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Stuart S. P. Parkin

Publications and source records attributed to Stuart S. P. Parkin.

At least 37 records · Page 2Linked to original sources
arXiv2024

Momentum-Resolved Fingerprint of Mottness in Layer-Dimerized Nb$_3$Br$_8$

In a well-ordered crystalline solid, insulating behaviour can arise from two mechanisms: electrons can either scatter off a periodic potential, thus forming band gaps that can lead to a band insulator, or they localize due to strong interactions, resulting in a Mott insulator. For an even number of electrons per unit cell, either band- or Mott-insulators can theoretically occur. However, unambiguously identifying an unconventional Mott-insulator with an even number of electrons experimentally has remained a longstanding challenge due to the lack of a momentum-resolved fingerprint. This challenge has recently become pressing for the layer dimerized van der Waals compound Nb$_3$Br$_8$, which exhibits a puzzling magnetic field-free diode effect when used as a weak link in Josephson junctions, but has previously been considered to be a band-insulator. In this work, we present a unique momentum-resolved signature of a Mott-insulating phase in the spectral function of Nb$_3$Br$_8$: the top of the highest occupied band along the out-of-plane dimerization direction $k_z$ has a momentum space separation of $Δk_z=2π/d$, whereas the valence band maximum of a band insulator would be separated by less than $Δk_z=π/d$, where $d$ is the average spacing between the layers. As the strong electron correlations inherent in Mott insulators can lead to unconventional superconductivity, identifying Nb$_3$Br$_8$ as an unconventional Mott-insulator is crucial for understanding its apparent time-reversal symmetry breaking Josephson diode effect. Moreover, the momentum-resolved signature employed here could be used to detect quantum phase transition between band- and Mott-insulating phases in van der Waals heterostructures, where interlayer interactions and correlations can be easily tuned to drive such transition.

cond-mat.str-el↗
arXiv2024

Switching of magnetic domains in a noncollinear antiferromagnet at the nanoscale

Antiferromagnets that display very small stray magnetic field are ideal for spintronic applications. Of particular interest are non-collinear, chiral antiferromagnets of the type Mn3X (X=Sn, Ge), which display a large magnetotransport response that is correlated with their antiferromagnetic ordering. The ability to read out and manipulate this ordering is crucial for their integration into spintronic devices. These materials exhibit a tiny unbalanced magnetic moment such that a large external magnetic field can, in principle, be used to set the material into a single antiferromagnetic domain. However, in thin films of Mn3Sn, we find that such fields induce only a partial magnetic ordering. By detecting two orthogonal in-plane components of the magnetic order vector, we find that the non-switchable fraction has a unidirectional anisotropy. This also enables us to visualize switching along multiple easy axes in Mn3Sn. Studying the switching at the nanoscale allows us to correlate the pining behavior to crystal grain boundaries in the Mn3Sn nanowire structures.

cond-mat.mes-hall↗
arXiv2024

Reduced decay in Josephson coupling across ferromagnetic junctions with spin-orbit coupling layers

The generation of $S_z=1$ triplet Cooper pairs has been predicted theoretically in superconducting-ferromagnetic hybrid heterostructures in the presence of spin-orbit coupling [F. S. Bergeret and I. V. Tokatly, Phys. Rev. B 89, 134517 (2014) and Jacobsen et al., Sci. Rep. 6, 23926 (2016)]. In this study, we experimentally investigate vertical Josephson junctions where the weak link is formed from a ferromagnetic layer with perpendicular magnetic anisotropy sandwiched by two non-magnetic layers with weak or strong spin-orbit coupling. We find that the decay of the Josephson coupling is reduced in the latter case, possibly indicating the presence of $S_z=1$ spin-triplet correlations. We speculate that the canted magnetization required for these correlations is provided by the interaction of magnetization with Meissner effect in the superconducting layers.

cond-mat.supr-con↗
arXiv2024

Long-range Phase Coherence and Tunable Second Order $ϕ_0$-Josephson Effect in a Dirac Semimetal $1T-PtTe_2$

