Physical Review Research

Journal: Physical Review Research

Abbreviation

Phys. Rev. Res.

Publisher

American Physical Society

ISSN

2643-1564

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Publications1 - 10 of 273

Magnetization fluctuations and magnetic after-effect probed via the anomalous Hall effect
Nabben, Nadine; Sala, Giacomo; Nowak, Ulrich; et al. (2024)
Physical Review Research
Taking advantage of the anomalous Hall effect, we electrically probe low-frequency magnetization fluctuations at room temperature in a thin ferromagnetic Pt/Co/AlOx layer stack with perpendicular magnetic anisotropy. We observe a strong enhancement of the Hall voltage fluctuations within the hysteretic region of the magnetization loop. Analyzing both the temporal evolution of the anomalous Hall voltage and its frequency-dependent noise power density, we identify two types of magnetic noise: abrupt changes in the magnetic domain configuration, evident as Barkhausen-like steps in the Hall voltage time trace, yield a noise power density spectrum scaling with frequency as 1/fβ with β≈2.0. In contrast, quasistationary magnetization configurations are connected with a magnetic noise power density with an exponent β≈1.0. The observation of Barkhausen steps and relaxation effects shows that the magnetic system is in a nonstationary state in the hysteresis region, such that the fluctuation-dissipation theorem cannot be expected to hold. However, the time-dependent change in the Hall voltage for constant magnetic field strength resembles the integrated noise power.
Attraction from kinetic frustration in ladder systems
Morera, Ivan; Bohrdt, Annabelle; Ho, Wen Wei; et al. (2024)
Physical Review Research
We analyze the formation of multiparticle bound states in ladders with frustrated kinetic energy in two-component bosonic and two-component fermionic systems. We focus on the regime of light doping relative to insulating states at half-filling, spin polarization close to 100%, and strong repulsive interactions. A special feature of these systems is that the binding energy scales with single-particle tunneling t rather than exchange interactions, since effective attraction arises from alleviating kinetic frustration. For two-component Fermi systems on a zigzag ladder we find a bound state between a hole and a flipped spin (magnon) with a binding energy that can be as large as 0.6t. We demonstrate that magnon-hole attraction leads to formation of clusters comprising several holes and magnons, and we expound on antiferromagentic correlations for the transverse spin components inside the clusters. We identify several many-body states that result from self-organization of multiparticle bound states, including a Luttinger liquid of hole-magnon pairs and a density wave state of two-hole-three-magnon composites. We establish a symmetry between the spectra of Bose and Fermi systems and use it to establish the existence of antibound states in two-component Bose mixtures with SU(2) symmetric repulsion on a zigzag ladder. We also consider Bose and Fermi systems on a square ladder with flux and demonstrate that both systems support bound states. We discuss experimental signatures of multiparticle bound states in both equilibrium and dynamical experiments. We point out intriguing connections between these systems and the quark bag model in QCD.
Spin currents in crystals with spin-orbit coupling: Multiband effects in an effective Hamiltonian formalism
Samokhin, Kirill V.; Sigrist, Manfred; Fischer, Mark H. (2026)
Physical Review Research
When focusing on a few essential bands in an effective description of a material to calculate observable quantities, the respective operators have to be adjusted accordingly. Ignoring contributions arising from integrating out remote bands can lead to qualitatively wrong results. We present a detailed analysis of the interband mixing effects on spin currents. Specifically, we calculate the intrinsic spin current in a time-reversal invariant noncentrosymmetric crystal in the presence of electron-lattice spin-orbit coupling. Starting from formally exact microscopic expressions, we derive the spin-current operator restricted to one or more essential bands by iterative elimination of the contributions from distant bands. We show that the standard definition of the spin-current operator in terms of the group velocity obtained from an effective band Hamiltonian cannot be justified using a microscopic theory. The modified expression for the spin-current operator contains additional terms, which dominate the equilibrium spin current in a uniform crystal. We show that the magnitude of these additional terms can considerably exceed the spin current obtained using the standard definition.
Unidirectional orbital magnetoresistance in light-metal-ferromagnet bilayers
Ding, Shilei; Noël, Paul; Krishnaswamy, Gunasheel Kauwtilyaa; et al. (2022)
Physical Review Research
We report the observation of a unidirectional magnetoresistance (UMR) that originates from the nonequilib-rium orbital momentum induced by an electric current in a naturally oxidized Cu/Co bilayer. The orbital UMR scales with the torque efficiency due to the orbital Rashba-Edelstein effect upon changing the Co thickness and temperature, reflecting their common origin. We attribute the UMR to orbital-dependent electron scattering and orbital to spin conversion in the ferromagnetic layer. In contrast to the spin current induced UMR, the magnon contribution to the orbital UMR is absent in thin Co layers, which we ascribe to the lack of coupling between low-energy magnons and orbital current. The magnon contribution to the UMR emerges in Co layers thicker than about 5 nm, which is comparable to the orbital to spin conversion length. Our results provide insight into orbital-to spin-momentum transfer processes relevant for the optimization of spintronic devices based on light metals and orbital transport.
Role of rare events in the pinning problem
Buchacek, Martin; Geshkenbein, Vadim B.; Blatter, Gianni (2020)
Physical Review Research
