arXiv subjects

Yun Suk Eo

Publications and source records attributed to Yun Suk Eo.

29 records · Page 2Linked to original sources

arXiv2020

Weyl Superconductivity in UTe2

The search for a material platform for topological quantum computation has recently focused on unconventional superconductors. Such material systems, where the superconducting order parameter breaks a symmetry of the crystal point group, are capable of hosting novel phenomena, including emergent Majorana quasiparticles. Unique among unconventional superconductors is the recently discovered UTe2, where spin-triplet superconductivity emerges from a paramagnetic normal state. Although UTe2 could be considered a relative of a family of known ferromagnetic superconductors, the unique crystal structure of this material and experimentally suggested zero Curie temperature pose a great challenge to determining the symmetries, magnetism, and topology underlying the superconducting state. These emergent properties will determine the utility of UTe2 for future spintronics and quantum information applications. Here, we report observations of a non-zero polar Kerr effect and of two transitions in the specific heat upon entering the superconducting state, which together show that the superconductivity in UTe2 is characterized by an order parameter with two components that breaks time reversal symmetry. These data allow us to place firm constraints on the symmetries of the order parameter, which strongly suggest that UTe2 is a Weyl superconductor that hosts chiral Fermi arc surface states.

cond-mat.str-el↗

arXiv2019

Definitive Surface Magnetotransport Study of SmB$_{6}$

After the theoretical prediction that SmB$_6$ is a topological Kondo insulator, there has been an explosion of studies on the SmB$_6$ surface. However, there is not yet an agreement on even the most basic quantities such as the surface carrier density and mobility. In this paper, we carefully revisit Corbino disk magnetotransport studies to find those surface transport parameters. We first show that subsurface cracks exist in the SmB$_6$ crystals, arising both from surface preparation and during the crystal growth. We provide evidence that these hidden subsurface cracks are additional conduction channels, and the large disagreement between earlier surface SmB$_6$ studies may originate from previous interpretations not taking this extra conduction path into account. We provide an update of a more reliable magnetotransport data than the previous one (Phys. Rev. B 92, 115110) and find that the orders-of-magnitude large disagreements in carrier density and mobility come from the surface preparation and the transport geometry rather than the intrinsic sample quality. From this magnetotransport study, we find an updated estimate of the carrier density and mobility of 2.71$\times$10$^{13}$ (1/cm$^2$) and 104.5 (cm$^{2}$/V$\cdot$sec), respectively. We compare our results with other studies of the SmB$_6$ surface. By this comparison, we provide insight into the disagreements and agreements of the previously reported angle-resolved photoemission spectroscopy, scanning tunneling microscopy, and magnetotorque quantum oscillations measurements.

cond-mat.mes-hall↗

arXiv2019

Point Node Gap Structure of Spin-Triplet Superconductor UTe2

Low-temperature electrical and thermal transport, magnetic penetration depth, and heat capacity measurements were performed on single crystals of the actinide superconductor UTe2 to determine the structure of the superconducting energy gap. Heat transport measurements performed with currents directed along both crystallographic a- and b-axes reveal a vanishingly small residual fermionic component of the thermal conductivity. The magnetic field dependence of the residual term follows a rapid, quasi-linear increase consistent with the presence of nodal quasiparticles, rising toward the a-axis upper critical field where the Wiedemann-Franz law is recovered. Together with a quadratic temperature dependence of the magnetic penetration depth up to T/T_c=0.3, these measurements provide evidence for an unconventional spin-triplet superconducting order parameter with point nodes. Millikelvin specific heat measurements performed on the same crystals used for thermal transport reveal an upturn below 300 mK that is well described by a divergent quantum-critical contribution to the density of states (DOS). Modeling this contribution with a T^{-1/3} power law allows restoration of the full entropy balance in the superconducting state and a resultant cubic power law for the electronic DOS below T_c, consistent with the point-node gap structure determined by thermal conductivity and penetration depth measurements.