Superconducting diode effects have recently attracted much attention for their potential applications in superconducting logic circuits. Several mechanisms such as magneto-chiral effects, finite momentum Cooper pairing, asymmetric edge currents have been proposed to give rise to a supercurrent diode effect in different materials. In this work, we establish the presence of a large intrinsic Josephson diode effect in a type-II Dirac semimetal $1T-PtTe_2$ facilitated by its helical spin-momentum locking and distinguish it from other extrinsic effects. The magnitude of the Josephson diode effect is shown to be directly correlated to the large second-harmonic component of the supercurrent that is induced by the significant contribution of the topological spin-momentum locked states that promote coherent Andreev processes in the junction. We denote such junctions, where the relative phase between the two harmonics corresponding to charge transfers of $2e$ and $4e$ can be tuned by a magnetic field, as second order $ϕ_0$-junctions. The direct correspondence between the second harmonic supercurrent component and the diode effect in $1T-PtTe_2$ junctions makes topological semimetals with high transparency an ideal platform to study and implement the Josephson diode effect, while also enabling further research on higher order supercurrent transport in Josephson junctions.

cond-mat.supr-con↗
arXiv2024

Anomalous Nernst effect based near field imaging of magnetic nanostructures

The anomalous Nernst effect (ANE) gives rise to an electrical response transverse to the magnetization and an applied temperature gradient in a magnetic metal. A nanoscale temperature gradient can be generated by the use of a laser beam applied to the apex of an atomic force microscope tip, thereby allowing for spatially-resolved ANE measurements beyond the optical diffraction limit. Such a method has been used previously to map in-plane magnetized magnetic textures. However, the spatial distribution of the out-of-plane temperature gradient, which is needed to fully interpret such an ANE-based imaging, was not studied. We therefore use a well-known magnetic texture, a magnetic vortex core, to demonstrate the reliability of the ANE method for the imaging of magnetic domains with nanoscale resolution. Moreover, since the ANE signal is directly proportional to the temperature gradient, we can also consider the inverse problem and deduce information about the nanoscale temperature distribution. Our results together with finite element modeling indicate that besides the out-of-plane temperature gradients, there are even larger in-plane temperature gradients. Thus we extend the ANE imaging to study out-of-plane magnetization in a racetrack nano-wire by detecting the ANE signal generated by in-plane temperature gradients. In all cases, a spatial resolution of about 80 nm is obtained. These results are significant for the rapidly growing field of thermo-electric imaging of antiferromagnetic spintronic device structures.

cond-mat.mes-hall↗
arXiv2024

Ferrimagnetic Heusler tunnel junctions with fast spin-transfer torque switching enabled by low magnetization

Magnetic random access memory that uses magnetic tunnel junction memory cells is a high performance, non-volatile memory technology that goes beyond traditional charge-based memories. Today its speed is limited by the high magnetization of the memory storage layer. Here we show that fast and highly reliable switching is possible using a very low magnetization ferrimagnetic Heusler alloy, Mn3Ge. Moreover, the tunneling magnetoresistance is the highest yet achieved for a ferrimagnetic material at ambient temperature. Furthermore, the devices were prepared on technologically relevant amorphous substrates using a novel combination of a nitride seed layer and a chemical templating layer. These results show a clear path to the lowering of switching currents using ferrimagnetic Heusler materials and, therefore, to the scaling of high performance magnetic random access memories beyond those nodes possible with ferromagnetic devices.

cond-mat.mtrl-sci↗
arXiv2024

Realization of a Spin Glass in a two-dimensional van der Waals material

Recent advances in van der Waals (vdW) materials have sparked renewed interest in the impact of dimensionality on magnetic phase transitions. While ordered magnetic phases have been demonstrated to survive in the two-dimensional (2D) limit, the quest for a spin-glass with quenched magnetic disorder in lower dimensions has proven elusive. Here we show evidence of a spin-glass emerging from randomly distributed Fe atoms in Fe3GeTe2, the first time such a state has been reported in a vdW material. AC magnetic susceptibility displays a strong frequency dependence indicative of slow spin dynamics. Additional distinctive phenomena, including ageing, chaos, and memory effects, further substantiate the existence of a glassy state. Remarkably, we find that this state persists even in single-unit-cell thick Fe3GeTe2, thereby confirming the existence of a 2D spin-glass. The formation of spin-glass states via intercalation in vdW systems allows for highly tunable spin-glass states that are otherwise difficult to realize.

cond-mat.mes-hall↗
arXiv2024

Two-dimensional Shiba lattices as possible platform for crystalline topological superconductivity

