arXiv subjects

Ryan Van Haren

Publications and source records attributed to Ryan Van Haren.

5 recordsLinked to original sources

arXiv2025

Direct Comparison of Static and Dynamic Measurements of Spin Generation in a Topological Insulator Thin Film

The competition between intrinsic spin-orbit physics, magnetic phenomena, and the quality of materials and interfaces governs the charge-to-spin conversion processes that are essential to the implementation of spintronic devices. Direct comparisons of spin parameters, which serve as metrics of device quality, obtained by different measurement techniques are scarce, leading to uncertainty regarding discrepancies and the reliability of the methods. Here, we directly compare the spin Hall coefficient ($θ_{SH}$) in molecular beam epitaxy grown films of $(Bi_{1-x}Sb_{x})_{2}Te_{3-y}Se_{y}$ (BSTS, x = 0.58, y = 1) at room temperature using two complementary techniques: a static method using non-local voltage (NLV) measurements in BSTS Hall bars with DC charge current, and a dynamic method using spin-torque ferromagnetic resonance (ST-FMR) measurement in $BSTS/Ni_{80}Fe_{20}$ heterostructures at GHz frequencies. We obtain comparable spin Hall coefficients in angular-dependent ST-FMR ($θ_{SH}$=$4.7\pm1.1$) and in NLV ($θ_{SH}$=$2.8\pm0.6$). The complex effects of ferromagnetic interfaces while determining spin Hall coefficients using static or dynamic techniques becomes evident by contrasting our results to literature.

cond-mat.mtrl-sci↗

arXiv2024

Direct Visualization of Relativistic Quantum Scars

Quantum scars refer to eigenstates with enhanced probability density along unstable classical periodic orbits (POs). First predicted 40 years ago, scars are special eigenstates that counterintuitively defy ergodicity in quantum systems whose classical counterpart is chaotic. Despite the importance and long history of scars, their direct visualization in quantum systems remains an open field. Here we demonstrate that, by using an in-situ graphene quantum dot (GQD) creation and wavefunction mapping technique, quantum scars are imaged for Dirac electrons with nanometer spatial resolution and meV energy resolution with a scanning tunneling microscope. Specifically, we find enhanced probability densities in the form of lemniscate-shaped and streak-like patterns within our stadium-shaped GQDs. Both features show equal energy interval recurrence, consistent with predictions for relativistic quantum scars. By combining classical and quantum simulations, we demonstrate that the observed patterns correspond to two unstable POs that exist in our stadium-shaped GQD, thus proving they are both quantum scars. In addition to providing the first unequivocal visual evidence of quantum scarring, our work offers insight into the quantum-classical correspondence in relativistic chaotic quantum systems and paves the way to experimental investigation of other recently proposed scarring species such as perturbation-induced scars, chiral scars, and antiscarring.

cond-mat.mes-hall↗

arXiv2024

Suppressed weak anti-localization in topological insulator - antiferromagnetic insulator (BiSb)$_2$Te$_3$ - MnF$_2$ thin film bilayers

Thin films of the topological insulator (BiSb)$_2$Te$_3$ oriented along the [0001] direction were grown via molecular beam epitaxy on substrates of Al$_2$O$_3$ (0001) and MgF$_2$ (110) single crystals, as well as on an epitaxial thin film of the antiferromagnetic insulator MnF$_2$ (110). Magnetoconductivity measurements of these samples showed close proximity of the Fermi level to the Dirac point and weak antilocalization at low temperature that was partially suppressed in the sample grown on the MnF$_2$ layer. The magnetoconductivity data were fit to a model that describes the quantum corrections to the conductivity for the Dirac surface state of a 3-dimensional topological insulator, from which values of the Fermi velocity and the phase coherence length of the surface state charge carriers were derived. The magnetoconductivity of the (BiSb)$_2$Te$_3$ - MnF$_2$ bilayer samples were fit to a model describing the crossover from weak antilocalization to weak localization due to magnetic doping. The results are consistent with the opening of an energy gap at the Dirac point in the (BiSb)$_2$Te$_3$ due to magnetic proximity interactions of the topological surface states with the antiferromagnetic MnF$_2$ insulator.

