arXiv subjects

Rudolf Gross

Publications and source records attributed to Rudolf Gross.

At least 127 records · Page 7Linked to original sources

arXiv2013

Current heating induced spin Seebeck effect

A new measurement technique for the spin Seebeck effect is presented, wherein the normal metal layer used for its detection is exploited simultaneously as a resistive heater and thermometer. We show how the various contributions to the measured total signal can be disentangled, allowing to extract the voltage signal solely caused by the spin Seebeck effect. To this end we performed measurements as a function of the external magnetic field strength and its orientation. We find that the effect scales linearly with the induced rise in temperature, as expected for the spin Seebeck effect.

cond-mat.mes-hall↗

arXiv2013

Unambiguous determination of spin dephasing times in ZnO

Time-resolved magneto-optics is a well-established optical pump probe technique to generate and to probe spin coherence in semiconductors. By this method, spin dephasing times T_2^* can easily be determined if their values are comparable to the available pump-probe-delays. If T_2^* exceeds the laser repetition time, however, resonant spin amplification (RSA) can equally be used to extract T_2^*. We demonstrate that in ZnO these techniques have several tripping hazards resulting in deceptive values for T_2^* and show how to avoid them. We show that the temperature dependence of the amplitude ratio of two separate spin species can easily be misinterpreted as a strongly temperature dependent T_2^* of a single spin ensemble, while the two spin species have T_2^* values which are nearly independent of temperature. Additionally, consecutive pump pulses can significantly diminish the spin polarization, which remains from previous pump pulses. While this barely affects T_2^* values extracted from delay line scans, it results in seemingly shorter T_2^* values in RSA.

cond-mat.mtrl-sci↗

arXiv2013

Zinc Oxide - From Dilute Magnetic Doping to Spin Transport

During the past years there has been renewed interest in the wide-bandgap II-VI semiconductor ZnO, triggered by promising prospects for spintronic applications. First, ferromagnetism was predicted for dilute magnetic doping. In comprehensive investigation of ZnO:Co thin films based on the combined measurement of macroscopic and microscopic properties, we find no evidence for carrier-mediated itinerant ferromagnetism. Phase-pure, crystallographically excellent ZnO:Co is uniformly paramagnetic. Superparamagnetism arises when phase separation or defect formation occurs, due to nanometer-sized metallic precipitates. Other compounds like ZnO:(Li,Ni) and ZnO:Cu do not exhibit indication of ferromagnetism. Second, its small spin-orbit coupling and correspondingly large spin coherence length makes ZnO suitable for transporting or manipulating spins in spintronic devices. From optical pump/optical probe experiments, we find a spin dephasing time of the order of 15 ns at low temperatures which we attribute to electrons bound to Al donors. In all-electrical magnetotransport measurements, we successfully create and detect a spin-polarized ensemble of electrons and transport this spin information across several nanometers. We derive a spin lifetime of 2.6 ns for these itinerant spins at low temperatures, corresponding well to results from an electrical pump/optical probe experiment.

cond-mat.mtrl-sci↗

arXiv2013

Magnon, phonon and electron temperature profiles and the spin Seebeck effect in magnetic insulator/normal metal hybrid structures

We calculate the phonon, electron and magnon temperature profiles in yttrium iron garnet/platinum bilayers by diffusive theory with appropriate boundary conditions, in particular taking into account interfacial thermal resistances. Our calculations show that in thin film hybrids, the interface magnetic heat conductance qualitatively affects the magnon temperature. Based on published material parameters we assess the degree of non-equilibrium at the yttrium iron garnet/platinum interface. The magnitude of the spin Seebeck effect derived from this approach compares well with experimental results for the longitudinal spin Seebeck effect. Additionally we address the temperature profiles in the transverse spin Seebeck effect.

cond-mat.mes-hall↗

arXiv2013

High cooperativity in coupled microwave resonator ferrimagnetic insulator hybrids

We report the observation of strong coupling between the exchange-coupled spins in gallium-doped yttrium iron garnet and a superconducting coplanar microwave resonator made from Nb. The measured coupling rate of 450 MHz is proportional to the square-root of the number of exchange-coupled spins and well exceeds the loss rate of 50 MHz of the spin system. This demonstrates that exchange coupled systems are suitable for cavity quantum electrodynamics experiments, while allowing high integration densities due to their extraordinary high spin densities. Our results furthermore show, that experiments with multiple exchange-coupled spin systems interacting via a single resonator are within reach.

quant-ph↗

arXiv2013

Comment on "Pt magnetic polarization on Y3Fe5O12 and magnetotransport characteristics"

