arXiv subjects

B. Beschoten

Publications and source records attributed to B. Beschoten.

At least 37 records · Page 2Linked to original sources

arXiv2016

Growth, characterization, and transport properties of ternary (Bi1-xSbx)2Te3 topological insulator layers

Ternary (Bi1-xSbx)2Te3 films with an Sb content between 0 and 100% were deposited on a Si(111) substrate by means of molecular beam epitaxy. X-ray diffraction measurements confirm single crystal growth in all cases. The Sb content is determined by X-ray photoelectron spectroscopy. Consistent values of the Sb content are obtained from Raman spectroscopy. Scanning Raman spectroscopy reveals that the (Bi1-xSbx)2Te3 layers with an intermediate Sb content show spatial composition inhomogeneities. The observed spectra broadening in angular-resolved photoemission spectroscopy (ARPES) is also attributed to this phenomena. Upon increasing the Sb content from x=0 to 1 the ARPES measurements show a shift of the Fermi level from the conduction band to the valence band. This shift is also confirmed by corresponding magnetotransport measurements where the conductance changes from n- to p-type. In this transition region, an increase of the resistivity is found, indicating a location of the Fermi level within the band gap region. More detailed measurements in the transition region reveals that the transport takes place in two independent channels. By means of a gate electrode the transport can be changed from n- to p-type, thus allowing a tuning of the Fermi level within the topologically protected surface states.

cond-mat.mtrl-sci↗

arXiv2015

Spin and charge transport in graphene-based spin transport devices with Co/MgO spin injection and spin detection electrodes

In this review we discuss spin and charge transport properties in graphene-based single-layer and few-layer spin-valve devices. We give an overview of challenges and recent advances in the field of device fabrication and discuss two of our fabrication methods in more detail which result in distinctly different device performances. In the first class of devices, Co/MgO electrodes are directly deposited onto graphene which results in rough MgO-to-Co interfaces and favor the formation of conducting pinholes throughout the MgO layer. We show that the contact resistance area product (R$_c$A) is a benchmark for spin transport properties as it scales with the measured spin lifetime in these devices indicating that contact-induced spin dephasing is the bottleneck for spin transport even in devices with large R$_c$A values. In a second class of devices, Co/MgO electrodes are first patterned onto a silicon substrate. Subsequently, a graphene-hBN heterostructure is directly transferred onto these prepatterned electrodes which provides improved interface properties. This is seen by a strong enhancement of both charge and spin transport properties yielding charge carrier mobilities exceeding 20000 cm$^2$/(Vs) and spin lifetimes up to 3.7 ns at room temperature. We discuss several shortcomings in the determination of both quantities which complicates the analysis of both extrinsic and intrinsic spin scattering mechanisms. Furthermore, we show that contacts can be the origin of a second charge neutrality point in gate dependent resistance measurements which is influenced by the quantum capacitance of the underlying graphene layer.

cond-mat.mes-hall↗

arXiv2015

Phase-coherent transport in catalyst-free vapor phase deposited Bi$_2$Se$_3$ crystals

Free-standing Bi$_2$Se$_3$ single crystal flakes of variable thickness are grown using a catalyst-free vapor-solid synthesis and are subsequently transferred onto a clean Si$^{++}$/SiO$_2$ substrate where the flakes are contacted in Hall bar geometry. Low temperature magneto-resistance measurements are presented which show a linear magneto-resistance for high magnetic fields and weak anti-localization (WAL) at low fields. Despite an overall strong charge carrier tunability for thinner devices, we find that electron transport is dominated by bulk contributions for all devices. Phase coherence lengths $ł_ϕ$ as extracted from WAL measurements increase linearly with increasing electron density exceeding $1 μ$m at 1.7 K. While $ł_ϕ$ is in qualitative agreement with electron electron interaction-induced dephasing, we find that spin flip scattering processes limit $ł_ϕ$ at low temperatures.

cond-mat.mes-hall↗

arXiv2014

Contact-induced charge contributions to non-local spin transport measurements in Co/MgO/graphene devices

