Colossal mid-infrared bulk photovoltaic effect in a type-I Weyl semimetal
Nature Materials 18; 471–475
04 March 2019
Broadband, efficient and fast conversion of light to electricity is crucial for sensing and clean energy. The bulk photovoltaic effect (BPVE) is a second-order nonlinear optical effect that intrinsically converts light into electrical current. Here, we demonstrate a large mid-infrared BPVE in microscopic devices of the Weyl semimetal TaAs. This discovery results from combining recent developments in Weyl semimetals, focused-ion beam fabrication and theoretical works suggesting a connection between BPVE and topology. We also present a detailed symmetry analysis that allows us to separate the shift current response from photothermal effects. The magnitude and wavelength range of the assigned shift current may impact optical detectors, clean energy and topology, and demonstrate the utility of Weyl semimetals for practical applications.
Thermodynamic Signatures of Weyl Fermions in NbP
Scientific Reports 9:2095
14 February 2019
We present a high magnetic field study of NbP—a member of the monopnictide Weyl semimetal (WSM) family. While the monoarsenides (NbAs and TaAs) have topologically distinct left and right-handed Weyl fermi surfaces, NbP is argued to be “topologically trivial” due to the fact that all pairs of Weyl nodes are encompassed by a single Fermi surface. We use torque magnetometry to measure the magnetic response of NbP up to 60 tesla and uncover a Berry paramagnetic response, characteristic of the topological Weyl nodes, across the entire field range. At the quantum limit B* (≈32 T), τ/B experiences a change in slope when the chemical potential enters the last Landau level. Our calculations confirm that this magnetic response arises from band topology of the Weyl pocket, even though the Fermi surface encompasses both Weyl nodes at zero magnetic field. We also find that the magnetic field pulls the chemical potential to the chiral n = 0 Landau level in the quantum limit, providing a disorder-free way of accessing chiral Weyl fermions in systems that are “not quite” WSMs in zero magnetic field.
Resonant torsion magnetometry in anisotropic quantum materials
Nature Communications 9:3975
Unusual behavior in quantum materials commonly arises from their effective low-dimensional physics, reflecting the underlying anisotropy in the spin and charge degrees of freedom. Here we introduce the magnetotropic coefficient k=∂2F/∂θ2, the second derivative of the free energy F with respect to the magnetic field orientation θ in the crystal. We show that the magnetotropic coefficient can be quantitatively determined from a shift in the resonant frequency of a commercially available atomic force microscopy cantilever under magnetic field. This detection method enables part per 100 million sensitivity and the ability to measure magnetic anisotropy in nanogram-scale samples, as demonstrated on the Weyl semimetal NbP. Measurement of the magnetotropic coefficient in the spin-liquid candidate RuCl3 highlights its sensitivity to anisotropic phase transitions and allows a quantitative comparison to other thermodynamic coefficients via the Ehrenfest relations.
Quantum limit transport and destruction of the Weyl nodes in TaAs
Nature Communications 9:2217
Weyl fermions are a recently discovered ingredient for correlated states of electronic matter. A key difficulty has been that real materials also contain non-Weyl quasiparticles, and disentangling the experimental signatures has proven challenging. Here we use magnetic fields up to 95 T to drive the Weyl semimetal TaAs far into its quantum limit, where only the purely chiral 0th Landau levels of the Weyl fermions are occupied. We find the electrical resistivity to be nearly independent of magnetic field up to 50 T: unusual for conventional metals but consistent with the chiral anomaly for Weyl fermions. Above 50 T we observe a two-order-of-magnitude increase in resistivity, indicating that a gap opens in the chiral Landau levels. Above 80 T we observe strong ultrasonic attenuation below 2 K, suggesting a mesoscopically textured state of matter. These results point the way to inducing new correlated states of matter in the quantum limit of Weyl semimetals.