cond-mat.supr-con↗

arXiv2019

Enhanced spin triplet superconductivity due to Kondo destabilization

In a Kondo lattice system, suppression of effective Kondo coupling leads to the breakdown of the heavy-electron metal and a change in the electronic structure. Spin triplet superconductivity in the Kondo lattice UTe2 appears to be associated with spin fluctuations originating from incipient ferromagnetic order. Here we show clear evidence of twofold enhancement of spin-triplet superconductivity near the pressure-driven suppression of the Kondo coherence, implying that superconductivity is strengthened by the affiliated growth of both spin and charge fluctuations. The coherent Kondo state discontinuously transitions to ferromagnetic order at higher pressures. Application of magnetic field tunes the system back across a first-order phase boundary. Straddling this phase boundary, we find another example of reentrant superconductivity in UTe2. In addition to spin fluctuations associated with ferromagnetism, our results show that a Kondo-driven Fermi surface instability may be playing a role in stabilizing spin triplet superconductivity.

cond-mat.supr-con↗

arXiv2019

Investigation of high-temperature bulk transport characteristics and skew scattering in samarium hexaboride

A well-known feature in transport data of the topological Kondo insulator SmB$_6$ is the sign change in the Hall coefficient at 65 K. Carriers in SmB$_6$ are known to be negative, but above 65 K, the Hall sign suggests that the carriers are positive. Here, we extend Hall measurements up to 400 K and observe that the Hall coefficient changes back to the correct (negative) sign at about 305 K. We interpret the anomalous sign of the Hall coefficient in the context of skew scattering arising from the strong correlations between the $f$ and $d$ electrons. At energy scales where the gap is closed, the number of $d$ electrons in resonance with the $f$ electrons at the Fermi energy varies. When a large proportion of $d$ and $f$ electrons are in resonance, skew scattering is dominant, leading to the observation of the positive sign, but when fewer are in resonance, conventional scattering mechanisms dominate instead.

cond-mat.str-el↗

arXiv2019

Extreme magnetic field-boosted superconductivity

Applied magnetic fields underlie exotic quantum states, such as the fractional quantum Hall effect and Bose-Einstein condensation of spin excitations. Superconductivity, on the other hand, is inherently antagonistic towards magnetic fields. Only in rare cases can these effects be mitigated over limited fields, leading to reentrant superconductivity. Here, we report the unprecedented coexistence of multiple high-field reentrant superconducting phases in the spin-triplet superconductor UTe2. Strikingly, we observe superconductivity in the highest magnetic field range identified for any reentrant superconductor, beyond 65 T. These extreme properties reflect a new kind of exotic superconductivity rooted in magnetic fluctuations and boosted by a quantum dimensional crossover.

cond-mat.supr-con↗

arXiv2018

Robustness of the Insulating Bulk in the Topological Kondo Insulator SmB$_{6}$

We used the inverted resistance method to extend the bulk resistivity of SmB$_{6}$ to a regime where the surface conduction overwhelms the bulk. Remarkably, the bulk resistivity shows an intrinsic thermally activated behavior that changes ten orders of magnitude, suggesting that it is an ideal insulator that is immune to disorder. Non-stoichiometrically-grown SmB$_{6}$ samples also show an almost identical thermally activated behavior. At low temperatures, however, these samples show a mysterious high bulk resistivity plateau, which may arise from extended defect conduction in a 3D TI.

cond-mat.str-el↗

arXiv2017

Understanding low-temperature bulk transport in samarium hexaboride without relying on in-gap bulk states

We present a new model to explain the difference between the transport and spectroscopy gaps in samarium hexaboride (SmB$_6$), which has been a mystery for some time. We propose that SmB$_6$ can be modeled as an intrinsic semiconductor with a depletion length that diverges at cryogenic temperatures. In this model, we find a self-consistent solution to Poisson's equation in the bulk, with boundary conditions based on Fermi energy pinning due to surface charges. The solution yields band bending in the bulk; this explains the difference between the two gaps because spectroscopic methods measure the gap near the surface, while transport measures the average over the bulk. We also connect the model to transport parameters, including the Hall coefficient and thermopower, using semiclassical transport theory. The divergence of the depletion length additionally explains the 10-12 K feature in data for these parameters, demonstrating a crossover from bulk dominated transport above this temperature to surface-dominated transport below this temperature. We find good agreement between our model and a collection of transport data from 4-40 K. This model can also be generalized to materials with similar band structure.

