44 research outputs found
First-Order Phase Transition in a Quantum Hall Ferromagnet
The single-particle energy spectrum of a two-dimensional electron gas in a
perpendicular magnetic field consists of equally-spaced spin-split Landau
levels, whose degeneracy is proportional to the magnetic field strength. At
integer and particular fractional ratios between the number of electrons and
the degeneracy of a Landau level (filling factors n) quantum Hall effects
occur, characterised by a vanishingly small longitudinal resistance and
quantised Hall voltage. The quantum Hall regime offers unique possibilities for
the study of cooperative phenomena in many-particle systems under
well-controlled conditions. Among the fields that benefit from quantum-Hall
studies is magnetism, which remains poorly understood in conventional material.
Both isotropic and anisotropic ferromagnetic ground states have been predicted
and few of them have been experimentally studied in quantum Hall samples with
different geometries and filling factors. Here we present evidence of
first-order phase transitions in n = 2 and 4 quantum Hall states confined to a
wide gallium arsenide quantum well. The observed hysteretic behaviour and
anomalous temperature dependence in the longitudinal resistivity indicate the
occurrence of a transition between the two distinct ground states of an Ising
quantum-Hall ferromagnet. Detailed many-body calculations allowed the
identification of the microscopic origin of the anisotropy field
Charge Hall effect driven by spin-dependent chemical potential gradients and Onsager relations in mesoscopic systems
We study theoretically the spin-Hall effect as well as its reciprocal
phenomenon (a transverse charge current driven by a spin-dependent chemical
potential gradient) in electron and hole finite size mesoscopic systems. The
Landauer-Buttiker-Keldysh formalism is used to model samples with mobilities
and Rashba coupling strengths which are experimentally accessible and to
demonstrate the appearance of measurable charge currents induced by the
spin-dependent chemical potential gradient in the reciprocal spin-Hall effect.
We also demonstrate that within the mesoscopic coherent transport regime the
Onsager relations are fulfilled for the disorder averaged conductances for
electron and hole mesoscopic systems.Comment: 5 pages, 6 figures, typos correcte
Beyond Conventional Ferromagnetism and Antiferromagnetism: A Phase with Nonrelativistic Spin and Crystal Rotation Symmetry
Recent series of theoretical and experimental reports have driven attention to time-reversal symmetry-breaking spintronic and spin-splitting phenomena in materials with collinear-compensated magnetic order incompatible with conventional ferromagnetism or antiferromagnetism. Here we employ an approach based on nonrelativistic spin-symmetry groups that resolves the conflicting notions of unconventional ferromagnetism or antiferromagnetism by delimiting a third basic collinear magnetic phase. We derive that all materials hosting this collinear-compensated magnetic phase are characterized by crystal-rotation symmetries connecting opposite-spin sublattices separated in the real space and opposite-spin electronic states separated in the momentum space. We describe prominent extraordinary characteristics of the phase, including the alternating spin-splitting sign and broken time-reversal symmetry in the nonrelativistic band structure, the planar or bulk d-, g-, or i-wave symmetry of the spin-dependent Fermi surfaces, spin-degenerate nodal lines and surfaces, band anisotropy of individual spin channels, and spin-split general, as well as time-reversal invariant momenta. Guided by the spin-symmetry principles, we discover in ab initio calculations outlier materials with an extraordinary nonrelativistic spin splitting, whose eV-scale and momentum dependence are determined by the crystal potential of the nonmagnetic phase. This spin-splitting mechanism is distinct from conventional relativistic spin-orbit coupling and ferromagnetic exchange, as well as from the previously considered anisotropic exchange mechanism in compensated magnets. Our results, combined with our identification of material candidates for the phase ranging from insulators and metals to a parent crystal of cuprate superconductors, underpin research of novel quantum phenomena and spintronic functionalities in high-temperature magnets with light elements, vanishing net magnetization, and strong spin coherence. In the discussion, we argue that the conflicting notions of unconventional ferromagnetism or antiferromagnetism, on the one hand, and our symmetry-based delimitation of the third phase, on the other hand, favor a distinct term referring to the phase. The alternating spin polarizations in both the real-space crystal structure and the momentum-space band structure characteristic of this unconventional magnetic phase suggest a term altermagnetism. We point out that d-wave altermagnetism represents a realization of the long-sought-after counterpart in magnetism of the unconventional d-wave superconductivity