Type II superconductors exhibit a fascinating phenomenology that is determined by the dynamical properties of the vortex matter hosted by the material. A crucial element in this phenomenology is vortex pinning by material defects, e.g., immobilizing vortices at small drives and thereby guaranteeing dissipation-free current flow. Pinning models for vortices and other topological defects, such as domain walls in magnets or dislocations in crystals, come in two standard variants: (1) weak-collective pinning, where individual weak defects are unable to pin, while the random accumulation of many force centers within a collective pinning volume combines into an effective pin, and (2) strong pinning, where strong defects produce large vortex displacements and bistabilities that lead to pinning on the level of individual defects. The transition between strong and weak pinning is quantified by the Labusch criterion κ≈fp/Cξ=1, where fp and C are the force of one defect and the effective elasticity of the vortex lattice, respectively (ξ is the coherence length). Here, we show that a third generic type of pinning becomes dominant when the pinning force fp enters the weak regime, the pinning by rare events. We find that within an intermediate regime 1/2<κ<1, compact pairs of weak defects define strong pinning clusters that extend the mechanism of strong pinning into the weak regime. We present a detailed analysis of this cluster-pinning mechanism and show that its pinning force density parametrically dominates over the weak pinning result. The present work is a first attempt to include correlations between defects into the discussion of strong pinning.
Universality of breath figures on two-dimensional surfaces: An experimental study
Stricker, Laura; Grillo, Fabio; Marquez, E.A.; et al. (2022)
Physical Review Research
Droplet condensation on surfaces produces patterns, called breath figures. Their evolution into self-similar structures is a classical example of self-organization. It is described by a scaling theory with scaling functions whose universality has recently been challenged by numerical work. Here, we provide thorough experimental testing, where we inspect substrates with vastly different chemical properties, stiffness, and condensation rates. We critically survey the size distributions and the related time-asymptotic scaling of droplet number and surface coverage. In the time-asymptotic regime, they admit a data collapse: the data for all substrates and condensation rates lie on universal scaling functions.
Statistical mechanics and machine learning of the α -Rényi ensemble
Jreissaty, Andrew; Carrasquilla, Juan (2025)
Physical Review Research
We study the statistical physics of the classical Ising model in the so-called α-Rényi ensemble, a finite-temperature thermal state approximation that minimizes a modified free energy based on the α-Rényi entropy. We begin by characterizing its critical behavior in mean-field theory in different regimes of the Rényi index α. Next, we re-introduce correlations and consider the model in one and two dimensions, presenting analytical arguments for the former and devising a Monte Carlo approach to the study of the latter. Remarkably, we find that while mean-field predicts a continuous phase transition below a threshold index value of α∼1.303 and a first-order transition above it, the Monte Carlo results in two dimensions point to a continuous transition at all α. We conclude by performing a variational minimization of the α-Rényi free energy using a recurrent neural network (RNN) Ansatz where we find that the RNN performs well in two dimensions when compared to the Monte Carlo simulations. Our work highlights the potential opportunities and limitations associated with the use of the α-Rényi ensemble formalism in probing the thermodynamic equilibrium properties of classical and quantum systems.
High-mobility transport in isotopically enriched ¹²C and ¹³C exfoliated graphene
Iwakiri, Shuichi; Miller, Jakob; Lang, Florian; et al. (2023)
Physical Review Research
Graphene quantum dots are promising candidates for qubits due to weak spin-orbit and hyperfine interactions. The hyperfine interaction, controllable via isotopic purification, could be the key to further improving the coherence. Here, we use isotopically enriched graphite crystals of both ¹²C and ¹³C grown by a high-pressure-high-temperature method to exfoliate graphene layers. We fabricated Hall bar devices and performed quantum transport measurements, revealing mobilities exceeding 10⁵ cm² /V s and a long mean free path of microns, which are as high as natural graphene. Shubnikov–de Haas oscillations, quantum Hall effect up to the filling factor of one, and Brown-Zak oscillations due to the alignment of hBN and graphene are observed thanks to the high mobility. These results constitute a material platform for physics and engineering of isotopically enriched graphene qubits.
Electronic g factor and magnetotransport in InSb quantum wells
Lei, Zijin; Lehner, Christian A.; Rubi, Km; et al. (2020)
Physical Review Research
High mobility InSb quantum wells with tunable carrier densities are investigated by transport experiments in magnetic fields tilted with respect to the sample normal. We employ the coincidence method and the temperature dependence of the Shubnikov–de Haas oscillations and find a value for the effective g factor of |g^∗| = 35±4 and a value for the effective mass of m^∗ ≈ 0.017 m_e, where m_e is the free electron mass. Our measurements are performed in a magnetic field and a density range where the enhancement mechanism of the effective g factor can be neglected. Accordingly, the obtained effective g factor and the effective mass can be explained in a single-particle picture. Additionally, we explore the magnetotransport up to magnetic fields of 35 T and do not find features related to the fractional quantum Hall effect.
Neural network evolution strategy for solving quantum sign structures
Chen, Ao; Choo, Kenny; Astrakhantsev, Nikita; et al. (2022)
Physical Review Research
Feed-forward neural networks are a novel class of variational wave functions for correlated many-body quantum systems. Here, we propose a specific neural network ansatz suitable for systems with real-valued wave functions. Its characteristic is to encode the all-important rugged sign structure of a quantum wave function in a convolutional neural network with discrete output. Its training is achieved through an evolutionary algorithm. We test our variational ansatz and training strategy on two spin-1/2 Heisenberg models, one on the two-dimensional square lattice and one on the three-dimensional pyrochlore lattice. In the former, our ansatz converges with high accuracy to the analytically known sign structures of ordered phases. In the latter, where such sign structures are a priori unknown, we obtain better variational energies than with other neural network states. Our results demonstrate the utility of discrete neural networks to solve quantum many-body problems.

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Journal: Physical Review Research

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