an equivalent circuit model to describe the device's current and mechanical dynamics, we quantitatively model the experiment to establish their relative contributions and compare with our results. We find that the stress susceptibility of the NV center spin single quantum transition is around $\sqrt{2}(0.5\pm0.2)$ times that for double quantum transition ($\left|+1\right>\leftrightarrow\left|-1\right>$). Although acoustic driving in the double quantum basis is valuable for quantum-enhanced sensing applications, double quantum driving lacks the ability to manipulate NV center spins out of the $\left|m_{s}=0\right>$ initialization state. Our results demonstrate that efficient all-acoustic quantum control over NV centers is possible, and is especially promising for sensing applications that benefit from the compact footprint and location selectivity of acoustic devices., Comment: 9 pages, 2-column format

The methods for controlling spin states of negatively charged nitrogen-vacancy (NV) centers using microwave (MW) or radiofrequency (RF) excitation fields for electron spin and nuclear spin transitions are effective in strong magnetic fields where a level anti-crossing (LAC) occurs. A LAC can also occur at zero field in the presence of transverse strain or electric fields in the diamond crystal, leading to mixing of the spin states. In this paper, we investigate zero-field LAC of NV centers using dual-frequency excitation spectroscopy. Under RF modulation of the spin states, we observe sideband transitions and Autler-Townes splitting in the optically detected magnetic resonance (ODMR) spectra. Numerical simulations show that the splitting originates from Landau-Zener transition between electron spin |$\pm$1> states, which potentially provides a new way of manipulating NV center spin states in zero or weak magnetic field., Comment: this article supersedes arXiv:1902.10481

We carried out point contact (PC) investigation of WTe2 single crystals. We measured Yanson d2V/dI2 PC spectra of the electron-phonon interaction (EPI) in WTe2. The spectra demonstrate a main phonon peak around 8 meV and a shallow second maximum near 16 meV. Their position is in line with the calculation of the EPI spectra of WTe2 in the literature, albeit phonons with higher energy are not resolved in our PC spectra. An additional contribution to the spectra is present above the phonon energy, what may be connected with the peculiar electronic band structure and need to be clarified. We detected tiny superconducting features in d2V/dI2 close to zero bias, which broadens by increasing temperature and blurs above 6K. Thus, (surface) superconductivity may exist in WTe2 with a topologically nontrivial state. We found a broad maximum in dV/dI at large voltages (>200 mV) indicating change of conductivity from metallic to semiconducting type. The latter might be induced by the high current density (~10^8 A/cm^2) in the PC and/or local heating, thus enabling the manipulation of the quantum electronic states at the interface in the PC core., Comment: 4 figs., 11 pages with supplement, will be presented at APS March Meeting, March 4-8, Boston

MoTe2 is a Weyl semimetal, which exhibits unique non-saturating magnetoresistance and strongly reinforced superconductivity under pressure. Here, we demonstrate that a novel mesoscopic superconductivity at ambient pressure arises on the surface of MoTe2 with a critical temperature up to 5 K significantly exceeding the bulk Tc=0.1K. We measured the derivatives of I-V curves for metallic hetero-contacts of MoTe2 with Ag or Cu, homo-contacts of MoTe2 as well as "soft" point contacts (PCs). Large number of these hetero-contacts exhibit a dV/dI dependence, which is characteristic for Andreev reflection. It allows to determine the superconducting gap \Delta. The average gap values are 2\Delta=1.30+/-0.15 meV with a 2\Delta/kBTc ratio of 3.7+/-0.4, which slightly exceeds the standard BCS value of 3.52. Furthermore, the temperature dependence of the gap follows a BCS-like behavior, which points to a nodeless superconducting order parameter with some strong-coupling renormalization. Remarkably, the observation of a "gapless-like" single minimum in the dV/dI of "soft" PCs may indicate a topological superconducting state of the MoTe2 surface as these contacts probe mainly the interface and avoid additional pressure effect. Therefore, MoTe2 might be a suitable material to study new forms of topological superconductivity., Comment: 9 pages, 7 figures. Presented at the 6th International Conference on Superconductivity and Magnetism (ICSM2018), April 29 - May 04, 2018 (Antalya, Turkey); V2: Discussion extended, seven Refs. added, accepted to "2D Materials" journal

We study the resonant optical transitions of a single nitrogen-vacancy (NV) center that is coherently dressed by a strong mechanical drive. Using a gigahertz-frequency diamond mechanical resonator that is strain-coupled to an NV center's orbital states, we demonstrate coherent Raman sidebands out to the ninth order and orbital-phonon interactions that mix the two excited-state orbital branches. These interactions are spectroscopically revealed through a multi-phonon Rabi splitting of the orbital branches which scales as a function of resonator driving amplitude, and is successfully reproduced in a quantum model. Finally, we discuss the application of mechanical driving to engineering NV center orbital states., Comment: 4 main text figures and 12 supporting figures. 20 pages total