Localized or propagating Majorana boundary modes are the key feature of topological superconductors. They are rare in naturally-occurring compounds, but the tailored manipulation of quantum matter offers opportunities for their realization. Specifically, lattices of Yu-Shiba-Rusinov bound states $-$ Shiba lattices $-$ that arise when magnetic adatoms are placed on the surface of a conventional superconductor can be used to create topological bands within the superconducting gap of the substrate. Here, using scanning tunnelling microscopy to create and probe adatom lattices with single atom precision we reveal two signatures consistent with the realization of two types of mirror symmetry protected topological superconductors. The first has edge modes as well as higher-order corner states, and the second has symmetry-protected bulk nodal points. In principle, their topological character and boundary modes should be protected by the spatial symmetries of the adatom lattice. Our results highlight the potential of Shiba lattices as a platform to design the topology and sample geometry of 2D superconductors.

cond-mat.mes-hall↗
arXiv2024

Population inversion and ultrafast terahertz nonlinearity of transient Dirac fermions in Cd$_3$As$_2$

Harmonic generation provides an efficient tool for the study of ultrafast nonlinear dynamics. We report on time-resolved optical-pump terahertz-harmonic-generation spectroscopic investigation of ultrafast nonlinearity in a prototypical three-dimensional Dirac semimetal Cd$_3$As$_2$. A transient population inversion characterized by excessive nonthermal Dirac electrons and holes is found to be very sensitive and responsive to a periodic terahertz drive, leading to very efficient terahertz third-harmonic generation. Based on the Boltzmann transport theory, we analyze the terahertz field-driven kinetics of the transient Dirac fermions that is responsible for the observed strong terahertz nonlinearity.

cond-mat.mtrl-sci↗
arXiv2023

Controllable orbital angular momentum monopoles in chiral topological semimetals

The emerging field of orbitronics aims at generating and controlling currents of electronic orbital angular momentum (OAM) for information processing. Structurally chiral topological crystals could be particularly suitable orbitronic materials because they have been predicted to host topological band degeneracies in reciprocal space that are monopoles of OAM. Around such a monopole, the OAM is locked isotopically parallel or antiparallel to the direction of the electron's momentum, which could be used to generate large and controllable OAM currents. However, OAM monopoles have not yet been directly observed in chiral crystals, and no handle to control their polarity has been discovered. Here, we use circular dichroism in angle-resolved photoelectron spectroscopy (CD-ARPES) to image OAM monopoles in the chiral topological semimetals PtGa and PdGa. Moreover, we also demonstrate that the polarity of the monopole can be controlled via the structural handedness of the host crystal by imaging OAM monopoles and anti-monopoles in the two enantiomers of PdGa, respectively. For most photon energies used in our study, we observe a sign change in the CD-ARPES spectrum when comparing positive and negative momenta along the light direction near the topological degeneracy. This is consistent with the conventional view that CD-ARPES measures the projection of the OAM monopole along the photon momentum. For some photon energies, however, this sign change disappears, which can be understood from our numerical simulations as the interference of polar atomic OAM contributions, consistent with the presence of OAM monopoles. Our results highlight the potential of chiral crystals for orbitronic device applications, and our methodology could enable the discovery of even more complicated nodal OAM textures that could be exploited for orbitronics.

cond-mat.str-el↗
arXiv2023

Quantum computing on magnetic racetracks with flying domain wall qubits

Domain walls (DWs) on magnetic racetracks are at the core of the field of spintronics, providing a basic element for classical information processing. Here, we show that mobile DWs also provide a blueprint for large-scale quantum computers. Remarkably, these DW qubits showcase exceptional versatility, serving not only as stationary qubits, but also performing the role of solid-state flying qubits that can be shuttled in an ultrafast way. We estimate that the DW qubits are long-lived because they can be operated at sweet spots to reduce potential noise sources. Single-qubit gates are implemented by moving the DW, and two-qubit entangling gates exploit naturally emerging interactions between different DWs. These gates, sufficient for universal quantum computing, are fully compatible with current state-of-the-art experiments on racetrack memories. Further, we discuss possible strategies for qubit readout and initialization, paving the way toward future quantum computers based on mobile topological textures on magnetic racetracks.

cond-mat.mes-hall↗
arXiv2023

Li iontronics in single-crystalline T-Nb2O5 thin films with vertical ionic transport channels

