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John Singleton

Publications and source records attributed to John Singleton.

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

Competing electronic ground states in the heavy-fermion superconductor CeRh2As2

CeRh2As2 is rare among superconductors, in that magnetic field tunes it between two distinct superconducting phases. Combined with a lack of local inversion symmetry and an upper critical field exceeding the Pauli paramagnetic limit, this excitingly suggests triplet multicomponent superconductivity. Preceding the superconducting onset, f-electron correlations cause long-range order, attributed both to local antiferromagnetism and itinerant (quadrupole) density-waves. Magnetic field provides a significant perturbation of the f-electron and may reveal the nature of the many-body correlations. Thus, we report comprehensive magnetization and magnetotransport studies on microstructured devices in fields of up to 73 T. Applied along the c-axis, field causes a low-temperature valence transition at μ0H ~ 24 T. By contrast, in-plane fields produce a cascade of phase transitions; the field-induced in-plane conductivity anisotropy and lack of accompanying magnetic features, plus the closed-dome nature of the overall phase boundary is consistent with a hierarchy of field-induced density-wave states.

cond-mat.str-el↗
arXiv2024

Unconventional Magnetic Oscillations in Kagome Mott Insulators

In metals, electrons in a magnetic field undergo cyclotron motion, leading to oscillations in physical properties called quantum oscillations. This phenomenon has never been seen in a robust insulator because there are no mobile electrons. We report the first exception to this rule. We study a Mott insulator on a kagome lattice which does not order magnetically down to milli-Kelvin temperatures despite antiferromagnetic interactions. We observe a plateau at magnetization equal to 1/9 Bohr magneton per magnetic ion, accompanied by oscillations in the magnetic torque, reminiscent of quantum oscillations in metals. The temperature dependence obeys Fermi distribution. These phenomena are consistent with a quantum spin liquid state whose excitations are fermionic spinons with a Dirac-like spectrum coupled to an emergent gauge field.

cond-mat.str-el↗
arXiv2024

Thermodynamic evidence of fermionic behavior in the vicinity of one-ninth plateau in a kagome antiferromagnet

The spin-1/2 kagome Heisenberg antiferromagnets are believed to host exotic quantum entangled states. Recently, the report of 1/9 magnetization plateau and magnetic oscillations in a kagome antiferromagnet YCu$_3$(OH)$_6$Br$_2$[Br$_x$(OH)$_{1-x}$] (YCOB) have made this material a promising candidate for experimentally realizing quantum spin liquid states. Here we present measurements of the specific heat $C_p$ in YCOB in high magnetic fields (up to 41.5 Tesla) down to 0.46 Kelvin, and the 1/9 plateau feature has been confirmed. Moreover, the temperature dependence of $C_p/T$ in the vicinity of 1/9 plateau region can be fitted by a linear in $T$ term which indicates the presence of a Dirac spectrum, together with a constant term, which indicates a finite density of states (DOS) contributed by other Fermi surfaces. Surprisingly the constant term is highly anisotropic in the direction of the magnetic field. Additionally, we observe a double-peak feature near $30$~T above the 1/9 plateau which is another hallmark of fermionic excitations in the specific heat.

cond-mat.str-el↗
arXiv2024

Large magnetoresistance and first-order phase transition in antiferromagnetic single-crystalline EuAg$_4$Sb$_2$

We present the results of a thorough investigation of the physical properties of EuAg$_4$Sb$_2$ single crystals using magnetization, heat capacity, and electrical resistivity measurements. High-quality single crystals, which crystallize in a trigonal structure with space group $R\bar{3}m$, were grown using a conventional flux method. Temperature-dependent magnetization measurements along different crystallographic orientations confirm two antiferromagnetic phase transitions around $T_{N1}$ = 10.5 K and $T_{N2}$ = 7.5 K. Isothermal magnetization data exhibit several metamagnetic transitions below these transition temperatures. Antiferromagnetic phase transitions in EuAg$_4$Sb$_2$ are further confirmed by two sharp peaks in the temperature-dependent heat capacity data at $T_{N1}$ and $T_{N2}$, which shift to the lower temperature in the presence of an external magnetic field. Our systematic heat capacity measurements utilizing a long-pulse and single-slope analysis technique allow us to detect a first-order phase transition in EuAg$_4$Sb$_2$ at 7.5 K. The temperature-dependent electrical resistivity data also manifest two features associated with magnetic order. The magnetoresistance exhibits a broad hump due to the field-induced metamagnetic transition. Remarkably, the magnetoresistance keeps increasing without showing any tendency to saturate as the applied magnetic field increases, and it reaches $\sim$20000\% at 1.6 K and 60 T. At high magnetic fields, several magnetic quantum oscillations are observed, indicating a complex Fermi surface. A large negative magnetoresistance of about -55\% is also observed near $T_{N1}$. Moreover, the $H$-$T$ phase diagram constructed using magnetization, heat capacity, and magnetotransport data indicates complex magnetic behavior in EuAg$_4$Sb$_2$.