FeSe single crystals have been studied by soft point-contact Andreev-reflection spectroscopy. Superconducting gap features in the differential resistance dV/dI(V) of point contacts such as a characteristic Andreev-reflection double-minimum structure have been measured versus temperature and magnetic field. Analyzing dV/dI within the extended two-gap Blonder-Tinkham-Klapwijk model allows to extract both the temperature and magnetic field dependence of the superconducting gaps. The temperature dependence of both gaps is close to the standard BCS behavior. Remarkably, the magnitude of the double-minimum structure gradually vanishes in magnetic field, while the minima position only slightly shifts with field indicating a weak decrease of the superconducting gaps. Analyzing the dV/dI(V) spectra for 25 point contacts results in the averaged gap values = 1.8+/-0.4meV and =1.0+/-0.2 meV and reduced values 2/kTc=4.2+/-0.9 and 2/kTc=2.3+/-0.5 for the large (L) and small (S) gap, respectively. Additionally, the small gap contribution was found to be within tens of percent decreasing with both temperature and magnetic field. No signatures in the dV/dI spectra were observed testifying a gapless superconductivity or presence of even smaller gaps., Comment: 8 pages, 4 figs., 3 tables. Shortened version without fig.4 and Table 3 is accepted for publication in Phys. Rev. B

By use of ultrasound studies we resolved magneto-acoustic quantum oscillation deep into the mixed state of the multiband nonmagnetic superconductor YNi2B2C. Below the upper critical field, only a very weak additional damping appears that can be well explained by the field inhomogeneity caused by the flux-line lattice in the mixed state. This is clear evidence for no or a vanishingly small gap for one of the bands, namely, the spheroidal alpha band. This contrasts de Haas--van Alphen data obtained by use of torque magnetometry for the same sample, with a rapidly vanishing oscillation signal in the mixed state. This points to a strongly distorted flux-line lattice in the latter case that, in general, can hamper a reliable extraction of gap parameters by use of such techniques., Comment: 6 pages, 6 figures

We propose a protocol to dissipatively cool a room temperature mechanical resonator using a nitrogen-vacancy (NV) center ensemble. The spin ensemble is coupled to the resonator through its orbitally-averaged excited state, which has a spin-strain interaction that has not been previously characterized. We experimentally demonstrate that the spin-strain coupling in the excited state is $13.5\pm0.5$ times stronger than the ground state spin-strain coupling. We then theoretically show that this interaction combined with a high-density spin ensemble enables the cooling of a mechanical resonator from room temperature to a fraction of its thermal phonon occupancy., Comment: Main text is 11 pages in preprint formatting, with 4 figures. Also included is 17 pages of supporting information including 7 supporting figures

Rise in superconducting critical temperature Tc more than two times (exceeding 20 K) is discov- ered in point-contacts created between iron-chalcogenide FeSe single crystal and Cu. The possible reasons of such Tc increase in point-contacts are discussed. The most probable cause for this may be the interfacial carriers doping and/or interfacial enhanced electron-phonon interaction., Comment: 4 pages, 3 Figs. V2: two Refs. added, Fig.3 updated

We have carried out detailed magnetic and transport studies of the new Sr0.5Ce0.5FBiS2-xSex (x = 0.5, 1) superconductors derived by doping Se in Sr0.5Ce0.5FBiS2. Se-doping produces several effects: it suppresses semiconducting like behavior observed in the undoped Sr0.5Ce0.5FBiS2, ferromagnetic ordering temperature, TFM, decreases considerably from 7.5 K (in Sr0.5Ce0.5FBiS2) to 3.5 K and superconducting transition temperature, Tc, gets enhanced slightly to 2.9 - 3.3 K. Thus in these Se-doped materials, TFM is just marginally higher than Tc. Magnetization studies provide an evidence of bulk superconductivity in Sr0.5Ce0.5FBiS2-xSex. Quite remarkably, as compared with the effective paramagnetic Ce-moment (~ 2.2 muB), the ferromagnetically ordered Ce-moment in the superconducting state is rather small (~ 0.1 muB). To the best of our knowledge, the title compounds are the first Ce-based superconducting itinerant ferromagnetic materials (Tc < TFM). We stress that Ce-4f electrons are responsible for both superconductivity and ferromagnetism just as U-5f electrons are in UCoGe. Furthermore, a novel feature of these materials is a dual hysteresis loop corresponding to both the ferromagnetism and the coexisting superconductivity. Such features of Sr0.5Ce0.5FBiS2-xSex put these materials apart from the well known U-containing superconducting ferromagnets reported so far., Comment: 12 pages, 5 figures + 4 supplementary figures

Nonlinear conductivity of point contacts (PCs) on the base of FeSe single crystals has been investigated. Measured dV/dI dependencies demonstrate the prevailing contribution to the PC conductivity caused by the degraded surface. Superconducting (SC) feature in dV/dI like a sharp zero-bias minimum develops for relatively low ohmic PCs, where the deep areas of FeSe are involved. Analysis of dV/dI has shown that the origin of the zero-bias minimum is connected with the Maxwell part of the PC resistance, what masks energy dependent spectral peculiarities. Even so, we have found the specific features in dV/dI - the sharp side maxima, which may have connection to the SC gap, since their position follows the BCS temperature dependence. Exploring the dV/dI spectra of the rare occurrence with Andreev-like structure, the two gaps with Delta = 2.5 and 3.5 meV were identified., Comment: 10 pages, 6 figs., accepted for publication in "Low Temperature Physics"