Beyond Conventional Ferromagnetism and Antiferromagnetism: A Phase with Nonrelativistic Spin and Crystal Rotation Symmetry
Recent series of theoretical and experimental reports have driven attention to time-reversal symmetry-breaking spintronic and spin-splitting phenomena in materials with collinear-compensated magnetic order incompatible with conventional ferromagnetism or antiferromagnetism. Here we employ an approach based on nonrelativistic spin-symmetry groups that resolves the conflicting notions of unconventional ferromagnetism or antiferromagnetism by delimiting a third basic collinear magnetic phase. We derive that all materials hosting this collinear-compensated magnetic phase are characterized by crystal-rotation symmetries connecting opposite-spin sublattices separated in the real space and opposite-spin electronic states separated in the momentum space. We describe prominent extraordinary characteristics of the phase, including the alternating spin-splitting sign and broken time-reversal symmetry in the nonrelativistic band structure, the planar or bulk d-, g-, or i-wave symmetry of the spin-dependent Fermi surfaces, spin-degenerate nodal lines and surfaces, band anisotropy of individual spin channels, and spin-split general, as well as time-reversal invariant momenta. Guided by the spin-symmetry principles, we discover in ab initio calculations outlier materials with an extraordinary nonrelativistic spin splitting, whose eV-scale and momentum dependence are determined by the crystal potential of the nonmagnetic phase. This spin-splitting mechanism is distinct from conventional relativistic spin-orbit coupling and ferromagnetic exchange, as well as from the previously considered anisotropic exchange mechanism in compensated magnets. Our results, combined with our identification of material candidates for the phase ranging from insulators and metals to a parent crystal of cuprate superconductors, underpin research of novel quantum phenomena and spintronic functionalities in high-temperature magnets with light elements, vanishing net magnetization, and strong spin coherence. In the discussion, we argue that the conflicting notions of unconventional ferromagnetism or antiferromagnetism, on the one hand, and our symmetry-based delimitation of the third phase, on the other hand, favor a distinct term referring to the phase. The alternating spin polarizations in both the real-space crystal structure and the momentum-space band structure characteristic of this unconventional magnetic phase suggest a term altermagnetism. We point out that d-wave altermagnetism represents a realization of the long-sought-after counterpart in magnetism of the unconventional d-wave superconductivity
Emerging Research Landscape of Altermagnetism
Magnetism is one of the largest, most fundamental, and technologically most relevant fields of condensed-matter physics. Traditionally, two basic magnetic phases have been distinguished ferromagnetism and antiferromagnetism. The spin polarization in the electronic band structure reflecting the magnetization in ferromagnetic crystals underpins the broad range of time-reversal symmetry-breaking responses in this extensively explored and exploited type of magnets. By comparison, antiferromagnets have vanishing net magnetization. Recently, there have been observations of materials in which strong time-reversal symmetry-breaking responses and spin-polarization phenomena, typical of ferromagnets, are accompanied by antiparallel magnetic crystal order with vanishing net magnetization, typical of antiferromagnets. A classification and description based on spin-symmetry principles offers a resolution of this apparent contradiction by establishing a third distinct magnetic phase, dubbed altermagnetism. Our perspective starts with an overview of the still emerging unique phenomenology of this unconventional d-wave (or higher even-parity wave) magnetic phase, and of the wide array of altermagnetic material candidates. We illustrate how altermagnetism can enrich our understanding of overarching condensedmatter physics concepts and how it can have impact on prominent condensed-matter research areas
Hydration of biologically relevant tetramethylammonium cation by neutron scattering and molecular dynamics
Neutron scattering and molecular dynamics studies were performed on a
concentrated aqueous tetramethylammonium (TMA) chloride solution to gain
insight into the hydration shell structure of TMA, which is relevant for
understanding its behavior in biological contexts of, e.g., properties of
phospholipid membrane headgroups or interactions between DNA and histones.