Focused Ion Beam Microstructuring of Quantum Matter
Annual Review of Condensed Matter Physics 9:147-162
Focused ion beam (FIB) machining promises exciting new possibilities for the study of quantum materials through precise control over the shape and geometry of single crystals on the submicrometer scale. It offers viable routes to fabricate high-quality mesoscale structures from materials that cannot yet be grown in thin-film form and to enhance the experimentally accessible signatures of new physical phenomena. Prototype devices can also be produced in a silicon-chip environment to investigate directly the materials application potential for future electronics. This review introduces the concepts of ion beam shaping of matter, discusses the role and extent of surface damage and material disorder inherentto these techniques, and gives an overview of recent experiments on FIB-structured crystals. Given the early stage of the field of FIB-fabricated quantum materials, much is yet to come, and emergent trends and future directions are also discussed.
Quantum oscillations in the type-II Dirac semi-metal candidate PtSe2
New Journal of Physics 20:043008
12 April 2018
Three-dimensional topological semi-metals carry quasiparticle states that mimic massless relativistic Dirac fermions, elusive particles that have never been observed in nature. As they appear in the solid body, they are not bound to the usual symmetries of space-time and thus new types of fermionic excitations that explicitly violate Lorentz-invariance have been proposed, the so-called type-II Dirac fermions. We investigate the electronic spectrum of the transition-metal dichalcogenide PtSe2 by means of quantum oscillation measurements in fields up to 65 T. The observed Fermi surfaces agree well with the expectations from band structure calculations, that recently predicted a type-II Dirac node to occur in this material. A hole- and an electron-like Fermi surface dominate the semi-metal at the Fermi level. The quasiparticle mass is significantly enhanced over the bare band mass value, likely by phonon renormalization. Our work is consistent with the existence of type-II Dirac nodes in PtSe2, yet the Dirac node is too far below the Fermi level to support free Dirac–fermion excitations.
Observation of two-dimensional Fermi surface and Dirac dispersion in YbMnSb2
Physical Review B 97, 045109
10 January 2018
We present the crystal structure, electronic structure, and transport properties of the material YbMnSb2, a candidate system for the investigation of Dirac physics in the presence of magnetic order. Our measurements reveal that this system is a low-carrier-density semimetal with a two-dimensional Fermi surface arising from a Dirac dispersion, consistent with the predictions of density-functional-theory calculations of the antiferromagnetic system. The low temperature resistivity is very large, suggesting that scattering in this system is highly efficient at dissipating momentum despite its Dirac-like nature.
Electronic in-plane symmetry breaking at field-tuned quantum criticality in CeRhIn5
Nature 548, 313-317
7 August 2017
Electronic nematic materials are characterized by a lowered symmetry of the electronic system compared to the underlying lattice, in analogy to the directional alignment without translational order in nematic liquid crystals. Such nematic phases appear in the copper- and iron-based high-temperature superconductors and their role in establishing superconductivity remains an open question. Nematicity may take an active part, cooperating or competing with superconductivity, or may appear accidentally in such systems. Here we present experimental evidence for a phase of fluctuating nematic character in a heavy-fermion superconductor, CeRhIn5. We observe a magnetic-field-induced state in the vicinity of a field-tuned antiferromagnetic quantum critical point at Hc ≈ 50 tesla. This phase appears above an out-of-plane critical field H* ≈ 28 tesla and is characterized by a substantial in-plane resistivity anisotropy in the presence of a small in-plane field component. The in-plane symmetry breaking has little apparent connection to the underlying lattice, as evidenced by the small magnitude of the magnetostriction anomaly at H*. Furthermore, no anomalies appear in the magnetic torque, suggesting the absence of metamagnetism in this field range. The appearance of nematic behaviour in a prototypical heavy-fermion superconductor highlights the interrelation of nematicity and unconventional superconductivity, suggesting nematicity to be common among correlated materials.