The niobium oxide polymorph T-Nb2O5 has been extensively investigated in its bulk form especially for applications in fast-charging batteries and electrochemical (pseudo)capacitors. Its crystal structure that has two-dimensional (2D) layers with very low steric hindrance allows for fast Li-ion migration. However, since its discovery in 1941, the growth of single-crystalline thin films and its electronic applications have not yet been realized, likely due to its large orthorhombic unit cell along with the existence of many polymorphs. Here we demonstrate the epitaxial growth of single-crystalline T-Nb2O5 thin films, critically with the ionic transport channels oriented perpendicular to the film's surface. These vertical 2D channels enable fast Li-ion migration which we show gives rise to a colossal insulator-metal transition where the resistivity drops by eleven orders of magnitude due to the population of the initially empty Nb 4d0 states by electrons. Moreover, we reveal multiple unexplored phase transitions with distinct crystal and electronic structures over a wide range of Li-ion concentrations by comprehensive in situ experiments and theoretical calculations, that allow for the reversible and repeatable manipulation of these phases and their distinct electronic properties. This work paves the way to the exploration of novel thin films with ionic channels and their potential applications.

cond-mat.mtrl-sci↗
arXiv2023

Yu-Shiba-Rusinov tips: imaging spins at the atomic scale with full magnetic sensitivity

Measurements of magnetic properties at the atomic scale require probes capable of combining high spatial resolution with spin sensitivity. Spin-polarized scanning tunneling microscopy (SP-STM) fulfills these conditions by using atomically sharp magnetic tips. The imaging of spin structures results from the tunneling magneto-conductance that depends on the imbalance in the local density of spin-up and spin-down electrons. Spin-sensitive tips are generally formed from bulk materials or by coating non-magnetic tips with a thin magnetic layer. However, ferromagnetic materials generate stray magnetic fields which can influence the magnetic structure of the probed system, while the magnetization of antiferromagnetic materials is difficult to set tip by externally applied magnetic fields. Here, we use functionalized Yu-Shiba-Rusinov (YSR) tips prepared by attaching magnetic adatoms at the apex of a superconducting cluster to image magnetic interactions at the atomic scale. We demonstrate that YSR tips are capable of sensing different magnetization directions, conferring them full magnetic sensitivity. We additionally show that the finite size of the tip superconducting cluster makes it robust against relatively strong magnetic fields, making YSR tips capable of visualizing magnetic field driven transitions of the spin texture.

cond-mat.supr-con↗
arXiv2023

Quantum information diode based on a magnonic crystal

Exploiting the effect of nonreciprocal magnons in a system with no inversion symmetry, we propose a concept of a quantum information diode, {\it i.e.}, a device rectifying the amount of quantum information transmitted in the opposite directions. We control the asymmetric left and right quantum information currents through an applied external electric field and quantify it through the left and right out-of-time-ordered correlation (OTOC). To enhance the efficiency of the quantum information diode, we utilize a magnonic crystal. We excite magnons of different frequencies and let them propagate in opposite directions. Nonreciprocal magnons propagating in opposite directions have different dispersion relations. Magnons propagating in one direction match resonant conditions and scatter on gate magnons. Therefore, magnon flux in one direction is damped in the magnonic crystal leading to an asymmetric transport of quantum information in the quantum information diode. A quantum information diode can be fabricated from an yttrium iron garnet (YIG) film. This is an experimentally feasible concept and implies certain conditions: low temperature and small deviation from the equilibrium to exclude effects of phonons and magnon interactions. We show that rectification of the flaw of quantum information can be controlled efficiently by an external electric field and magnetoelectric effects.

quant-ph↗
arXiv2023

A Cu3BHT-Graphene van der Waals Heterostructure with Strong Interlayer Coupling

Two dimensional van der Waals heterostructures (2D are of significant interest due to their intriguing physical properties that are critically defined by the constituent monolayers and their interlayer coupling . However, typical inorganic 2 D vdWhs fall into the weakly coupled region, limiting efficient interfacial charge flow crucial for developing high performance quantum opto electronics. Here, we demonstrate strong interlayer coupling in Cu3 BHT (BHT = benzenehexathiol) graphene vdWhs an organic inorganic bilayer characterized by prominent interlayer charge transfer Monolayer Cu3 BHT with a Kagome lattice is synthesized on the water surface and then coupled with graphene to produce a cm2 scale 2D vdWh. Spectroscopic and electrical studies, along with theoretical calculation s show significant hole transfer from monolayer Cu3 BHT to graphene upon contact , being characteristic fingerprints for strong interlayer coupling This study unveils the great potential of integrating highly pi-conjugated 2D coordination polymers (2DCPs) into 2D vdWhs to explor e intriguing physical phenomena.