In a recent Letter [Y.M. Lu et al., Phys. Rev. Lett. 110, 147207 (2013)], Lu et al. reported on "ferromagneticlike transport properties" of thin films of Pt, deposited ex situ via sputtering on the ferrimagnetic insulator Y3Fe5O12. The authors found a magnetoresistance in Pt displaying a hysteresis corresponding to the coercive field of Y3Fe5O12, consistent with the findings of other groups. While the latter interpreted their data in terms of the recently proposed spin-Hall magnetoresistance, Lu et al. attributed their observation to a magnetic proximity effect. To support this interpretation, they measured the X-ray magnetic circular dichroism (XMCD) at the Pt L2,3 edges from a Pt/Y3Fe5O12 sample with a Pt thickness of 1.5 nm and derived an average induced magnetic moment of 0.054 Bohr magnetons per Pt atom. This is contradictory to the results of our previous comprehensive XMCD study of three different Pt/Y3Fe5O12 samples with Pt thicknesses of 3, 7, and 10 nm from which we identified an upper limit of (0.003 +/- 0.001) Bohr magnetons per Pt [Geprägs et al., Appl. Phys. Lett. 101, 262407 (2012), arXiv:1211.0916].

cond-mat.mtrl-sci↗

arXiv2013

Strain-controlled nonvolatile magnetization switching

We investigate different approaches towards a nonvolatile switching of the remanent magnetization in single-crystalline ferromagnets at room temperature via elastic strain using ferromagnetic thin film/piezoelectric actuator hybrids. The piezoelectric actuator induces a voltage-controllable strain along different crystalline directions of the ferromagnetic thin film, resulting in modifications of its magnetization by converse magnetoelastic effects. We quantify the magnetization changes in the hybrids via ferromagnetic resonance spectroscopy and superconducting quantum interference device magnetometry. These measurements demonstrate a significant strain-induced change of the magnetization, limited by an inefficient strain transfer and domain formation in the particular system studied. To overcome these obstacles, we address practicable engineering concepts and use a model to demonstrate that a strain-controlled, nonvolatile magnetization switching should be possible in appropriately engineered ferromagnetic/piezoelectric actuator hybrids.

cond-mat.mtrl-sci↗

arXiv2013

Laser molecular beam epitaxy of ZnO thin films and heterostructures

We report on the growth of epitaxial ZnO thin films and ZnO based heterostructures on sapphire substrates by laser molecular beam epitaxy (MBE). We first discuss some recent developments in laser-MBE such as flexible ultra-violet laser beam optics, infrared laser heating systems or the use of atomic oxygen and nitrogen sources, and describe the technical realization of our advanced laser-MBE system. Then we describe the optimization of the deposition parameters for ZnO films such as laser fluence and substrate temperature and the use of buffer layers. The detailed structural characterization by x-ray analysis and transmission electron microscopy shows that epitaxial ZnO thin films with high structural quality can be achieved, as demonstrated by a small out-of-plane and in-plane mosaic spread as well as the absence of rotational domains. We also demonstrate the heteroepitaxial growth of ZnO based multilayers as a prerequisite for spin transport experiments and the realization of spintronic devices. As an example, we show that TiN/Co/ZnO/Ni/Au multilayer stacks can be grown on (0001)-oriented sapphire with good structural quality of all layers and well defined in-plane epitaxial relations.

cond-mat.mtrl-sci↗

arXiv2013

Experimental test of the spin mixing interface conductivity concept

We perform a quantitative, comparative study of the spin pumping, spin Seebeck and spin Hall magnetoresistance effects, all detected via the inverse spin Hall effect in a series of over 20 yttrium iron garnet/Pt samples. Our experimental results fully support present, exclusively spin current-based, theoretical models using a single set of plausible parameters for spin mixing conductance, spin Hall angle and spin diffusion length. Our findings establish the purely spintronic nature of the aforementioned effects and provide a quantitative description in particular of the spin Seebeck effect.

cond-mat.mtrl-sci↗

arXiv2013

Quantitative study of the spin Hall magnetoresistance in ferromagnetic insulator/normal metal hybrids

We experimentally investigate and quantitatively analyze the spin Hall magnetoresistance effect in ferromagnetic insulator/platinum and ferromagnetic insulator/nonferromagnetic metal/platinum hybrid structures. For the ferromagnetic insulator we use either yttrium iron garnet, nickel ferrite or magnetite and for the nonferromagnet copper or gold. The spin Hall magnetoresistance effect is theoretically ascribed to the combined action of spin Hall and inverse spin Hall effect in the platinum metal top layer. It therefore should characteristically depend upon the orientation of the magnetization in the adjacent ferromagnet, and prevail even if an additional, nonferromagnetic metal layer is inserted between Pt and the ferromagnet. Our experimental data corroborate these theoretical conjectures. Using the spin Hall magnetoresistance theory to analyze our data, we extract the spin Hall angle and the spin diffusion length in platinum. For a spin mixing conductance of $4\times10^{14}\;\mathrm{Ω^{-1}m^{-2}}$ we obtain a spin Hall angle of $0.11\pm0.08$ and a spin diffusion length of $(1.5\pm0.5)\;\mathrm{nm}$ for Pt in our thin film samples.

