Pallavi Kushwaha

Evidence for hydrodynamic electron flow in PdCoO2

Electrons that flow like a fluid Electrons inside a conductor are often described as flowing in response to an electric field. This flow rarely resembles anything like the familiar flow of water through a pipe, but three groups describe counterexamples (see the Perspective by Zaanen). Moll et al. found that the viscosity of the electron fluid in thin wires of PdCoO2 had a major effect on the flow, much like what happens in regular fluids. Bandurin et al. found evidence in graphene of electron whirlpools similar to those formed by viscous fluid flowing through a small opening. Finally, Crossno et al. observed a huge increase of thermal transport in graphene, a signature of so-called Dirac fluids. Science, this issue p. 1061, 1055, 1058; see also p. 1026 Transport measurements in thin wires of PdCoO2 reveal a regime wherein electron-electron collisions play a major role. [Also see Perspective by Zaanen] Electron transport is conventionally determined by the momentum-relaxing scattering of electrons by the host solid and its excitations. Hydrodynamic fluid flow through channels, in contrast, is determined partly by the viscosity of the fluid, which is governed by momentum-conserving internal collisions. A long-standing question in the physics of solids has been whether the viscosity of the electron fluid plays an observable role in determining the resistance. We report experimental evidence that the resistance of restricted channels of the ultrapure two-dimensional metal palladium cobaltate (PdCoO2) has a large viscous contribution. Comparison with theory allows an estimate of the electronic viscosity in the range between 6 × 10–3 kg m–1 s–1 and 3 × 10–4 kg m–1 s–1, versus 1 × 10–3 kg m–1 s–1 for water at room temperature.

Low-temperature study of field-induced antiferromagnetic-ferromagnetic transition in Pd-doped Fe-Rh

The first-order antiferromagnetic (AFM) to ferromagnetic (FM) transition in the functional material

Room temperature giant baroresistance and magnetoresistance and its tunability in Pd doped FeRh

{\text{Fe}}_{49}{({\text{Rh}}_{0.93}{\text{Pd}}_{0.07})}_{51}

Nearly free electrons in a 5d delafossite oxide metal.

has been studied at low temperatures and high magnetic fields. We have addressed the nonmonotonic variation in lower critical field required for FM to AFM transition. It is shown that critically slow dynamics of the transition dominates below 50 K. At low temperature and high magnetic field, state of the system depends on the measurement history resulting in tunable coexistence of AFM and FM phases. By following cooling and heating in unequal magnetic field protocol it is shown that equilibrium state at 6 T magnetic field is AFM state. Glasslike FM state at 6 T (obtained after cooling in 8 T) shows reentrant transition with increasing temperature; viz., devitrification to AFM state followed by melting to FM state.

Real-space visualization of thermomagnetic irreversibility within supercooling and superheating spinodals in Mn 1.85 Co 0.15 Sb using scanning Hall probe microscopy

We report room temperature giant baroresistance (≈128%) in Fe49(Rh0.93Pd0.07)51. With the application of external pressure (P) and magnetic field (H), the temperature range of giant baroresistance (≈600% at 5 K, 19.9 kilobars and 8 T) and magnetoresistance (≈ −85% at 5 K and 8 T) can be tuned from 5 K to well above room temperature. It is shown that under external pressure, antiferromagnetic state is stabilized at room temperature and shows giant magnetoresistance (≈−55%). Due to coupled magnetic and lattice changes, the isothermal change in resistivity at room temperature under pressure (at constant H) as well as magnetic field (at constant P) can be scaled together to a single curve, when plotted as a function of X = T + 12.8 × H − 7.3 × P.

Magnetic properties and magnetocaloric effect in Pt doped Ni-Mn-Ga

Transport and ARPES reveal extremely good metallicity arising from almost free-electron behavior in single-crystal PtCoO2. Understanding the role of electron correlations in strong spin-orbit transition-metal oxides is key to the realization of numerous exotic phases including spin-orbit–assisted Mott insulators, correlated topological solids, and prospective new high-temperature superconductors. To date, most attention has been focused on the 5d iridium-based oxides. We instead consider the Pt-based delafossite oxide PtCoO2. Our transport measurements, performed on single-crystal samples etched to well-defined geometries using focused ion beam techniques, yield a room temperature resistivity of only 2.1 microhm·cm (μΩ-cm), establishing PtCoO2 as the most conductive oxide known. From angle-resolved photoemission and density functional theory, we show that the underlying Fermi surface is a single cylinder of nearly hexagonal cross-section, with very weak dispersion along kz. Despite being predominantly composed of d-orbital character, the conduction band is remarkably steep, with an average effective mass of only 1.14me. Moreover, the sharp spectral features observed in photoemission remain well defined with little additional broadening for more than 500 meV below EF, pointing to suppressed electron-electron scattering. Together, our findings establish PtCoO2 as a model nearly-free–electron system in a 5d delafossite transition-metal oxide.