cond-mat.str-el↗
arXiv2024

Shubnikov-de Haas oscillations of biaxial-strain-tuned superconductors in pulsed magnetic field up to 60 T

Two-dimensional (2D) materials have gained increasing prominence not only in fundamental research but also in daily applications. However, to fully harness their potential, it is crucial to optimize their properties with an external parameter and track the electronic structure simultaneously. Magnetotransport over a wide magnetic field range is a powerful method to probe the electronic structure and, for metallic 2D materials, quantum oscillations superimposed on the transport signals encode Fermi surface parameters. In this manuscript, we utilize biaxial strain as an external tuning parameter and investigate the effects of strain on the electronic properties of two quasi-2D superconductors, MoTe$_2$ and RbV$_3$Sb$_5$, by measuring their magnetoresistance in pulsed magnetic fields up to 60 T. With a careful selection of insulating substrates, we demonstrate the possibility of both the compressive and tensile biaxial strain, imposed on MoTe$_2$ and RbV$_3$Sb$_5$, respectively. For both systems, the applied strain has led to superconducting critical temperature enhancement compared to their free-standing counterparts, proving the effectiveness of this biaxial strain method at cryogenic temperatures. Clear quantum oscillations in the magnetoresistance -- the Shubnikov-de Haas (SdH) effect -- are obtained in both samples. In strained MoTe$_2$, the magnetoresistance exhibits a nearly quadratic dependence on the magnetic field and remains non-saturating even at the highest field. Whereas in strained RbV$_3$Sb$_5$, two SdH frequencies showed a substantial enhancement in effective mass values, hinting at a possible enhancement of charge fluctuations. Our results demonstrate that combining biaxial strain and pulsed magnetic field paves the way for studying 2D materials under unprecedented conditions.

cond-mat.supr-con↗
arXiv2024

Coexistence of Superconductivity and Antiferromagnetism in Topological Magnet MnBi2Te4 Films

The interface of two materials can harbor unexpected emergent phenomena. One example is interface-induced superconductivity. In this work, we employ molecular beam epitaxy to grow a series of heterostructures formed by stacking together two non-superconducting antiferromagnetic materials, an intrinsic antiferromagnetic topological insulator MnBi2Te4 and an antiferromagnetic iron chalcogenide FeTe. Our electrical transport measurements reveal interface-induced superconductivity in these heterostructures. By performing scanning tunneling microscopy and spectroscopy measurements, we observe a proximity-induced superconducting gap on the top surface of the MnBi2Te4 layer, confirming the interaction between superconductivity and antiferromagnetism in the MnBi2Te4 layer. Our findings will advance the fundamental inquiries into the topological superconducting phase in hybrid devices and provide a promising platform for the exploration of chiral Majorana physics in MnBi2Te4-based heterostructures.

cond-mat.supr-con↗
arXiv2023

Direct evidence from high-field magnetotransport for a dramatic change of quasiparticle character in van der Waals ferromagnet Fe$_{3-x}$GeTe$_2$

Magnetometry and magnetoresistance (MR) data taken on the van der Waals ferromagnet Fe$_{3-x}$GeTe$_2$ (FGT) reveal three distinct contributions to the MR: a linear negative component, a contribution from closed Fermi-surface orbits, and a $H^2$ enhancement linked to a non-coplanar spin arrangement. Contrary to earlier studies on FGT, by accounting for the field dependence of the anomalous Hall effect, we find that the ordinary Hall coefficient decreases markedly below 80 K, indicating a significant change in character of the electrons and holes on the Fermi surface at this temperature. The resulting altered ground state eventually causes the Hall coefficient to reverse sign at 35 K. Our Hall data support the proposal that Kondo-lattice behavior develops in this $d$-electron material below 80 K. Additional evidence comes from the negative linear component of the MR, which arises from electron-magnon scattering with an atypical temperature dependence attributable to the onset of Kondo screening.

cond-mat.str-el↗
arXiv2023

Interface-Induced Superconductivity in Magnetic Topological Insulator-Iron Chalcogenide Heterostructures