Inducing superconductivity in Weyl semimetal microstructures by selective ion sputtering
Science Advances Vol. 3, no. 5, e1602983
24 May 2017
By introducing a superconducting gap in Weyl or Dirac semimetals, the superconducting state inherits the nontrivial topology of their electronic structure. As a result, Weyl superconductors are expected to host exotic phenomena, such as nonzero-momentum pairing due to their chiral node structure, or zero-energy Majorana modes at the surface. These are of fundamental interest to improve our understanding of correlated topological systems, and, moreover, practical applications in phase-coherent devices and quantum applications have been proposed. Proximity-induced superconductivity promises to allow these experiments on nonsuperconducting Weyl semimetals. We show a new route to reliably fabricate superconducting microstructures from the nonsuperconducting Weyl semimetal NbAs under ion irradiation. The significant difference in the surface binding energy of Nb and As leads to a natural enrichment of Nb at the surface during ion milling, forming a superconducting surface layer (Tc ~ 3.5 K). Being formed from the target crystal itself, the ideal contact between the superconductor and the bulk may enable an effective gapping of the Weyl nodes in the bulk because of the proximity effect. Simple ion irradiation may thus serve as a powerful tool for the fabrication of topological quantum devices from monoarsenides, even on an industrial scale.
Emergent magnetic anisotropy in the cubic heavy-fermion metal CeIn3
Nature Quantum Materials 2:46
21 August 2017
Metals containing cerium exhibit a diverse range of fascinating phenomena including heavy fermion behavior, quantum criticality, and novel states of matter such as unconventional superconductivity. The cubic system CeIn3 has attracted significant attention as a structurally isotropic Kondo lattice material possessing the minimum required complexity to still reveal this rich physics. By using magnetic fields with strengths comparable to the crystal field energy scale, we illustrate a strong field-induced anisotropy as a consequence of non-spherically symmetric spin interactions in the prototypical heavy fermion material CeIn3. This work demonstrates the importance of magnetic anisotropy in modeling f-electron materials when the orbital character of the 4f wavefunction changes (e.g., with pressure or composition). In addition, magnetic fields are shown to tune the effective hybridization and exchange interactions potentially leading to new exotic field tuned effects in f-based materials.
Single Crystal Growth, Structure, and Electronic Properties of Metallic Delafossite PdRhO2
Cryst. Growth Des. 17, 8, 4144-4150
30 June 2017
We report growth of single crystals of the nonmagnetic metallic delafossite PdRhO2. Complete crystallographic data were obtained by single crystal X-ray diffraction and used for the electronic structure calculations. Focused-ion beam microstructuring was used to prepare a sample for resistivity measurements revealing the large anisotropy consistent with the calculation results.
Thermodynamic anomaly above the superconducting critical temperature in the quasi-one-dimensional superconductor Ta4Pd3Te16
Physical Review B 95, 075121
13 February 2017
We study the intrinsic electronic anisotropy and fermiology of the quasi-one-dimensional superconductor Ta4Pd3Te16. Below T∗=20 K, we detect a thermodynamic phase transition that predominantly affects the conductivity perpendicular to the quasi-one-dimensional chains. The transition relates to the presence of charge order that precedes superconductivity. Remarkably, the Fermi surface pockets detected by de Haas–van Alphen oscillations are unaffected by this transition, suggesting that the ordered state does not break any translational symmetries but rather alters the scattering of the quasiparticles themselves.
