arXiv subjects

M. Valiska

Publications and source records attributed to M. Valiska.

10 recordsLinked to original sources

arXiv2025

Strange metallicity encompasses high magnetic field-induced superconductivity in UTe2

A strange metallic state - characterized by an electrical resistivity that rises linearly in temperature $T$ at a rate governed by Planckian energetic dissipation - is present in a number of unconventional superconductors. Despite widespread investigative efforts, the microscopic properties of strange metallicity continue to defy elementary description. Here we investigate heavy fermion UTe$_2$, which hosts a suite of exotic spin-triplet superconducting phases, the most extreme of which resides in a narrow angular window of intense magnetic fields $>$ 40 T. Through angle-dependent magnetotransport measurements in pulsed magnetic fields, we find that this superconductive phase emerges from a strange metallic state with Planckian $T$-linear resistivity confined to a small region of phase space where the field-induced superconductivity is strongest, suggesting a shared underlying mechanism. These findings reveal a novel setting for strange metallicity - in contrast to the general case of singlet superconductivity emerging on the border of antiferromagnetism, here strange metallicity accompanies field-induced triplet superconductivity within a spin-polarized magnetic state. This result demonstrates the remarkable ubiquity of strange metallicity across diverse materials settings, and highlights UTe$_2$ as a unique platform for exploring the interplay between unconventional superconductivity and quantum criticality.

cond-mat.str-el↗

arXiv2024

Pressure-enhanced $f$-electron orbital weighting in UTe2 mapped by quantum interferometry

The phase landscape of UTe$_2$ features a remarkable diversity of superconducting phases under applied pressure and magnetic field. Recent quantum oscillation studies at ambient pressure have revealed the quasi-2D Fermi surface of this material. However, the pressure-dependence of the Fermi surface remains an open question. Here we track the evolution of the UTe$_2$ Fermi surface as a function of pressure up to 19.5 kbar by measuring quantum interference oscillations. We find that in sufficient magnetic field to suppress both superconductivity at low pressures and incommensurate antiferromagnetism at higher pressures, the quasi-2D Fermi surface found at ambient pressure smoothly connects to that at 19.5 kbar, with no signs of a reconstruction over this pressure interval. We observe a smooth increase in oscillatory frequency with increasing pressure, indicating that the warping of the cylindrical Fermi sheets continuously increases with pressure. By computing a tight-binding model, we show that this enhanced warping indicates increased $f$-orbital contribution at the Fermi level - up to and beyond the critical pressure at which superconductivity is truncated. These findings highlight the value of high-pressure quantum interference measurements as a new probe of the electronic structure in heavy fermion materials.

cond-mat.supr-con↗

arXiv2024

A quantum critical line bounds the high field metamagnetic transition surface in UTe$_2$

Quantum critical phenomena are widely studied across various materials families, from high temperature superconductors to magnetic insulators. They occur when a thermodynamic phase transition is suppressed to zero temperature as a function of some tuning parameter such as pressure or magnetic field. This generally yields a point of instability - a so-called quantum critical point - at which the phase transition is driven exclusively by quantum fluctuations. Here we show that the heavy fermion metamagnet UTe$_2$ possesses a quantum phase transition at extreme magnetic field strengths of over 70 T. Rather than terminating at one singular point, we find that the phase boundary is sensitive to magnetic field components in each of the three Cartesian axes of magnetic field space. This results in the three-dimensional transition surface being bounded by a continuous ring of quantum critical points, the locus of which forms an extended line of quantum criticality - a novel form of quantum critical phase boundary. Within this quantum critical line sits a magnetic field-induced superconducting state in a toroidal shape, which persists to fields over 70~T. We model our data by a phenomenological free energy expansion, and show how a three-dimensional quantum critical phase boundary - rather than a more conventional singular point of instability - anchors the remarkable high magnetic field phase landscape of UTe$_2$.

cond-mat.supr-con↗

arXiv2023

Quantum interference between quasi-2D Fermi surface sheets in UTe2

UTe$_2$ is a spin-triplet superconductor candidate for which high quality samples with long mean free paths have recently become available, enabling quantum oscillation measurements to probe its Fermi surface and effective carrier masses. It has recently been reported that UTe$_2$ possesses a 3D Fermi surface component [Phys. Rev. Lett. 131, 036501 (2023)]. The distinction between 2D and 3D Fermi surface sections in triplet superconductors can have important implications regarding the topological properties of the superconductivity. Here we report the observation of oscillatory components in the magnetoconductance of UTe$_2$ at high magnetic fields. We find that these oscillations are well described by quantum interference between quasiparticles traversing semiclassical trajectories spanning magnetic breakdown networks. Our observations are consistent with a quasi-2D model of this material's Fermi surface based on prior dHvA-effect measurements. Our results strongly indicate that UTe$_2$ -- which exhibits a multitude of complex physical phenomena -- possesses a remarkably simple Fermi surface consisting exclusively of two quasi-2D cylindrical sections.