Point-defects in wide bandgap semiconductors are promising candidates for future applications that necessitate quantum light sources. Recently, defect-based single photon sources have been observed in ZnO that are very bright and remain photoactive from 4.5 K to room temperature. Despite several investigations, the structure and electronic states of these emitters remain unknown. In this work, we establish a procedure to distinguish a Z dipole from an XY dipole when studying quantum emitters that are randomly oriented. Our cryogenic and room temperature polarization measurements collectively establish that these unidentified ZnO quantum emitters have a Z dipole. We show that the associated absorption and emission dipoles are parallel within experimental uncertainty for all 32 individuals studied. Additionally, we apply group theory and find that assuming the defect symmetry belongs to a point-group relevant to the ZnO wurtzite lattice, the ground and excited states are orbital singlets. These results are a significant step in identifying the structure and electronic states of defect-based single photon sources in ZnO., Comment: 19 pages in manuscript format, 4 figures. Also a 9 page supplementary information

Inhomogeneous dephasing from uncontrolled environmental noise can limit the coherence of a quantum sensor or qubit. For solid state spin qubits such as the nitrogen-vacancy (NV) center in diamond, a dominant source of environmental noise is magnetic field fluctuations due to nearby paramagnetic impurities and instabilities in a magnetic bias field. In this work, we use ac stress generated by a diamond mechanical resonator to engineer a dressed spin basis in which a single NV center qubit is less sensitive to its magnetic environment. For a qubit in the thermally isolated subspace of this protected basis, we prolong the dephasing time $T_2^*$ from $2.7\pm0.1$ $\mu$s to $15\pm1$ $\mu$s by dressing with a $\Omega=581\pm2$ kHz mechanical Rabi field. Furthermore, we develop a model that quantitatively predicts the relationship between $\Omega$ and $T_2^*$ in the dressed basis. Our model suggests that a combination of magnetic field fluctuations and hyperfine coupling to nearby nuclear spins limits the protected coherence time over the range of $\Omega$ accessed here. We show that amplitude noise in $\Omega$ will dominate the dephasing for larger driving fields.

We investigated the influence of different Ag additions (up to 10 wt %) on the superconducting properties of FeSe0.94. The structural investigations (XRD and SEM) indicated that Ag is present in three different forms. Ag at grain boundaries supports the excellent intergrain connections and reduces superconducting transition width to values smaller than 1K at B=0 and smaller than 2.74 K at B=14 T. Ag insertion in the crystal lattice unit cell provides additional carriers and changes the electron hole imbalance in FeSe0.94. This results in an increase in the magnetoresistive effect (MR) and critical temperature (Tc). Reacted Ag forms a small amount (~1%) of Ag2Se impurity phase, which may increase the pinning energy in comparison with that of the undoped sample. The enhanced upper critical field (Bc2) is also a result of the increased impurity scattering. Thus, unlike cuprates Ag addition enhances the Tc, Bc2, pinning energy and MR making the properties of polycrystalline FeSe0.94 similar to those of single crystals., Comment: 19 pages, 8 figures, submited to Supercond. Sci. Technol

Measuring local magnetization dynamics and its spatial variation is essential for advancements in spintronics and relevant applications. Here we demonstrate a phase-sensitive imaging technique for studying patterned magnetic structures based on picosecond laser heating. With the time-resolved anomalous Nernst effect (TRANE) and extensions, we simultaneously image the dynamic magnetization and RF driving current density. The stroboscopic detection implemented in TRANE microscopy provides access to both amplitude and phase information of ferromagnetic resonance (FMR) and RF current. Using this approach, we measure the spatial variation of the Oersted driving field angle across a uniform channel. In a spatially nonuniform sample with a cross shape, a strong spatial variation for the RF current as well as FMR precession is observed. We find that both the amplitude and the phase of local FMR precession are closely related to those of the RF current., Comment: 7 pages, 8 figures

Recent advances in nanoscale magnetism have demonstrated the potential for spin-based technology including magnetic random access memory, nanoscale microwave sources, and ultra-low power signal transfer. Future engineering advances and new scientific discoveries will be enabled by research tools capable of examining local magnetization dynamics at length and time scales fundamental to spatiotemporal variations in magnetic systems - typically 10 - 200 nm and 5 - 50 ps. A key problem is that current table-top magnetic microscopy cannot access both of these scales simultaneously. In this letter, we introduce a spatiotemporal magnetic microscopy that uses magneto-thermoelectric interactions to measure local magnetization via the time-resolved anomalous Nernst effect (TRANE). By generating a short-lived, local thermal gradient, the magnetic moment is transduced into an electrical signal. Experimentally, we show that TRANE microscopy has time resolution below 30 ps and spatial resolution limited by the thermal excitation area. Furthermore, we present numerical simulations to show that the thermal spot size sets the limits of the spatial resolution, even at 50 nm. The thermal effects used for TRANE microscopy have no fundamental limit on their spatial resolution, therefore a future TRANE microscope employing a scanning plasmon antenna could enable measurements of nanoscale magnetic dynamics., Comment: 31 pages including supplementary information

Coherent control of the nitrogen-vacancy (NV) center in diamond's triplet spin state has traditionally been accomplished with resonant ac magnetic fields under the constraint of the magnetic dipole selection rule, which forbids direct control of the $|-1>\leftrightarrow |+1>$ spin transition. We show that high-frequency stress resonant with the spin state splitting can coherently control NV center spins within this subspace. Using a bulk-mode mechanical microresonator fabricated from single-crystal diamond, we apply intense ac stress to the diamond substrate and observe mechanically driven Rabi oscillations between the $|-1>$ and $|+1>$ states of an NV center spin ensemble. Additionally, we measure the inhomogeneous spin dephasing time ($T_{2}^{*}$) of the spin ensemble using a mechanical Ramsey sequence and compare it to the dephasing times measured with a magnetic Ramsey sequence for each of the three spin qubit combinations available within the NV center ground state. These results demonstrate coherent spin driving with a mechanical resonator and could enable the creation of a phase-sensitive $\Delta$-system within the NV center ground state., Comment: 19 pages double spaced, 4 figures and Supplementary Information