Recently, it has been shown that oxide barriers in graphene-based non-local spin-valve structures can be the bottleneck for spin transport. The barriers may cause spin dephasing during or right after electrical spin injection which limit spin transport parameters such as the spin lifetime of the whole device. An important task is to evaluate the quality of the oxide barriers of both spin injection and detection contacts in a fabricated device. To address this issue, we discuss the influence of spatially inhomogeneous oxide barriers and especially conducting pinholes within the barrier on the background signal in non-local measurements of graphene/MgO/Co spin-valve devices. By both simulations and reference measurements on devices with non-ferromagnetic electrodes, we demonstrate that the background signal can be caused by inhomogeneous current flow through the oxide barriers. As a main result, we demonstrate the existence of charge accumulation next to the actual spin accumulation signal in non-local voltage measurements, which can be explained by a redistribution of charge carriers by a perpendicular magnetic field similar to the classical Hall effect. Furthermore, we present systematic studies on the phase of the low frequency non-local ac voltage signal which is measured in non-local spin measurements when applying ac lock-in techniques. This phase has so far widely been neglected in the analysis of non-local spin transport. We demonstrate that this phase is another hallmark of the homogeneity of the MgO spin injection and detection barriers. We link backgate dependent changes of the phase to the interplay between the capacitance of the oxide barrier to the quantum capacitance of graphene.

cond-mat.mes-hall↗

arXiv2014

Limitations to Carrier Mobility and Phase-Coherent Transport in Bilayer Graphene

We present transport measurements on high-mobility bilayer graphene fully encapsulated in hexagonal boron nitride. We show two terminal quantum Hall effect measurements which exhibit full symmetry broken Landau levels at low magnetic fields. From weak localization measurements, we extract gate-tunable phase coherence times $τ_ϕ$ as well as the inter- and intra-valley scattering times $τ_i$ and $τ_*$. While $τ_ϕ$ is in qualitative agreement with an electron-electron interaction mediated dephasing mechanism, electron spin-flip scattering processes are limiting $τ_ϕ$ at low temperatures. The analysis of $τ_i$ and $τ_*$ points to local strain fluctuation as the most probable mechanism for limiting the mobility in high-quality bilayer graphene.

cond-mat.mes-hall↗

arXiv2014

Suppression of contact-induced spin dephasing in graphene/MgO/Co spin-valve devices by successive oxygen treatments

By successive oxygen treatments of graphene non-local spin-valve devices we achieve a gradual increase of the contact resistance area products ($R_cA$) of Co/MgO spin injection and detection electrodes and a transition from linear to non-linear characteristics in the respective differential dV-dI-curves. With this manipulation of the contacts both spin lifetime and amplitude of the spin signal can significantly be increased by a factor of seven in the same device. This demonstrates that contact-induced spin dephasing is the bottleneck for spin transport in graphene devices with small $R_cA$ values. With increasing $R_cA$ values, we furthermore observe the appearance of a second charge neutrality point (CNP) in gate dependent resistance measurements. Simultaneously, we observe a decrease of the gate voltage separation between the two CNPs. The strong enhancement of the spin transport properties as well as the changes in charge transport are explained by a gradual suppression of a Co/graphene interaction by improving the oxide barrier during oxygen treatment.

cond-mat.mes-hall↗

arXiv2013

All-electrical time-resolved spin generation and spin manipulation in n-InGaAs

We demonstrate all-electrical spin generation and subsequent manipulation by two successive electric field pulses in an n-InGaAs heterostructure in a time-resolved experiment at zero external magnetic field. The first electric field pulse along the $[1\bar10]$ crystal axis creates a current induced spin polarization (CISP) which is oriented in the plane of the sample. The subsequent electric field pulse along [110] generates a perpendicular magnetic field pulse leading to a coherent precession of this spin polarization with 2-dimensional electrical control over the final spin orientation. Spin precession is probed by time-resolved Faraday rotation. We determine the build-up time of CISP during the first field pulse and extract the spin dephasing time and internal magnetic field strength during the spin manipulation pulse.

cond-mat.mes-hall↗

arXiv2013

Role of MgO barriers for spin and charge transport in Co/MgO/graphene non-local spin-valve devices