When two different electronic materials are brought together, the resultant interface often shows unexpected quantum phenomena, including interfacial superconductivity and Fu-Kane topological superconductivity (TSC). Here, we use molecular beam epitaxy (MBE) to synthesize heterostructures formed by stacking together two magnetic materials, a ferromagnetic topological insulator (TI) and an antiferromagnetic iron chalcogenide (FeTe). We discover emergent interface-induced superconductivity in these heterostructures and demonstrate the trifecta occurrence of superconductivity, ferromagnetism, and topological band structure in the magnetic TI layer, the three essential ingredients of chiral TSC. The unusual coexistence of ferromagnetism and superconductivity can be attributed to the high upper critical magnetic field that exceeds the Pauli paramagnetic limit for conventional superconductors at low temperatures. The magnetic TI/FeTe heterostructures with robust superconductivity and atomically sharp interfaces provide an ideal wafer-scale platform for the exploration of chiral TSC and Majorana physics, constituting an important step toward scalable topological quantum computation.

cond-mat.mes-hall↗
arXiv2023

Dirac-Fermion-Assisted Interfacial Superconductivity in Epitaxial Topological Insulator/Iron Chalcogenide Heterostructures

Over the last decade, the possibility of realizing topological superconductivity (TSC) has generated much excitement, mainly due to the potential use of its excitations (Majorana zero modes) in a fault-tolerant topological quantum computer 1,2. TSC can be created in electronic systems where the topological and superconducting orders coexist3, motivating the continued exploration of candidate material platforms to this end. Here, we use molecular beam epitaxy (MBE) to synthesize heterostructures that host emergent interfacial superconductivity when a non-superconducting antiferromagnet (FeTe) is interfaced with a topological insulator (TI) (Bi, Sb)2Te3 wherein the chemical potential can be tuned through varying the Bi/Sb ratio. By performing in-vacuo angle-resolved photoemission spectroscopy (ARPES) and ex-situ electrical transport measurements, we find that the superconducting transition temperature and the upper critical magnetic field are suppressed when the chemical potential approaches the Dirac point. This observation implies a direct correlation between the interfacial superconductivity and Dirac electrons of the TI layer. We provide evidence to show that the observed interfacial superconductivity and its chemical potential dependence is the result of the competition between the Ruderman-Kittel-Kasuya-Yosida-type ferromagnetic coupling mediated by Dirac surface states and antiferromagnetic exchange couplings that generate the bicollinear antiferromagnetic order in the FeTe layer. The Dirac-fermion-assisted interfacial superconductivity in (Bi,Sb)2Te3/FeTe heterostructures provides a new approach to probe TSC and Majorana physics in hybrid devices and potentially constitutes an alternative platform for topological quantum computation.

cond-mat.mes-hall↗
arXiv2023

Revealing a 3D Fermi Surface Pocket and Electron-Hole Tunneling in UTe$_{2}$ with Quantum Oscillations

Spin triplet superconductor UTe$_{2}$ is widely believed to host a quasi-two-dimensional Fermi surface, revealed by first principal calculations, photoemission and quantum oscillation measurements. An outstanding question still remains as to the existence of a three-dimensional Fermi surface pocket, which is crucial for our understanding of the exotic superconducting and topological properties of UTe$_{2}$. This 3D Fermi surface pocket appears in various theoretical models with different physics origins but has not been detected experimentally. Here for the first time, we provide concrete evidence for a relatively isotropic, small Fermi surface pocket of UTe$_{2}$ via quantum oscillation measurements. In addition, we observed high frequency quantum oscillations corresponding to electron-hole tunneling between adjacent electron and hole pockets. The coexistence of 2D and 3D Fermi surface pockets, as well as the breakdown orbits, provides a test bed for theoretical models and aid the realization of a unified understanding of superconducting state of UTe$_{2}$ from the first-principles approach.

cond-mat.str-el↗
arXiv2023

Sudden adiabaticity entering field-induced state in UTe2

There has been a recent surge of interest in UTe$_2$ due to its unconventional magnetic field (H) reinforced spin-triplet superconducting phases persisting at fields far above the simple Pauli limit for H $\parallel$ [010]. Magnetic fields in excess of 35 T then induce a field-polarized magnetic state via a first-order-like phase transition. More controversially, for field orientations close to H $\parallel$ [011] and above 40 T, electrical resistivity measurements suggest that a further superconducting state may exist. However, no Meissner effect or thermodynamic evidence exists to date for this phase making it difficult to exclude a simple low-resistance metallic state. In this paper, we describe a study using thermal, electrical, and magnetic probes in magnetic fields of up to 55 T applied between the [010] ($b$) and [001] ($c$) directions. Our MHz conductivity data reveal the field-induced state of low or vanishing electrical resistance; simultaneous magnetocaloric effect measurements (i.e. changes in sample temperature due to changing magnetic field), show the first definitive evidence for adiabaticity and thermal behavior characteristic of bulk field-induced superconductivity.