physics.app-ph↗
arXiv2023

Observation of Fluctuation Spin Hall Effect in Antiferromagnet

The spin Hall effect (SHE) can generate a pure spin current by an electric current, which is promisingly used to electrically control magnetization. To reduce power consumption of this control, a giant spin Hall angle (SHA) in the SHE is desired in low-resistivity systems for practical applications. Here, critical spin fluctuation near the antiferromagnetic (AFM) phase-transition is proved as an effective mechanism to create an additional part of SHE, named as fluctuation spin Hall effect (FSHE). This FSHE enhances the SHA due to the AFM spin fluctuation between conduction electrons and local spins. We detect the FSHE with the inverse and direct spin Hall effect (ISHE and DSHE) set-up and their temperature (T) dependences in the Cr/MgO/Fe magnetic tunnel junctions (MTJs). The SHA is significantly enhanced when temperature is approached to the Néel temperature (T_N) and has a peak value of -0.34 at 200 K near T_N. This value is higher than the room-temperature value by 240% and comparable to that of heavy metals Ta and W. Furthermore, the spin Hall resistivity of Cr well fits the modeled T-dependence when T approaches T_N from low temperatures, implying the AFM spin fluctuation nature of strong SHA enhancement. Thus, this study demonstrates the critical spin fluctuation as a prospective way of increasing SHA and enriches the AFM material candidates for spin-orbitronic devices.

cond-mat.mes-hall↗
arXiv2023

Intrinsic supercurrent non-reciprocity coupled to the crystal structure of a van der Waals Josephson barrier

Non-reciprocal electronic transport in a spatially homogeneous system arises from the simultaneous breaking of inversion and time-reversal symmetries. Superconducting and Josephson diodes, a key ingredient for future non-dissipative quantum devices, have recently been realized. Only a few examples of a vertical superconducting diode effect have been reported and its mechanism, especially whether intrinsic or extrinsic, remains elusive. Here we demonstrate a substantial supercurrent non-reciprocity in a van der Waals vertical Josephson junction formed with a Td-WTe2 barrier and NbSe2 electrodes that clearly reflects the intrinsic crystal structure of Td-WTe2. The Josephson diode efficiency increases with the Td-WTe2 thickness up to critical thickness, and all junctions, irrespective of the barrier thickness, reveal magneto-chiral characteristics with respect to a mirror plane of Td-WTe2. Our results, together with the twist-angle-tuned magneto-chirality of a Td-WTe2 double-barrier junction, show that two-dimensional materials promise vertical Josephson diodes with high efficiency and tunability.

cond-mat.supr-con↗
arXiv2023

Anisotropic proximity-induced superconductivity and edge supercurrent in Kagome metal, K1-xV3Sb5

Materials with transition metals in triangular lattices are of great interest for their potential combination of strong correlation, exotic magnetism and electronic topology. Kagome nets are of particular importance since the discovery of geometrically frustrated magnetism and topological band structures in crystals like Herbertsmithite and Fe3Sn2, respectively. KV3Sb5 was discovered to be a layered topological metal with a Kagome net of vanadium. Here, we fabricated Josephson Junctions (JJ) of K1-xV3Sb5 and induced superconductivity over long junction lengths. Through magnetoresistance and current vs. phase measurements, we observed magnetic field sweeping direction dependent magnetoresistance, and an anisotropic interference pattern with a Fraunhofer pattern for in-plane magnetic field, but a suppression of critical current for out-of-plane magnetic field. These results indicate an anisotropic internal magnetic field in K1-xV3Sb5 which influences the superconducting coupling in the junction, possibly giving rise to spin-triplet superconductivity. In addition, the observation of long-lived fast oscillations shows evidence of spatially localized conducting channels arising from edge states. These observations pave the way for studying unconventional superconductivity and Josephson device based on Kagome metals with electron correlation and topology.

cond-mat.supr-con↗