cond-mat.mes-hall↗

arXiv2023

Emergent Magnetic Phases and Piezomagnetic Effects in Mn$_x$Ni$_{1-x}$F$_2$ Thin Film Alloys

The effect of random competing single-ion anisotropies in antiferromagnets was studied using epitaxial Mn$_x$Ni$_{1-x}$F$_2$ antiferromagnetic thin film alloys grown via molecular beam epitaxy. The crystal structure of this material is tetragonal for all values of $x$, and the Mn sites have a magnetic easy axis single-ion anisotropy while the Ni sites have an easy plane anisotropy perpendicular to the Mn easy axis. Crystallographic and magnetization measurements demonstrated that the thin film alloys were homogeneously mixed and did not phase-separate into their constituent parts. Pure MnF$_2$ thin films epitaxially grown on MgF$_2$ exhibited compressive strain along all three crystallographic axes which resulted in piezomagnetic effects. The piezomagnetism disappeared if the film was grown on a (MnNi)F$_2$ graded buffer layer. A mean-field theory fit to the transition temperature as a function of the Mn concentration $x$, which takes into account piezomagnetic effects, gave a magnetic exchange constant between Mn and Ni ions of $J_{\text{MnNi}} = 0.305 \pm 0.003$~meV. Mean-field theory calculations also predicted the existence of an oblique antiferromagnetic phase in the Mn$_x$Ni$_{1-x}$F$_2$ alloy which agreed with the experimental data. A magnetic phase diagram for Mn$_x$Ni$_{1-x}$F$_2$ thin film alloys was constructed and showed evidence for the existence of two unique magnetic phases, in addition to the ordinary antiferromagnetic and paramagnetic phases: an oblique antiferromagnetic phase, and an emergent magnetic phase proposed to be either a magnetic glassy phase or a helical phase. The phase diagram is quantitatively different from that of Fe$_x$Ni$_{1-x}$F$_2$ because of the much larger single-ion anisotropy of Fe$^{2+}$ compared to Mn$^{2+}$.

cond-mat.mtrl-sci↗

arXiv2023

Surface state mediated ferromagnetism in Mn$_{0.14}$Bi$_{1.86}$Te$_3$ thin films

A spontaneous ferromagnetic moment can be induced in Bi$_{2}$Te$_{3}$ thin films below a temperature T $\approx$ 16 K by the introduction of Mn dopants. We demonstrate that films grown via molecular beam epitaxy with the stoichiometry Mn$_{0.14}$Bi$_{1.86}$Te$_3$ maintain the crystal structure of pure Bi$_{2}$Te$_{3}$. The van der Waals nature of inter-layer forces in the Mn$_{0.14}$Bi$_{1.86}$Te$_3$ crystal causes lattice mismatch with the underlayer to have a limited effect on the resulting crystal structure, as we demonstrate by thin film growth on tetragonal MgF$_{2}$ (110) and NiF$_{2}$ (110). Electronic transport and magnetic moment measurements show that the ferromagnetic moment of the Mn$_{0.14}$Bi$_{1.86}$Te$_3$ thin films is enhanced as the Fermi level moves from the bulk conduction band and towards the bulk band gap, suggesting that electronic surface states play an important role in mediating the ferromagnetic order. Ferromagnetic Mn$_{0.14}$Bi$_{1.86}$Te$_3$/antiferromagnetic NiF$_{2}$ bilayers show evidence that the ferromagnetic moment of the Mn$_{0.14}$Bi$_{1.86}$Te$_3$ film is suppressed, suggesting the existence of an interface effect between the two magnetic layers.

cond-mat.mtrl-sci↗