Transport evidence for Fermi-arc-mediated chirality transfer in the Dirac semi-metal Cd3As2
Nature 535, p.266–270
14 July 2016
The dispersion of charge carriers in a metal is distinctly different from that of free electrons owing to their interactions with the crystal lattice. These interactions may lead to quasiparticles mimicking the massless relativistic dynamics of high-energy particle physics and they can twist the quantum phase of electrons into topologically non-trivial knots—producing protected surface states with anomalous electromagnetic properties. These effects intertwine in materials known as Weyl semimetals, and in their crystal-symmetry-protected analogues, Dirac semimetals. The latter show a linear electronic dispersion in three dimensions described by two copies of the Weyl equation (a theoretical description of massless relativistic fermions). At the surface of a crystal, the broken translational symmetry creates topological surface states, so-called Fermi arcs, which have no counterparts in high-energy physics or conventional condensed matter systems. Here we present Shubnikov–de Haas oscillations in focused-ion-beam-prepared microstructures of Cd3As2 that are consistent with the theoretically predicted ‘Weyl orbits’, a kind of cyclotron motion that weaves together Fermi-arc and chiral bulk states. In contrast to conventional cyclotron orbits, this motion is driven by the transfer of chirality from one Weyl node to another, rather than momentum transfer of the Lorentz force. Our observations provide evidence for direct access to the topological properties of charge in a transport experiment, a first step towards their potential application.
Magnetic torque anomaly in the quantum limit of Weyl semi-metals
Nature Communications 7:12492
22 August 2016
Electrons in materials with linear dispersion behave as massless Weyl- or Dirac-quasiparticles, and continue to intrigue due to their close resemblance to elusive ultra-relativistic particles as well as their potential for future electronics. Yet the experimental signatures of Weyl-fermions are often subtle and indirect, in particular if they coexist with conventional, massive quasiparticles. Here we show a pronounced anomaly in the magnetic torque of the Weyl semimetal NbAs upon entering the quantum limit state in high magnetic fields. The torque changes sign in the quantum limit, signalling a reversal of the magnetic anisotropy that can be directly attributed to the topological nature of the Weyl electrons. Our results establish that anomalous quantum limit torque measurements provide a direct experimental method to identify and distinguish Weyl and Dirac systems.
Evidence for hydrodynamic electron flow in PdCoO2
Science Vol.351, Issue 6277, p.1061-1064
4 March 2016
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.
Scaling between magnetic field and temperature in the high temperature superconductor BaFe2(As1-xPx)2
Nature Physics 12, p.916–919
16 May 2016
Many exotic metallic systems have a resistivity that varies linearly with temperature, and the physics behind this is thought to be connected to high-temperature superconductivity in the cuprates and iron pnictides. Although this phenomenon has attracted considerable attention, it is unclear how the relevant physics manifests in other transport properties, for example their response to an applied magnetic field. We report measurements of the high-field magnetoresistance of the iron pnictide superconductor BaFe2(As1-xPx)2 and find that it obeys an unusual scaling relationship between applied magnetic field and temperature, with a conversion factor given simply by the ratio of the Bohr magneton and the Boltzmann constant. This suggests that magnetic fields probe the same physics that gives rise to the T-linear resistivity, providing a new experimental clue to this long-standing puzzle.
Field-induced density wave in the heavy-fermion compound CeRhIn5
Nature Communications 6:6663
23 March 2015
Strong electron correlations lead to a variety of distinct ground states, such as magnetism, charge order or superconductivity. Understanding the competitive or cooperative interplay between neighbouring phases is an outstanding challenge in physics. CeRhIn5 is a prototypical example of a heavy-fermion superconductor: it orders anti-ferromagnetically below 3.8 K, and moderate hydrostatic pressure suppresses the anti-ferromagnetic order inducing unconventional superconductivity. Here we show evidence for a phase transition to a state akin to a density wave (DW) under high magnetic fields (>27 T) in high-quality single crystal microstructures of CeRhIn5. The DW is signalled by a hysteretic anomaly in the in-plane resistivity accompanied by non-linear electrical transport, yet remarkably thermodynamic measurements suggest that the phase transition involves only small portions of the Fermi surface. Such a subtle order might be a common feature among correlated electron systems, reminiscent of the similarly subtle charge DW state in the cuprates.
Nearly free electrons in a 5d delafossite oxide metal
Science Advances Vol.1, no.9, e1500692
23 October 2015
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.