We report an experimental and theoretical investigation of the electron-boson interaction in KFe2As2 by point-contact (PC) spectroscopy, model, and ab-initio LDA-based calculations for the standard electron-phonon Eliashberg function. The PC spectrum viz. the second derivative of the I - V characteristic of representative PC exhibits a pronounced maximum at about 20meV and surprisingly a featureless behavior at lower and higher energies. We discuss phonon and non-phonon (excitonic) mechanisms for the origin of this peak. Analysis of the underlying source of this peak may be important for the understanding of serious puzzles of superconductivity in this type of compounds., Comment: 10 pages, 6 figs., to be published in PRB

We investigated the superconducting transition and the pinning properties of undoped and Ag-doped FeSe0.94 at magnetic fields up to 14 T. It was established that due to Ag addition the hexagonal phase formation in melted FeSe0.94 samples is suppressed and the grain connectivity is strongly improved. The obtained superconducting zero-field transition becomes sharp (with a transition width below 1 K), Tc and the upper critical field were found to increase, whereas the normal state resistivity significantly reduces becoming comparable with those of FeSe single crystals. In addition, a considerable magnetoresistance was observed due to Ag doping. The resistive transition of undoped and Ag-doped FeSe0.94 is dominated by thermally activated flux flow. From the activation energy U vs H dependence, a crossover from single-vortex pinning to a collective creep pinning behavior was found with increasing the magnetic field., Comment: 19 pages, 11 figures

We present study of derivatives of current-voltage I(V) characteristics of point-contacts (PCs) based on Ba{1-x}Na{x}Fe2As2 (x=0.25) in the normal and superconducting state. The detailed analysis of dV/dI(V) data (also given in Appendix A) shows that the thermal regime, when temperature increases with a voltage at a rate of about 1.8 K/mV, is realized in the investigated PCs at least at high biases V above the superconducting (SC) gap \Delta. In this case, specific resistivity \rho (T) in PC core is responsible for a peculiar dV/dI(V) behavior, while a pronounced asymmetry of dV/dI(V) is caused by large value of thermopower in this material. A reproducible zero-bias minima detected on dV/dI(V) at low biases in the range \pm(6--9)mV well below the SC critical temperature T_c could be connected with the manifestation of the SC gap \Delta. Evaluation of these Andreev-reflection-like structures on dV/dI(V) points out to the preferred value of 2\Delta/kT_c \approx 6. The expected second gap features on dV/dI(V) are hard to resolve unambiguously, likely due to impurity scattering, spatial inhomogeneity and transition to the mentioned thermal regime as the bias further increases. Suggestions are made how to separate spectroscopic features in dV/dI(V) from those caused by the thermal regime., Comment: 10 pages, 15 figs, two Apps, to be published in PRB

Investigations that probe defects one at a time offer a unique opportunity to observe properties and dynamics that are washed out of ensemble measurements. Here we present confocal fluorescence measurements of individual defects in Al-doped ZnO nanoparticles and undoped ZnO sputtered films that are excited with sub-bandgap energy light. Photon correlation measurements yield both antibunching and bunching, indicative of single-photon emission from isolated defects that possess a metastable shelving state. The single-photon emission is in the range 560 - 720 nm and typically exhibits two broad spectral peaks separated by approximately 150 meV. The excited state lifetimes range from 1 - 13 ns, consistent with the finite-size and surface effects of nanoparticles and small grains. We also observe discrete jumps in the fluorescence intensity between a bright state and a dark state. The dwell times in each state are exponentially distributed and the average dwell time in the bright (dark) state does (may) depend on the power of the exciting laser. Taken together, our measurements demonstrate the utility of a single-molecule approach to semiconductor defect studies and highlight ZnO as a potential host material for single-defect based applications., Comment: 33 pages, 7 figures

The tetragonal FeSe phase is an intensively investigated iron based superconductor. In this study we examined the influence of Ag addition on the superconducting properties of selenium deficient polycrystalline FeSe0.94. The samples were obtained by solid state reaction and melting methods. XRD analysis shows the presence of tetragonal phase and EDX analysis establishes inhomogeneous Ag distribution in the grains. The superconducting properties were investigated by fundamental and third harmonic AC magnetic susceptibility. The intergranular critical current determined from AC magnetic susceptibility in the Ag doped sample is several times higher than that in the undoped one, obtained by melting at approximately the same temperatures. Intra-granular current is field independent up to almost 1000 Oe. Using the temperature dependence of third harmonic AC magnetic susceptibility at different DC magnetic fields, the irreversibility lines were obtained for all samples. It is found that Ag addition increased the irreversibility field in comparison with undoped melted and powder sintered samples. All results show that the Ag addition in selenium deficient (FeSe0.94) samples leads to improvement of inter- and intra- granular properties: screening ability, pinning, activation energy and critical current and improves the irreversibility line., Comment: 10 pages, 8 figures

As spin-based quantum technology evolves, the ability to manipulate spin with non-magnetic fields is critical - both for the development of hybrid quantum systems and for compatibility with conventional technology. Particularly useful examples are electric fields, optical fields, and mechanical lattice vibrations. The last of these represents direct spin-phonon coupling, which has garnered fundamental interest as a potential mediator of spin-spin interactions, but could also find applications in high-stability inertial sensing. In this Letter, we demonstrate direct coupling between phonons and nitrogen-vacancy (NV) center spins in diamond by inducing spin transitions with mechanically-driven harmonic strain. The ability to control NV spins mechanically can enhance NV-based quantum metrology, grant access to all transitions within the spin-1 quantum state of the NV center, and provide a platform to study spin-phonon interactions at the level of a few interacting spins.