cond-mat.supr-con↗

arXiv2023

Enhanced triplet superconductivity in next generation ultraclean UTe2

The unconventional superconductor UTe$_2$ exhibits numerous signatures of spin-triplet superconductivity -- a rare state of matter which could enable quantum computation protected against decoherence. UTe$_2$ possesses a complex phase landscape comprising two magnetic field-induced superconducting phases, a metamagnetic transition to a field-polarised state, along with pair- and charge-density wave orders. However, contradictory reports between studies performed on UTe$_2$ specimens of varying quality have severely impeded theoretical efforts to understand the microscopic origins of the exotic superconductivity. Here, we report a comprehensive suite of high magnetic field measurements on a new generation of pristine quality UTe$_2$ crystals. Our experiments reveal a significantly revised high magnetic field superconducting phase diagram in the ultraclean limit, showing a pronounced sensitivity of field-induced superconductivity to the presence of crystalline disorder. We employ a Ginzburg-Landau model that excellently captures this acute dependence on sample quality. Our results suggest that in close proximity to a field--induced metamagnetic transition the enhanced role of magnetic fluctuations -- that are strongly suppressed by disorder -- is likely responsible for tuning UTe$_2$ between two distinct spin-triplet superconducting phases.

cond-mat.supr-con↗

arXiv2023

Quasi-2D Fermi surface in the anomalous superconductor UTe2

The heavy fermion paramagnet UTe$_2$ exhibits numerous characteristics of spin-triplet superconductivity. Efforts to understand the microscopic details of this exotic superconductivity have been impeded by uncertainty regarding the underlying electronic structure. Here we directly probe the Fermi surface of UTe$_2$ by measuring magnetic quantum oscillations in pristine quality crystals. We find an angular profile of quantum oscillatory frequency and amplitude that is characteristic of a quasi-2D Fermi surface, which we find is well described by two cylindrical Fermi sheets of electron- and hole-type respectively. Additionally, we find that both cylindrical Fermi sheets possess considerable undulation but negligible small-scale corrugation, which may allow for their near-nesting and therefore promote magnetic fluctuations that enhance the triplet pairing mechanism. Importantly, we find no evidence for the presence of any 3D Fermi surface sections. Our results place strong constraints on the possible symmetry of the superconducting order parameter in UTe$_2$.

cond-mat.supr-con↗

arXiv2020

Magnetic and electronic phases of U$_\mathbf{2}$Rh$_\mathbf{3}$Si$_\mathbf{5}$

We present a detailed study of the magnetic and electronic properties of U$_2$Rh$_3$Si$_5$, a material that has been demonstrated to exhibit a first order antiferromagnetic phase transition. From a high magnetic field study, together with extensive experiments in moderate fields, we establish the magnetic phase diagrams for all crystallographic directions. The possibility of an electronic phase in a narrow interval above the N\'eel temperature as a precursor of a magnetic phase is discussed.

cond-mat.str-el↗

arXiv2020

Comparison of two superconducting phases induced by a magnetic field in UTe2

Superconductivity induced by a magnetic field near metamagnetism is a striking manifestation of magnetically-mediated superconducting pairing. After being observed in itinerant ferromagnets, this phenomenon was recently reported in the orthorhombic paramagnet UTe$_2$. Under a magnetic field applied along the hard magnetization axis b, superconductivity is reinforced on approaching metamagnetism at $\mu_0H_m$ = 35 T, but it abruptly disappears beyond $H_m$. On the contrary, field-induced superconductivity was reported beyond $\mu_0H_m$ = 40-50 T in a magnetic field tilted by $\simeq25-30\deg$ from b in the (b,c) plane. Here we explore the phase diagram of UTe2 under these two magnetic-field directions. Zero-resistance measurements permit to confirm unambiguously that superconductivity is established beyond Hm in the tilted-field direction. While superconductivity is locked exactly at fields either smaller (for a H || b), or larger (for H tilted by $\simeq27\deg$ from b to c), than Hm, the variations of the Fermi-liquid coefficient in the electrical resistivity and of the residual resistivity are surprisingly similar for the two field directions. The resemblance of the normal states for the two field directions puts constraints for theoretical models of superconductivity and implies that some subtle ingredients must be in play.

cond-mat.supr-con↗

arXiv2019

Multiple superconducting phases in a nearly ferromagnetic system

Multiple superconducting order parameters are extremely rare. Here we show that a very small pressure can induce this phenomenon in the recently discovered heavy fermion superconductor UTe2. This nearly ferromagnetic system shows several intriguing phenomena, including an extraordinary reinforcement of superconductivity in very strong magnetic fields. We find that pressure can tune the system to a more correlated state and probable magnetic order. The superconducting critical temperature is strongly enhanced, reaching almost 3K, a new record for Ce- and U-based heavy fermion superconductors. Most spectacularly under pressure we find a transition within the superconducting state, putting UTe2 among the very rare systems having multiple superconducting phases. UTe2 under pressure is a treasure trove of several of the most fascinating phenomena in unconventional superconductivity and may well be a keystone in their understanding.

cond-mat.str-el↗

arXiv2016

Topological insulator homojunctions including magnetic layers: the example of n-p type ($n$-QLs Bi$_2$Se$_3$/Mn-Bi$_2$Se$_3$) heterostructures

Homojunctions between Bi$_2$Se$_3$ and its Mn-doped phase are investigated as a sample geometry to study the influence of spin degrees of freedom on topological insulator properties. $n$ quintuple layers (QLs) of Bi$_2$Se$_3$ are grown ontop of Mn-doped Bi$_2$Se$_3$ by molecular beam epitaxy for $0\le n \le 30\,$QLs, allowing to unhamperedly monitor the development of electronic and topological properties by surface sensitive techniques like angle resolved photoemission spectroscopy. With increasing $n$, a Mn-induced gap at the Dirac point is gradually filled in an "hourglass" fashion to reestablish a topological surface state at $n \sim 9\,$QLs. Our results suggest a competition of upwards and downwards band bending effects due to the presence of an n-p type interface, which can be used to tailor topological and quantum well states independently.

cond-mat.mtrl-sci↗