We investigate spin and charge transport in both single and bilayer graphene non-local spin-valve devices. Similar to previous studies on bilayer graphene, we observe an inverse dependence of the spin lifetime on the carrier mobility in our single layer devices. This general trend is only observed in devices with large contact resistances. Furthermore, we observe a second Dirac peak in devices with long spin lifetimes. This results from charge transport underneath the contacts. In contrast, all devices with low ohmic contact resistances only exhibit a single Dirac peak. Additionally, the spin lifetime is significantly reduced indicating that an additional spin dephasing occurs underneath the electrodes.

cond-mat.mes-hall↗

arXiv2012

Electric field-driven coherent spin reorientation of optically generated electron spin packets in InGaAs

Full electric-field control of spin orientations is one of the key tasks in semiconductor spintronics. We demonstrate that electric field pulses can be utilized for phase-coherent +/- pi spin rotation of optically generated electron spin packets in InGaAs epilayers detected by time-resolved Faraday rotation. Through spin-orbit interaction, the electric-field pulses act as local magnetic field pulses (LMFP). By the temporal control of the LMFP, we can turn on and off electron spin precession and thereby rotate the spin direction into arbitrary orientations in a 2-dimensional plane. Furthermore, we demonstrate a spin echo-type spin drift experiment and find an unexpected partial spin rephasing, which is evident by a doubling of the spin dephasing time.

cond-mat.mtrl-sci↗

arXiv2012

Triggering and probing of phase-coherent spin packets by time-resolved spin transport across an Fe/GaAs Schottky barrier

Time-resolved electrical spin transport is used to generate and probe spin currents in GaAs electrically. We use high bandwidth current pulses to inject phase-coherent spin packets from Fe into n-GaAs. By means of time-resolved Faraday rotation we demonstrate that spins are injected with a clearly defined phase by the observation of multiple Larmor precession cycles. We furthermore show that spin precession of optically created spin packets in n-GaAs can be probed electrically by spin-polarized photo-current pulses. The injection and detection experiments are not direct reciprocals of each other. In particular, we find that interfacial spin accumulation generated by the photocurrent pulse plays a critical role in time-resolved electrical spin detection.

cond-mat.mtrl-sci↗

arXiv2012

Intermediate state switching dynamics in magnetic double layer nanopillars grown by molecular beam epitaxy

We observe a stable intermediate resistance switching state in the current perpendicular to plane geometry for all Co/Cu/Co double layer nanopillar junctions grown by molecular beam epitaxy. This novel state has a resistance between the resistances of the parallel and antiparallel alignment of both Co-layer magnetizations. The state, which originates from an additional in-plane magnetic easy axis, can be reached by spin transfer torque switching or by an external magnetic field. In addition to spin torque-induced coherent small-angle spin wave modes we observe a broad microwave emission spectrum. The latter is attributed to incoherent magnetic excitations that lead to a switching between the intermediate state and the parallel or antiparallel alignment of both ferromagnetic layers. We conclude that the additional magnetic easy axis suppresses a stable trajectory of coherent large-angle precession, which is not observed in our samples.

cond-mat.mes-hall↗

arXiv2012

Frequency domain studies of current-induced magnetization dynamics in single magnetic-layer nanopillars

Spin transfer torque-induced high-frequency dynamics of single thin cobalt-layer nanopillars of circular and elliptical shape have been observed directly. Two types of precessional modes can be identified as a function of magnetic field perpendicular to the layer plane, excited for negative current polarity only. They are assigned to vortex-core and transverse spin-wave excitations, which corroborate recent model predictions. The observed narrow linewidth of 4 MHz at room temperature indicates the high coherence of the magnetic excitations.

cond-mat.mtrl-sci↗

arXiv2012

Origin of training effect of exchange bias in Co/CoO due to irreversible thermoremanent magnetization of the magnetically diluted antiferromagnet