Using the microscopic formalism of Eilenberger equations, a three-band Ginzburg-Landau theory for the intraband dirty limit and clean interband scattering case is derived. Within the framework of this three-band Ginzburg-Landau theory, expressions for the critical temperature Tc and the temperature dependence of the upper critical field H_c2 are obtained. Based on some special cases of the matrix of interaction constants, we demonstrate the influence of the sign of the interband interaction on the critical temperature and the upper critical field as compared with a two-band superconductor where it plays no role. We study also analytically and numerically the effect of its magnitude., Comment: 24 pages, 5 figures 2 new references numbered as [35], [36] in the revised version of the paper have been added; corrected typos and some wording in the whole text

High-quality KFe2As2 (K122) single crystals synthesized by different techniques have been studied by magnetization and specific heat (SH) measurements. There are 2 types of samples both affected by disordered magnetic phases: (i) cluster-glass (CG) like or (ii) Griffiths phase (G) like. For (i) at low applied magnetic fields the T-dependence of the zero field cooled (ZFC) linear susceptibility (chi_l) exhibits an anomaly with an irreversible behavior in ZFC and field cooled (FC) data. This anomaly is related to the freezing temperature T_f. The extrapolated T_f to B=0 varies between 50 K and 90 K. Below T_f we observed a magnetic hysteresis in the field dependence of the isothermal magnetization (M(B)). The frequency shift of the freezing temperature delta T_f=Delta T_f/[T_f\Delta(\ln \nu)]\sim 0.05$ has an intermediate value, which provides evidence for the formation of a CG-like state in the K122 samples of type (i). The frequency dependence of their T_f follows a conventional power-law divergence of critical slowing down: tau=tau_0 [T_f(nu)/T_f(0)-1]^{-z\nu^{'}} with the critical exponent z\nu^{'}=10(2) and a relatively long characteristic time constant tau_0 =6.9 x10^{-11}$s also supporting a CG behavior. The large value of the Sommerfeld coefficient was related to magnetic contribution from a CG. Samples from (ii) did not show a hysteresis behavior for chi_l(T) and M(B). Below a crossover temperature T^* sim 40 K a power-law dependence in the chi_l propto T^[lambda_G-1}], with a non-universal lambda_G was observed, suggesting a quantum G-like behavior. In this case chi_l and M(B) can be scaled using the scaling function M_s(T,B)= B^{1-\lambda_{\tiny G}}Y(mu B/k_BT) with the scaling moment mu of the order of 3.5mu_b. The same non-universal exponent was found also in SH measurements, where the magnetic contribution C/T propto T^(lambda_G-1)., Comment: 6 pages, 6 figures

High-quality KFe2As2 single crystals have been studied by transport, magnetization and low-T specific heat measurements. Their analysis shows that superconductivity occurs (in some cases coexists) in the vicinity of disordered magnetic phases (Griffiths and spin-glass type) depending of the amount of local magnetic moments (probably excess Fe derived)and sample inhomogeneity. The achieved phenomenological description of our data including also data from the literature provides a consistent explanation of the observed non-Fermi-liquid behavior and of the nominally large experimental Sommerfeld coefficient gamma_n about 94 mJ/mol K^2. We suggest that the intrinsic value (directly related to the itinerant quasi-particles) gamma_el about 60(10)mJ/mol K^2 is significantly reduced compared with gamma_n. Then an enhanced relative jump of the electronic specific heat Delta C_el/gamma_el T_c about 0.8 and a weak total electron-boson coupling constant lambda less or about 1 follow., Comment: 6 pages, 4 figures (including supplement)

We examine the quantum spin state of a single nitrogen-vacancy (NV) center in diamond at room temperature as it makes a transition from the orbital ground-state (GS) to the orbital excited-state (ES) during non-resonant optical excitation. While the fluorescence read-out of NV-center spins relies on conservation of the longitudinal spin projection during optical excitation, the question of quantum phase preservation has not been examined. Using Ramsey measurements and quantum process tomography, we establish limits on NV center spin decoherence induced during optical excitation. Treating the optical excitation and ES spin precession as a quantum process, we measure a process fidelity of F=0.87\pm0.03, which includes ES spin dephasing during measurement. Extrapolation to the moment of optical excitation yields F\approx0.95. This result demonstrates that ES spin interactions may be used as a resource for quantum control because the quantum spin state can survive incoherent orbital transitions., Comment: 12 pages, 3 figures