Influence of thermal annealing and magnetic field on first order magnetic transition in Pd substituted FeRh

Phase coexistence across disorder-broadened and magnetic-field-induced first-order antiferromagnetic to ferrimagnetic transition in polycrystalline Mn 1.85 Co 0.15 Sb has been studied mesoscopically by scanning Hall probe microscope at 120 K and up to 5 T magnetic fields. We have observed hysteresis with varying magnetic fields and evolution of coexisting antiferromagnetic and ferrimagnetic states on mesoscopic length scale. These studies show that the magnetic state of the system at low field depends on the path followed to reach 120 K. The low-field magnetic states are mesoscopically different for virgin and second field increasing cycle when 120 K is reached by warming from 5 K but are the same within measurement accuracy when the measuring temperature of 120 K is reached from 300 K by cooling.

Maximal Rashba-like spin splitting via kinetic energy-coupled inversion symmetry breaking

Large magnetocaloric effect is observed in Ni1.8Pt0.2MnGa close to room temperature. The entropy change shows a crossover from positive to negative sign at the martensite transition. It is negative above 1.6 T and its magnitude increases linearly with magnetic field. An increase in the saturation magnetic moment is observed with Pt doping in Ni2MnGa. Ab initio theoretical calculations show that the increase in magnetic moment with Pt doping in Ni2MnGa is associated with increase in the Mn and Pt local moments in the ferromagnetic ground state. The Curie temperature calculated from the exchange interaction parameters is in good agreement with experiment, showing the absence of any antiferromagnetic correlation due to Pt doping.

Magnetic field dependence of magnetic domains in Co doped Mn2Sb using magnetic force microscopy

Influence of successive thermal annealing and magnetic field on First order antiferro (AFM) to ferromagnetic (FM) transition in the Pd substituted FeRh has been studied. With successive thermal annealing CsCl type bcc phase increases at the expense of fct (pseudo fcc) phase. Resistivity measurements do not show any transition in as-cast sample in contrast to annealed samples. AFM to FM transition temperature (TN)is found to decrease with higher annealing temperature. With the application of magnetic field, TN shift to lower temperature. These measurements show anomalous thermomagnetic irreversibility besides showing giant magnetoresistance across magnetic field induced first order AFM to FM transition.

Direct visualization of first-order magnetic transition in La 5 / 8 − y Pr y Ca 3 / 8 MnO 3 ( y = 0.45 ) thin films

Engineering and enhancing the breaking of inversion symmetry in solids—that is, allowing electrons to differentiate between ‘up’ and ‘down’—is a key goal in condensed-matter physics and materials science because it can be used to stabilize states that are of fundamental interest and also have potential practical applications. Examples include improved ferroelectrics for memory devices and materials that host Majorana zero modes for quantum computing. Although inversion symmetry is naturally broken in several crystalline environments, such as at surfaces and interfaces, maximizing the influence of this effect on the electronic states of interest remains a challenge. Here we present a mechanism for realizing a much larger coupling of inversion-symmetry breaking to itinerant surface electrons than is typically achieved. The key element is a pronounced asymmetry of surface hopping energies—that is, a kinetic-energy-coupled inversion-symmetry breaking, the energy scale of which is a substantial fraction of the bandwidth. Using spin- and angle-resolved photoemission spectroscopy, we demonstrate that such a strong inversion-symmetry breaking, when combined with spin–orbit interactions, can mediate Rashba-like spin splittings that are much larger than would typically be expected. The energy scale of the inversion-symmetry breaking that we achieve is so large that the spin splitting in the CoO2- and RhO2-derived surface states of delafossite oxides becomes controlled by the full atomic spin–orbit coupling of the 3d and 4d transition metals, resulting in some of the largest known Rashba-like spin splittings. The core structural building blocks that facilitate the bandwidth-scaled inversion-symmetry breaking are common to numerous materials. Our findings therefore provide opportunities for creating spin-textured states and suggest routes to interfacial control of inversion-symmetry breaking in designer heterostructures of oxides and other material classes.