cond-mat.str-el↗

arXiv2016

Evidence of electronic cloaking from chiral electron transport in bilayer graphene nanostructures

The coupling of charge carrier motion and pseudospin via chirality for massless Dirac fermions in monolayer graphene has generated dramatic consequences, such as the unusual quantum Hall effect and Klein tunneling. In bilayer graphene, charge carriers are massive Dirac fermions with a finite density of states at zero energy. Because of their non-relativistic nature, massive Dirac fermions can provide an even better test bed with which to clarify the importance of chirality in transport measurement than massless Dirac fermions in monolayer graphene. Here, we report an electronic cloaking effect as a manifestation of chirality by probing phase coherent transport in chemical-vapor-deposited bilayer graphene. Conductance oscillations with different periodicities were observed on extremely narrow bilayer graphene heterojunctions through electrostatic gating. Using a Fourier analysis technique, we identified the origin of each individual interference pattern. Importantly, the electron waves on the two sides of the potential barrier can be coupled through the evanescent waves inside the barrier, making the confined states underneath the barrier invisible to electrons. These findings provide direct evidence for the electronic cloaking effect and hold promise for the realization of pseudospintronics based on bilayer graphene.

cond-mat.mes-hall↗

arXiv2015

Magnetotransport Measurements of the Surface States of Samarium Hexaboride using Corbino Structures

The recent conjecture of a topologically-protected surface state in SmB$_6$ and the verification of robust surface conduction below 4 K have prompted a large effort to understand the surface states. Conventional Hall transport measurements allow current to flow on all surfaces of a topological insulator, so such measurements are influenced by contributions from multiple surfaces of varying transport character. Instead, we study magnetotransport of SmB$_6$ using a Corbino geometry, which can directly measure the conductivity of a single, independent surface. Both (011) and (001) crystal surfaces show a strong negative magnetoresistance at all magnetic field angles measured. The (011) surface has a carrier mobility of $122\text{ cm}^2/\text{V}\cdot\text{sec}$ with a carrier density of $2.5\times10^{13} \text{ cm}^{-2}$, which are significantly smaller than indicated by Hall transport studies. This mobility value can explain a failure so far to observe Shubnikov-de Haas oscillations. Analysis of the angle-dependence of conductivity on the (011) surface suggests a combination of a field-dependent enhancement of the carrier density and a suppression of Kondo scattering from native oxide layer magnetic moments as the likely origin of the negative magnetoresistance. Our results also reveal a hysteretic behavior whose magnitude depends on the magnetic field sweep rate and temperature. Although this feature becomes smaller when the field sweep is slower, does not disappear or saturate during our slowest sweep-rate measurements, which is much slower than a typical magnetotransport trace. These observations cannot be explained by quantum interference corrections such as weak anti-localization, but are more likely due to an extrinsic magnetic effect such as the magnetocaloric effect or glassy ordering.

cond-mat.str-el↗

arXiv2014

Influence of Helical Spin Structure on the Magnetoresistance of an Ideal Topological Insulator

In an ideal topological insulator, the helical spin structure of surface electrons suppresses backscattering and thus can enhance surface conductivity. We investigate the effect of perpendicular magnetic field on the spin structure of electrons at the Fermi energy and calculate a magnetic-field dependent topological enhancement factor for different disorder potentials, ranging from short-range disorder to screened Coulomb potential. Within the Boltzmann approximation, the topological enhancement factor reaches its maximum value of 4 for a short-range disorder at zero magnetic field and approaches a value of 1 at high magnetic fields independent of the nature of the disorder potential.

cond-mat.mes-hall↗