Critical current oscillations in the intrinsic hybrid vortex state of SmFeAs(O,F)
Phys. Rev. Lett. 113, 186402
30 October 2014
In layered superconductors the order parameter may be modulated within the unit cell, leading to nontrivial modifications of the vortex core if the interlayer coherence length ξc(T) is comparable to the interlayer spacing. In the iron pnictide SmFeAs(O,F) (Tc≈50 K) this occurs below a crossover temperature T⋆≈41 K, which separates two regimes of vortices: anisotropic Abrikosov-like at high and Josephson-like at low temperatures. Yet in the transition region around T⋆, hybrid vortices between these two characteristics appear. Only in this region around T⋆ and for magnetic fields well aligned with the FeAs layers, we observe oscillations of the c-axis critical current jc(H) periodic in 1/√H due to a delicate balance of intervortex forces and interaction with the layered potential. jc(H) shows pronounced maxima when a hexagonal vortex lattice is commensurate with the underlying crystal structure. The narrow temperature window in which oscillations are observed suggests a significant suppression of the order parameter between the superconducting layers in SmFeAs(O,F), despite its low coherence length anisotropy (γξ≈3–5).
Intrinsic Josephson junctions in the iron-based multi-band superconductor (V2Sr4O6)Fe2As2
Nature Physics vol.10, p.644–647
27 July 2014
In layered superconductors, Josephson junctions may be formed within the unit cell as a result of sufficiently low inter-layer coupling. These intrinsic Josephson junction (iJJ) systems have attracted considerable interest for their application potential in quantum computing as well as efficient sources of THz radiation, closing the famous ‘THz gap’. So far, iJJ have been demonstrated in single-band, copper-based high-Tc superconductors, mainly in Bi–Sr–Ca–Cu–O. Here we report clear experimental evidence for iJJ behaviour in the iron-based superconductor V2Sr4O6)Fe2As2. The intrinsic junctions are identified by periodic oscillations of the flux-flow voltage on increasing a well-aligned in-plane magnetic field. The periodicity is explained by commensurability effects between the Josephson vortex lattice and the crystal structure, which is a hallmark signature of Josephson vortices confined into iJJ stacks. This finding adds the pnictide V2Sr4O6)Fe2As2 to the copper-based iJJ materials of interest for Josephson junction applications. In particular, novel devices based on multi-band Josephson coupling may be realized.
Transition from slow Abrikosov to fast moving Josephson vortices in iron-pnictide superconductors
Nature Materials 12, p.134–138
Iron pnictides are layered high Tc superconductors with moderate material anisotropy and thus Abrikosov vortices are expected in the mixed state. Yet, we have discovered a distinct change in the nature of the vortices from Abrikosov-like to Josephson-like in the pnictide superconductor SmFeAs(O,F) with Tc~48–50 K on cooling below a temperature T*~41–42 K, despite its moderate electronic anisotropy γ~4–6. This transition is hallmarked by a sharp drop in the critical current and accordingly a jump in the flux-flow voltage in a magnetic field precisely aligned along the FeAs layers, indicative of highly mobile vortices. T* coincides well with the temperature where the coherence length ξc perpendicular to the layers matches half of the FeAs-layer spacing. For fields slightly out-of-plane (> 0.1°– 0.15°) the vortices are completely immobilized as well-pinned Abrikosov segments are introduced when the vortex crosses the FeAs layers. We interpret these findings as a transition from well-pinned, slow moving Abrikosov vortices at high temperatures to weakly pinned, fast flowing Josephson vortices at low temperatures. This vortex dynamics could become technologically relevant as superconducting applications will always operate deep in the Josephson regime.