cond-mat.mes-hall↗

arXiv2013

On-chip time resolved detection of quantum dot emission using integrated superconducting single photon detectors

We report the routing of quantum light emitted by self-assembled InGaAs quantum dots (QDs) into the optical modes of a GaAs ridge waveguide and its efficient detection on-chip via evanescent coupling to NbN superconducting nanowire single photon detectors (SNSPDs). Individual QD light sources embedded within such integrated nano-photonic circuits are highly attractive for the realization of quantum photonic circuits for many applications in photonic information science. Here, we demonstrate that the waveguide coupled SNSPDs primarily detect QD luminescence with scattered photons from the excitation laser being negligible by comparison. The SNSPD detection efficiency from the evanescently coupled waveguide modes is shown to be two orders of magnitude higher when compared with operation under normal incidence illumination. Furthermore, in-situ time resolved measurements show an average exciton lifetime of 0.93 \pm 0.03 ns when recorded with the integrated detector with an ultrafast timing jitter of only 72 \pm 2 ps showing the great potential of this highly integrated quantum optics system.

cond-mat.mes-hall↗

arXiv2012

Optimisation of NbN thin films on GaAs substrates for in-situ single photon detection in structured photonic devices

We prepare NbN thin films by DC magnetron sputtering on [100] GaAs substrates, optimise their quality and demonstrate their use for efficient single photon detection in the near-infrared. The interrelation between the Nb:N content, growth temperature and crystal quality is established for 4-22nm thick films. Optimised films exhibit a superconducting critical temperature of 12.6\pm0.2K for a film thickness of 22\pm0.5nm and 10.2\pm0.2K for 4\pm0.5nm thick films. The optimum growth temperature is shown to be ~475°C reflecting a trade-off between enhanced surface diffusion, which improves the crystal quality, and arsenic evaporation from the GaAs substrate. Analysis of the elemental composition of the films provides strong evidence that the δ-phase of NbN is formed in optimised samples, controlled primarily via the nitrogen partial pressure during growth. By patterning optimum 4nm and 22nm thick films into a 100nm wide, 369μm long nanowire meander using electron beam lithography and reactive ion etching, we fabricated single photon detectors on GaAs substrates. Time-resolved studies of the photo-response, absolute detection efficiency and dark count rates of these detectors as a function of the bias current reveal maximum single photon detection efficiencies as high as 21\pm2% at 4.3\pm0.1K with ~50k dark counts per second for bias currents of 98%Ic at a wavelength of 950nm. As expected, similar detectors fabricated from 22nm thick films exhibit much lower efficiencies (0.004%) with very low dark count rates <=3cps. The maximum lateral extension of a photo-generated hotspot is estimated to be 30\pm8nm, clearly identifying the low detection efficiency and dark count rate of the thick film detectors as arising from hotspot cooling via the heat reservoir provided by the NbN film.

cond-mat.supr-con↗

arXiv2012

Investigation of induced Pt magnetic polarization in Pt/Y3Fe5O12 bilayers

Using X-ray magnetic circular dichroism (XMCD) measurements, we explore the possible existence of induced magnetic moments in thin Pt films deposited onto the ferrimagnetic insulator yttrium iron garnet (Y3Fe5O12). Such a magnetic proximity effect is well established for Pt/ferromagnetic metal heterostructures. Indeed, we observe a clear XMCD signal at the Pt L3 edge in Pt/Fe bilayers, while no such signal can be discerned in XMCD traces of Pt/Y3Fe5O12 bilayers. Integrating the XMCD signals allows to estimate an upper limit for the induced Pt magnetic polarization in Pt/Y3Fe5O12 bilayers.

cond-mat.mtrl-sci↗

arXiv2012

Electromechanically induced absorption in a circuit nano-electromechanical system