The irreversible thermoremanent magnetization of a sole, magnetically diluted epitaxial antiferromagnetic Co$_{1-y}$O(100) layer is determined by the mean of its thermoremanent magnetizations at positive and negative remanence. During hysteresis-loop field cycling, thermoremanent magnetization exhibits successive reductions, consistent with the training effect (TE) of the exchange bias measured for the corresponding Co$_{1-y}$O(100)/Co bilayer. The TE of exchange bias is shown to have its microscopic origin in the TE of the irreversible thermoremanent magnetization of the magnetically diluted AFM.

cond-mat.mtrl-sci↗

arXiv2011

Observation of Long Spin Relaxation Times in Bilayer Graphene at Room Temperature

We report on the first systematic study of spin transport in bilayer graphene (BLG) as a function of mobility, minimum conductivity, charge density and temperature. The spin relaxation time $τ_s$ scales inversely with the mobility $μ$ of BLG samples both at room temperature and at low temperature. This indicates the importance of D'yakonov - Perel' spin scattering in BLG. Spin relaxation times of up to 2 ns are observed in samples with the lowest mobility. These times are an order of magnitude longer than any values previously reported for single layer graphene (SLG). We discuss the role of intrinsic and extrinsic factors that could lead to the dominance of D'yakonov-Perel' spin scattering in BLG. In comparison to SLG, significant changes in the density dependence of $τ_s$ are observed as a function of temperature.

cond-mat.mes-hall↗

arXiv2010

Two-dimensional optical control of electron spin orientation by linearly polarized light in InGaAs

Optical absorption of circularly polarized light is well known to yield an electron spin polarization in direct band gap semiconductors. We demonstrate that electron spins can even be generated with high efficiency by absorption of linearly polarized light in InGaAs. By changing the incident linear polarization direction we can selectively excite spins both in polar and transverse directions. These directions can be identified by the phase during spin precession using time-resolved Faraday rotation. We show that the spin orientations do not depend on the crystal axes suggesting an extrinsic excitation mechanism.

cond-mat.mtrl-sci↗

arXiv2007

Mapping of spin lifetimes to electronic states in n-type GaAs near the metal-insulator transition

The longest spin lifetimes in bulk n-GaAs exceed 100 ns for doping concentrations near the metal-insulator transition (J.M. Kikkawa, D.D. Awschalom, Phys. Rev. Lett. 80, 4313 (1998)). The respective electronic states have yet not been identified. We therefore investigate the energy dependence of spin lifetimes in n-GaAs by time-resolved Kerr rotation. Spin lifetimes vary by three orders of magnitude as a function of energy when occupying donor and conduction band states. The longest spin lifetimes (>100 ns) are assigned to delocalized donor band states, while conduction band states exhibit shorter spin lifetimes. The occupation of localized donor band states is identified by short spin lifetimes (~300 ps) and a distinct Overhauser shift due to dynamic nuclear polarization.

cond-mat.mtrl-sci↗

arXiv2007

Anisotropic Electron Spin Lifetime in (In,Ga)As/GaAs (110) Quantum Wells

Anisotropic electron spin lifetimes in strained undoped (In,Ga)As/GaAs (110) quantum wells of different width and height are investigated by time-resolved Faraday rotation and time-resolved transmission and are compared to the (001)-orientation. From the suppression of spin precession, the ratio of in-plane to out-of-plane spin lifetimes is calculated. Whereas the ratio increases with In concentration in agreement with theory, a surprisingly high anisotropy of 480 is observed for the broadest quantum well, when expressed in terms of spin relaxation times.

cond-mat.mtrl-sci↗

arXiv2006

Dilution dependence of blocking temperature distribution in the exchange bias system CoO(111)/Co(111)

In epitaxially grown CoO(111)/Co(111) bilayers we have determined the blocking temperature distribution f(Tb), which is correlated with the antiferromagnetic domain size (z) distribution f(z). The latter is probed by reversing antiferromagnetic domains at successively higher temperatures in the reversed cooling field. The implementation of nonmagnetic defects throughout the antiferromagnet CoO is found to give rise to a broadening of the domain size distribution f(z) within CoO as evidenced by a broadened f(Tb). This broadening is responsible for an enhancement of the exchange bias field.

cond-mat.mtrl-sci↗