Point-contact (PC) investigations on the title compound in the normal and superconducting (SC) state (Tc=10,6 K) are presented. The temperature dependence of the SC gap of TmNi2B2C determined from Andreev-reflection (AR) spectra using the standard single-gap approximation (SGA) deviates from the BCS behavior in displaying a maximum at about Tc/2. A refined analysis within the two-gap approximation provides evidence for the presence of a second gap twice as large as the main gap (the first one), while the latter is close to that within the SGA. This way, TmNi2B2C expands the number of nickel borocarbide superconductors which exhibit a clear multiband character. Additionally, for the first time "reentrant" features were found in the AR spectra for some PCs measured in a magnetic field. The PC spectroscopy of the electron-boson interaction in TmNi2B2C in the normal state reveals a pronounced phonon maximum at 9.5meV and a more smeared one around 15 meV, while at higher energies the PC spectra are almost featureless. Additionally, the most intense peak slightly above 3meV observed in the PC spectra of TmNi2B2C is presumably caused by crystalline-electric-field (CEF) excitations. The peak near 1meV detected for some PC spectra is connected with a modification of the CEF probably due to boron or carbon vacancies, allowing to probe the local stoichiometry by PC spectroscopy., Comment: 10 two-column pages, 9 figs, to be published in PRB

Point-contact (PC) Andreev-reflection (AR) measurements of the superconducting gap in iron-oxipnictide ReFeAsO_{1-x}F_x (Re=La, Sm) films have been carried out. The value of the gap is distributed in the range 2\Delta \simeq 5-10 meV (for Re=Sm) with a maximum in the distribution around 6 meV. Temperature dependence of the gap \Delta(T) can be fitted well by BCS curve giving reduced gap ratio 2\Delta /kT_c^*\simeq 3.5 (here T_c^* is the critical temperature from the BCS fit). At the same time, an expected second larger gap feature was difficult to resolve distinctly on the AR spectra making determination reliability of the second gap detection questionable. Possible reasons for this and the origin of other features like clear-cut asymmetry in the AR spectra and current regime in PCs are discussed., Comment: 6 two-column pages, 6 figs., 26 Refs., to be published in Superconductor Science and Technology

We show that the zero field normal-state resistivity above Tc for various levels of electron doping - both for LaO1-xFxFeAs (La-1111) and SmO1-xFxFeAs (Sm-1111) members of the 1111-iron-pnictide superconductor family - can be scaled in a broad temperature range from 20 to 300 K onto single curves for underdoped La-1111 (x=0.05-0.075), for optimally and overdoped La-1111 (x=0.1-0.2) and for underdoped Sm-1111 (x=0.06-0.1) compounds. The scaling was performed using the energy scale {\Delta}, the resistivity {\rho}_{\Delta} and the residual resistivity {\rho}_0 as scaling parameters as well as by applying a recently proposed model-independent scaling method (H. G. Luo, Y. H. Su, and T. Xiang, Phys. Rev. B 77, 014529 (2008)). The scaling parameters have been calculated and the compositional variation of {\Delta} has been determined. The observed scaling behaviour for {\rho}(T) is interpreted as an indication of a common mechanism which dominates the scattering of the charge carriers in underdoped La-1111, in optimally and overdoped La-1111 and in underdoped Sm-1111 compounds.., Comment: 17 pages, 5 figures, 1 Table

We present point-contact (PC) Andreev-reflection measurements of a superconducting epitaxial c-axis oriented nickel borocarbide film LuNi2B2C (Tc=15.9 K). The averaged value of the superconducting gap is found to be 2.6 +/-0.2 meV in the one-gap approach, whereas the two-gap approach results in 2.14+/-0.36 meV and 3.0+/-0.27 meV. The better fit of the Andreev-reflection spectra for the LuNi2B2C - Cu PC obtained by the two-gap approach provides evidence for multiband superconductivity in LuNi2B2C. For the first time, PC electron-phonon interaction (EPI) spectra have been measured for this compound. They demonstrate pronounced phonon maximum at 8.5+/-0.4meV and a second shallow one at 15.8+/-0.6 meV. The electron-phonon coupling constant estimated from the PC EPI spectra turned out to be small (~ 0.1), like in other superconducting rare-earth nickel borocarbides. Possible reasons for this are discussed., Comment: 5 pages, 5 figures, V2: figs. 2 & 5 captions are corrected, and new Refs. 4, 6, 12, 13, 14 are added

We demonstrate methods to locally control the spin rotation of moving electrons in a GaAs channel. The Larmor frequency of optically-injected spins is modulated when the spins are dragged through a region of spin-polarized nuclei created at a MnAs/GaAs interface. The effective field created by the nuclei is controlled either optically or electrically using the ferromagnetic proximity polarization effect. Spin rotation is also tuned by controlling the carrier traverse time through the polarized region. We demonstrate coherent spin rotations exceeding 4 pi radians during transport., Comment: 15 pages, 4 figures

We show that the zero field normal-state resistivity above Tc for various levels of electron doping-both for underdoped, optimally, and overdoped LaO1-xFxFeAs, i.e. (x=0.05-0.075 and 0.1-0.2) and underdoped SmO1-xFxFeAs (x=0.06- 0.1) members of the 1111-iron-pnictide superconductor family-can be scaled in a broad temperature range from 20 to 300 K onto a single universal curve. The scaling was performed using the energy scale Delta, the resistivity rho(Delta) and the residual resistivity rho0 as scaling parameters as well as applying a recently proposed model-independent scaling method (H. G. Luo, Y. H. Su, and T. Xiang, Phys. Rev. B 77, 014529 (2008)). The scaling parameters have been calculated and the compositional variation of Delta has been determined. The existence of a universal metallic rho(T) curve is interpreted as an indication of a single mechanism which dominates the scattering of the charge carriers in LaO1-xFxFeAs (x= 0.05 to 0.075 and 0.1 to 0.2) and SmO1-xFxFeAs (x = 0.06 to 0.1) ., Comment: This paper has been withdrawn by the authors. 18 pages, 5 figures, the number of authors has been changed considerably, a similar slightly shortened paper with four previous authors, only, has been submitted as a new preprint