A detailed analysis of electromechanically induced absorption (EMIA) in a circuit nano-electromechanical hybrid system consisting of a superconducting microwave resonator coupled to a nanomechanical beam is presented. By performing two-tone spectroscopy experiments we have studied EMIA as a function of the drive power over a wide range of drive and probe tone detunings. We find good quantitative agreement between experiment and theoretical modeling based on the Hamiltonian formulation of a generic electromechanical system. We show that the absorption of microwave signals in an extremely narrow frequency band (Δω/2π<5 Hz) around the cavity resonance of about 6 GHz can be adjusted over a range of more than 25 dB on varying the drive tone power by a factor of two. Possible applications of this phenomenon include notch filters to cut out extremely narrow frequency bands (< Hz) of a much broader band of the order of MHz defined by the resonance width of the microwave cavity. The amount of absorption as well as the filtered frequency is tunable over the full width of the microwave resonance by adjusting the power and frequency of the drive field. At high drive power we observe parametric microwave amplification with the nanomechanical resonator. Due to the very low loss rate of the nanomechanical beam the drive power range for parametric amplification is narrow, since the beam rapidly starts to perform self-oscillations.

cond-mat.mes-hall↗

arXiv2012

Spin transport and spin dephasing in zinc oxide

The wide bandgap semiconductor ZnO is interesting for spintronic applications because of its small spin-orbit coupling implying a large spin coherence length. Utilizing vertical spin valve devices with ferromagnetic electrodes (TiN/Co/ZnO/Ni/Au), we study the spin-polarized transport across ZnO in all-electrical experiments. The measured magnetoresistance agrees well with the prediction of a two spin channel model with spin-dependent interface resistance. Fitting the data yields spin diffusion lengths of 10.8nm (2K), 10.7nm (10K), and 6.2nm (200K) in ZnO, corresponding to spin lifetimes of 2.6ns (2K), 2.0ns (10K), and 31ps (200K).

cond-mat.mtrl-sci↗

arXiv2012

Giant Magnetoelastic Effects in BaTiO3-based Extrinsic Multiferroic Hybrids

Extrinsic multiferroic hybrid structures consisting of ferromagnetic and ferroelectric layers elastically coupled to each other are promising due to their robust magnetoelectric effects even at room temperature. For a quantitative analysis of these magnetoelectric effects, a detailed knowledge of the piezoelectric and magnetoelastic behavior of both constituents as well as their mutual elastic coupling is mandatory. We here report on a theoretical and experimental study of the magnetic behavior of BaTiO3-based extrinsic multiferroic structures. An excellent agreement between molecular dynamics simulations and the experiments was found for Fe50Co50/BaTiO3 and Ni/BaTiO3 hybrid structures. This demonstrates that the magnetic behavior of extrinsic multiferroic hybrid structures can be determined by means of ab-initio calculations, allowing for the design of novel multiferroic hybrids.

cond-mat.mtrl-sci↗

arXiv2012

Networks of nonlinear superconducting transmission line resonators

We investigate a network of coupled superconducting transmission line resonators, each of them made nonlinear with a capacitively shunted Josephson junction coupling to the odd flux modes of the resonator. The resulting eigenmode spectrum shows anticrossings between the plasma mode of the shunted junction and the odd resonator modes. Notably, we find that the combined device can inherit the complete nonlinearity of the junction, allowing for a description as a harmonic oscillator with a Kerr nonlinearity. Using a dc SQUID instead of a single junction, the nonlinearity can be tuned between 10 kHz and 4 MHz while maintaining resonance frequencies of a few gigahertz for realistic device parameters. An array of such nonlinear resonators can be considered a scalable superconducting quantum simulator for a Bose-Hubbard Hamiltonian. The device would be capable of accessing the strongly correlated regime and be particularly well suited for investigating quantum many-body dynamics of interacting particles under the influence of drive and dissipation.

cond-mat.supr-con↗

arXiv2012

Control of microwave signals using circuit nano-electromechanics

Waveguide resonators are crucial elements in sensitive astrophysical detectors [1] and circuit quantum electrodynamics (cQED) [2]. Coupled to artificial atoms in the form of superconducting qubits [3, 4], they now provide a technologically promising and scalable platform for quantum information processing tasks [2, 5-8]. Coupling these circuits, in situ, to other quantum systems, such as molecules [9, 10], spin ensembles [11, 12], quantum dots [13] or mechanical oscillators [14, 15] has been explored to realize hybrid systems with extended functionality. Here, we couple a superconducting coplanar waveguide resonator to a nano-coshmechanical oscillator, and demonstrate all-microwave field controlled slowing, advancing and switching of microwave signals. This is enabled by utilizing electromechanically induced transparency [16-18], an effect analogous to electromagnetically induced transparency (EIT) in atomic physics [19]. The exquisite temporal control gained over this phenomenon provides a route towards realizing advanced protocols for storage of both classical and quantum microwave signals [20-22], extending the toolbox of control techniques of the microwave field.

cond-mat.mes-hall↗