Specifically, neutron diffraction with isotopic substitution experiments were
performed on TMA and water hydrogens to extract the specific correlation
between hydrogens in TMA () and hydrogens in water
(). Classical molecular dynamics simulations were performed to
help interpret the experimental neutron scattering data. Comparison of the
hydration structure and simulated neutron signals obtained with various force
field flavors (e.g. overall charge, charge distribution, polarity of the CH
bonds and geometry) allowed us to gain insight into how sensitive the TMA
hydration structure is to such changes and how much the neutron signal can
capture them. We show that certain aspects of the hydration, such as the
correlation of the hydrogen on TMA to hydrogen on water, showed little
dependence on the force field. In contrast, other correlations, such as the
ion-ion interactions, showed more marked changes. Strikingly, the neutron
scattering signal cannot discriminate between different hydration patterns.
Finally, ab initio molecular dynamics was used to examine the three-dimensional
hydration structure and thus to benchmark force field simulations. Overall,
while neutron scattering has been previously successfully used to improve force
fields, in the particular case of TMA we show that it has only limited value to
fully determine the hydration structure, with other techniques such as ab
initio MD being of a significant help
Calcium ions in aqueous solutions: Accurate force field description aided by ab initio molecular dynamics and neutron scattering
International audienc
Giant and Tunneling Magnetoresistance in Unconventional Collinear Antiferromagnets with Nonrelativistic Spin-Momentum Coupling
Giant and tunneling magnetoresistance are physical phenomena used for reading information in commercial spintronic devices. The effects rely on a conserved spin current passing between a reference and a sensing ferromagnetic electrode in a multilayer structure. Recently, we have proposed that these fundamental spintronic effects can be realized in unconventional collinear antiferromagnets with nonrelativistic alternating spin-momentum coupling. Here, we elaborate on the proposal by presenting archetype model mechanisms for the giant and tunneling magnetoresistance effects in multilayers composed of these unconventional collinear antiferromagnets. The models are based, respectively, on anisotropic and valley-dependent forms of the alternating spin-momentum coupling. Using first-principles calculations, we link these model mechanisms to real materials and predict an approximately 100% scale for the effects. We point out that, besides the giant or tunneling magnetoresistance detection, the alternating spin-momentum coupling can allow for magnetic excitation by the spin-transfer torque
Frequency-independent terahertz anomalous Hall effect in DyCo5, Co32Fe68 and Gd27Fe73 thin films from DC to 40 THz
The anomalous Hall effect (AHE) is a fundamental spintronic charge‐to‐charge‐current conversion phenomenon and closely related to spin‐to‐charge‐current conversion by the spin Hall effect. Future high‐speed spintronic devices will crucially rely on such conversion phenomena at terahertz (THz) frequencies. Here, it is revealed that the AHE remains operative from DC up to 40 THz with a flat frequency response in thin films of three technologically relevant magnetic materials: DyCo5, Co32Fe68, and Gd27Fe73. The frequency‐dependent conductivity‐tensor elements σxx and σyx are measured, and good agreement with DC measurements is found. The experimental findings are fully consistent with ab initio calculations of σyx for CoFe and highlight the role of the large Drude scattering rate (≈100 THz) of metal thin films, which smears out any sharp spectral features of the THz AHE. Finally, it is found that the intrinsic contribution to the THz AHE dominates over the extrinsic mechanisms for the Co32Fe68 sample. The results imply that the AHE and related effects such as the spin Hall effect are highly promising ingredients of future THz spintronic devices reliably operating from DC to 40 THz and beyond
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Imaging and writing magnetic domains in the non-collinear antiferromagnet Mn3Sn
Non-collinear antiferromagnets are revealing many unexpected phenomena and they became crucial for the field of antiferromagnetic spintronics. To visualize and prepare a well-defined domain structure is of key importance. The spatial magnetic contrast, however, remains extraordinarily difficult to be observed experimentally. Here, we demonstrate a magnetic imaging technique based on a laser induced local thermal gradient combined with detection of the anomalous Nernst effect. We employ this method in one the most actively studied representatives of this class of materials—Mn3Sn. We demonstrate that the observed contrast is of magnetic origin. We further show an algorithm to prepare a well-defined domain pattern at room temperature based on heat assisted recording principle. Our study opens up a prospect to study spintronics phenomena in non-collinear antiferromagnets with spatial resolution