Magnetic field-tuned localization of the 5f-electrons in URu2Si2
Phys. Rev. B v88, 241108(R)
20 December 2013
We report Shubnikov-de Haas oscillation measurements within the high magnetic field (μ0H>39 T) magnetically polarized regime of URu2Si2, made possible using mesoscopic samples prepared by means of focused ion beam lithography. A significant change in the Fermi surface topology relative to the “hidden-order” phase is observed, signaling a transformation into a high magnetic field regime in which 5f-electrons are removed from the Fermi surface. URu2Si2 is therefore a rare example of an actinide compound in which a transformation of 5f-electrons can be directly observed at low temperatures, setting the stage for the unconventional ordering and high magnetic field quantum criticality in this material.
Single crystal study of the heavy-fermion antiferromagnet CePt2In7
Journal of Physics: Condensed Matter Vol.24, Nb.1
We report the synthesis, structure, and physical properties of single crystals of CePt2In7. Single crystal x-ray diffraction analysis confirms the tetragonal I4/mmm structure of CePt2In7 with unit cell parameters a = 4.5886(6) Å, c = 21.530(6) Å and V = 453.32(14) Å3. The magnetic susceptibility, heat capacity, Hall effect and electrical resistivity measurements are all consistent with CePt2In7 undergoing an antiferromagnetic order transition at TN = 5.5 K, which is field independent up to 9 T. Above TN, the Sommerfeld coefficient of specific heat is γ ≈ 300 mJ mol−1 K−2, which is characteristic of an enhanced effective mass of itinerant charge carriers. The electrical resistivity is typical of heavy-fermion behavior and gives a residual resistivity ρ0 ~ 0.2 µΩ cm, indicating good crystal quality. CePt2In7 also shows moderate anisotropy of the physical properties that is comparable to structurally related CeMIn5 (M = Co, Rh, Ir) heavy-fermion superconductors.
High pressure flux growth, structural, and superconducting properties of LnFeAsO (Ln = Pr, Nd, Sm) single crystals
Phys. Rev. B 86, 214509
17 December 2012
Single crystals of the LnFeAsO (Ln1111, Ln = Pr, Nd, and Sm) family with lateral dimensions up to 1 mm were grown from NaAs and KAs flux at high pressure. The crystals are of good structural quality and become superconducting when O is partially substituted by F (PrFeAsO1−xFx and NdFeAsO1−xFx) or when Fe is substituted by Co (SmFe1−xCoxAsO). From magnetization measurements, we estimate the temperature dependence and anisotropy of the upper critical field and the critical current density of underdoped PrFeAsO0.7F0.3 crystal with Tc ≈ 25 K. Single crystals of SmFe1−xCoxAsO with maximal Tc up to 16.3 K for x ≈ 0.08 were grown. From transport and magnetic measurements, we estimate the critical fields and their anisotropy and find these superconducting properties to be quite comparable to the ones in SmFeAsO1−xFx with a much higher Tc ≈ 50 K. The magnetically measured critical current densities are as high as 109 A/m2 at 2 K up to 7 T, with indication of the usual fishtail effect. The upper critical field estimated from resistivity measurements is anisotropic with slopes of ∼−8.7 T/K (H||ab plane) and ∼−1.7 T/K (H||c axis). This anisotropy (∼5) is similar to that in other Ln1111 crystals with various higher Tc‘s.
Tetrathiofulvalene and tetracyanoquinodimethane crystals: Conducting surface versus interface
Applied Physics Letters 101, 023302
10 July 2012
When a tetrathiofulvalene (TTF) crystal is placed onto a 7,7,8,8‐tetracyanoquinodimethane (TCNQ) crystal at room temperature, a highly conducting layer is formed. In this study, we explore to what degree this is due to physical contact or transfer by sublimation of one species onto the other crystal. We have performed a variety of time‐dependent surface conductivity measurements, including TTF lamination on TCNQ at room temperature and low temperatures, as well as deposition of TTF molecules from the gas phase. Crystal-to-crystal contact insignificantly modifies material conductivity while TTF sublimation onto TCNQ is shown to dominate electronic modification.