We report 75As NMR measurements of the spin-lattice relaxation in the superconducting state of LaFeAsO_(0.9)F_(0.1) and As-deficient LaFeAs_(1-d)O_0.9F_0.1. The temperature behavior of 1/T_1 below T_c changes drastically from a T^3-dependence for LaFeAsO_(0.9)F_(0.1) to a T^5-dependence for the As-deficient sample. These results, together with the previously reported unexpected increase of T_c and the slope of the upper critical field near T_c for the As-deficient sample, are discussed in terms of non-universal SC gaps in Fe-pnictides and the effect of As deficiency as an exotic case where nonmagnetic 'smart' impurities even stabilize an s+- wave superconductor or within a scenario of a disorder-driven change to s++ superconductivity., Comment: 4 pages, 3 figures

The localized spin triplet ground state of a nitrogen vacancy (NV) center in diamond can be used in atomic-scale detection of local magnetic fields. Here we present a technique using these defects in diamond to image fields around magnetic structures. We extract the local magnetic field vector by probing resonant transitions of the four fixed tetrahedral NV orientations. In combination with confocal microscopy techniques, we construct a 2-dimensional image of the local magnetic field vectors. Measurements are done in external fields less than 50 G and under ambient conditions., Comment: 9 pages, 3 figures

We report resistivity and upper critical field B_c2(T) data for disordered (As deficient) LaO_0.9F_0.1FeAs_1-delta in a wide temperature and high field range up to 60 T. These samples exhibit a slightly enhanced superconducting transition at T_c = 28.5 K and a significantly enlarged slope dB_c2/dT = -5.4 T/K near T_c which contrasts with a flattening of B_c2(T) starting near 23 K above 30 T. The latter evidences Pauli limiting behaviour (PLB) with B_c2(0) approximately 63 T. We compare our results with B_c2(T)-data from the literature for clean and disordered samples. Whereas clean samples show almost no PLB for fields below 60 to 70 T, the hitherto unexplained pronounced flattening of B_c2(T) for applied fields H II ab observed for several disordered closely related systems is interpreted also as a manifestation of PLB. Consequences are discussed in terms of disorder effects within the frames of (un)conventional superconductivity, respectively., Comment: 2 pages, 3 figures, submitted to M2S Tokyo 09

Hydrogenation reactions of CO in inter- and circumstellar ices are regarded as an important starting point in the formation of more complex species. Previous laboratory measurements by two groups on the hydrogenation of CO ices resulted in controversial results on the formation rate of methanol. Our aim is to resolve this controversy by an independent investigation of the reaction scheme for a range of H-atom fluxes and different ice temperatures and thicknesses. Reaction rates are determined by using a state-of-the-art ultra high vacuum experimental setup to bombard an interstellar CO ice analog with room temperature H atoms. The reaction of CO + H into H2CO and subsequently CH3OH is monitored by a Fourier transform infrared spectrometer in a reflection absorption mode. In addition, after each completed measurement a temperature programmed desorption experiment is performed to identify the produced species. Different H-atom fluxes, morphologies, and ice thicknesses are tested. The formation of both formaldehyde and methanol via CO hydrogenation is confirmed at low temperature (12-20 K). We confirm, as proposed by Hidaka et al., that the discrepancy between the two Japanese studies is mainly due to a difference in the applied hydrogen atom flux. The production rate of formaldehyde is found to decrease and the penetration column to increase with temperature. In order to fully understand the laboratory data, the experimental results are interpreted using Monte Carlo simulations. This technique takes into account the layered structure of CO ice. Temperature-dependent reaction barriers and diffusion rates are inferred using this model. The model is extended to interstellar conditions to compare with observational H2CO/CH3OH data., Comment: accepted by A. & A., 21 pages, 15 figures

We report resistivity and upper critical field B_c2(T) data for As deficient LaO_(0.9)F_(0.1)FeAs_(1-delta) in a wide temperature and high field range up to 60 T. These disordered samples exhibit a slightly enhanced superconducting transition at T_c = 29 K and a significantly enlarged slope dB_(c2))/dT = -5.4 T/K near T_c which contrasts with a flattening of B_(c2)(T) starting near 23 K above 30 T. This flattening is interpreted as Pauli limiting behaviour (PLB) with B_(c2)(0) approx 63 T. We compare our results with B_(c2)(T)-data reported in the literature for clean and disordered samples. Whereas clean samples show no PLB for fields below 60 to 70 T, the hitherto unexplained flattening of B_(c2)(T) for applied fields H || ab observed for several disordered closely related systems is interpreted also as a manifestation of PLB. Consequences of our results are discussed in terms of disorder effects within the frame of conventional and unconventional superconductivity., Comment: 4 pages, 3 figures, submitted to RHMF09 (9th International Conference on the Research in High Magnetic Fields), Dresden, July 22-25, 2009

DyRh4B4 has been known to crystallize in the primitive tetragonal (pt)-structure and to exhibit a ferromagnetic transition at 12 K, the highest magnetic transition temperature in the entire series of the RRh4B4 materials [1]. We show here that our silicon-added samples of the nominal composition DyRh4B4Si0.2 exhibit superconductivity below Tc ~ 4.5 K and an antiferromagnetic transition below TN ~ 2.7 K. The 12 K transition observed in the pt-DyRh4B4 is completely suppressed. Our annealed samples mainly consist of domains of the chemical composition DyRh3.9B4.2Si0.08. These domains contain two crystallographic phases belonging to the body-centred tetragonal (bct)-structure and the orthorhombic (o)-structure. We have reasons to suggest that superconductivity and antiferromagnetic ordering arise from bct- DyRh4B4 phase and, therefore, coexist below TN ~ 2.7 K., Comment: 11 pages, 6 figures, Accepted for publication in Journal of Alloys and Compounds

We report theoretical values for the unscreened plasma frequencies Omega_p of several Fe pnictides obtained from DFT based calculations within the LDA and compare them with experimental plasma frequencies obtained from reflectivity data. The sizable renormalization observed for all considered compounds points to the presence of many-body effects beyond the LDA. From the large empirical background dielectric constant of about 12-15, we estimate a large arsenic polarizability of about 9.5 +- 1.2 Angstroem^3 where the details depend on the polarizabilities of the remaining ions taken from the literature. This large polarizability can significantly reduce the value of the Coulomb repulsion U_d about 4 eV on iron known from iron oxides to a level of 2 eV or below. In general, this result points to rather strong polaronic effects as suggested by G.A. Sawatzky et al., in Refs. arXiv:0808.1390 and arXiv:0811.0214 (Berciu et al.). Possible consequences for the conditions of a formation of bipolarons are discussed, too. From the extrapolated muon spin rotation penetration depth data at T= 0 and the experimental Omega_p we estimate the total coupling constant lambda_tot for the el-boson interaction within the Eliashberg-theory adopting a single band approximation. For LaFeAsO_0.9F_0.1 a weak to intermediately strong coupling regime and a quasi-clean limit behaviour are found. For a pronounced multiband case we obtain a constraint for various intraband coupling constants which in principle allows for a sizable strong coupling in bands with either slow electrons or holes., Comment: 34 pages, 10 figures, submitted to New Journal of Physics (30.01.2009)

We report B_c2 data for LaO_{0.9}F_{0.1}FeAs_{1-delta} in a wide T and field range up to 60 Tesla. The large slope of B_c2 approx ~ -6 Tesla/K near an improved T_c = 28.5 K of the in-plane B_c2(T) contrasts with a flattening starting at 23 K above 30 Tesla we regard as the onset of Pauli-limited behavior (PLB) with B_c2(0) about 65 Tesla. We interpret a similar hitherto unexplained flattening of the B_c2(T) curves reported for at least three other disordered closely related systems as also as a manifestation of PLB. Their Maki parameters have been estimated analyzing their B_c2(T) data within the WHH approach. The pronounced PLB of (Ba,K)Fe_2As_2 single crystals from a tin-flux is attributed also to a significant As deficiency. Consequences of our results are discussed in terms of disorder effects within conventional (CSC) and unconventional superconductivity (USC). USC scenarios with nodes on individual Fermi surface sheets (FSS), can be discarded for our samples. The increase of dB_c2/dT|_{T_c} by sizeable disorder provides evidence for an important intraband (intra-FSS) contribution to the orbital upper critical field. We suggest that it can be ascribed either to an impurity driven transition from s_{+-} USC to CSC of an extended s_{++}-wave state or to a stabilized s_{+-}-state provided As-vacancies cause predominantly strong intraband scattering in the unitary limit. We compare our results with B_c2 data from the literature with no PLB for fields below 60 to 70 Tesla probed so far. A novel disorder related scenario of a complex interplay of SC with two different competing magnetic instabilities is suggested., Comment: 30 pages, 14 figures (1 removed, 1 added), discussions about s_{+-} superconductivity and the related interband scattering added, shortened version with a slightly changed title, references updated, two new authors added

We report time-dependent photocurrent and transport measurements of sub-bandgap photoexcited carriers in nitrogen-rich (type Ib), single-crystal diamond. Transient carrier dynamics are characteristic of trapping conduction with long charge storage lifetimes of ~3 hours. By measuring the photoexcited Hall effect we confirm that the charge carriers are electrons and by varying the excitation energy we observe a strong turn-on in the photoconduction at ~1.9 eV. These findings shed new light on sub-bandgap states in nitrogen doped single-crystal diamond., Comment: 12 pages, 3 figures

In this study pseudoquaternary rare-earth nickel borocarbide superconductors RxR'1-xNi2B2C have been investigated predominantly in the diluted limit x << 1 or (1 x) << 1. In all of these materials structural disorder results in a reduction of the superconducting transition temperature Tc . Depending on the selection of the rare earth elements R and R' this disorder induced deterioration of superconductivity is combined with magnetic pair breaking of Abrikosov-Gorkov type or pair breaking of non-magnetic impurities in antiferromagnetic superconductors (Morozov-type of pair breaking)., Comment: 10 pages, 5 figures

An general survey of the superconducting (SC) gap study in the title compounds by point-contact (PC) spectroscopy is presented. The SC gap was determined from dV/dI of PCs employing the well-known theory of conductivity for normal metal-superconductor PCs accounting Andreev reflection. The theory was modified by including pair-breaking effects considering the presence of magnetic rare-earth ions. A possible multiband structure of these compounds was also taken into account. The PC study of the gap in the Er-compound (TN=6KTc=6.5K) the SC gap has a BCS-like dependence in the AF state. The SC gap for R = Ho (TN=5.2K