cond-mat.supr-con↗
arXiv2023

Fermi Surface Evolution and Anomalous Hall Effect in an Ideal Type-II Weyl Semimetal

Weyl semimetals (WSMs) are three-dimensional topological materials that exhibit fascinating properties due to the presence of Weyl nodes in their band structure. However, existing WSMs discovered so far often possess multiple pairs of Weyl nodes, posing a challenge in disentangling the contributions to transport phenomena from different energy bands. To overcome this challenge, we have identified field-induced ferromagnetic MnBi$_{2-x}$Sb$_{x}$Te$_{4}$ as an ideal type-II WSM with a single pair of Weyl nodes. By employing a combination of quantum oscillations and high-field Hall measurements, we have resolved the evolution of Fermi-surface sections as the Fermi level is tuned across the charge neutrality point, precisely matching the band structure of an ideal type-II WSM. Furthermore, the anomalous Hall conductivity exhibits a heartbeat-like behavior as the Fermi level is tuned across the Weyl nodes, a unique feature previously predicted for a type-II WSM. Our findings establish MnBi$_{2-x}$Sb$_{x}$Te$_{4}$ as an ideal platform for further investigation into Weyl physics.

cond-mat.mes-hall↗
arXiv2023

Orphan High Field Superconductivity in Non-Superconducting Uranium Ditelluride

Reentrant superconductivity is a phenomenon in which the destructive effects of magnetic field on superconductivity are mitigated, allowing a zero-resistance state to survive under conditions that would otherwise destroy it. Typically, the reentrant superconducting region derives from a zero-field parent superconductor. Here, we show that in specifically-prepared UTe$_2$ crystals, extremely large magnetic field gives rise to an unprecedented high field superconductor that lacks a zero-field parent phase. This orphan superconductivity exists at fields between 37 T and 52 T, over a smaller angular range than observed in superconducting UTe$_2$. The stability of field-induced orphan superconductivity is a challenge to existing theoretical explanations, and underscores the likelihood of a field-induced modification of the electronic structure of UTe$_2$.

cond-mat.supr-con↗
arXiv2023

Quantum-limit phenomena and bandstructure in the magnetic topological semimetal EuZn2As2

We have experimentally investigated the low-temperature (0.6 K) electronic and magnetic properties of the layered antiferromagnet EuZn2As2 in pulsed magnetic fields of up to 60 T at a temperature of 0.6 K, giant positive magnetoresistance (MR) is observed above μ_{0}H ~ 20 T, a regime in which the spins are already fully polarized. Both magnetic torque and proximity detector oscillator (PDO) data show no corresponding anomaly at or close to this field. By analyzing the quantum oscillations observed in the MR and PDO frequency, we find that (1) the oscillation frequency F = 46 \pm 6 T for H // c and 42 \pm 2 T for H // ab; (2) the corresponding Berry phase is close to πfor H // c, implying a nontrivial topology; and (3) the large linear MR at high fields corresponds to the quantum limit (i.e., only the last Landau level being occupied). From these observations we conclude that the linear MR can be understood by considering diffusing cyclotron centers in the quantum limit. Our findings help understand the intimate relationship between magnetism and electronic topology in EuZn2As2 under extremely high fields and suggest reasons for the emergent behavior in the quantum limit.

cond-mat.mtrl-sci↗
arXiv2023

Giant spin-valve effect and chiral anomaly in antiferromagnetic topological insulators Mn(Bi1-xSbx)2Te4

We report c-axis transport studies on magnetic topological insulators Mn(Bi1-xSbx)2Te4. We performed systematic c-axis magnetoresistivity measurements under high magnetic fields (up to 35 T) on several representative samples. We find the lightly hole- and lightly electron-doped samples, while both having the same order of magnitude of carrier density and similar spin-flop transitions, exhibit sharp contrast in electronic anisotropy and transport mechanism. The electronic anisotropy is remarkably enhanced for the lightly hole-doped sample relative to pristine MnBi2Te4 but not for the lightly electron-doped sample. The lightly electron-doped sample displays a giant negative longitudinal magnetoresistivity (LMR) induced by the spin-valve effect at the spin-flop transition field, whereas the lightly hole-doped sample exhibits remarkable negative LMR consistent with the chiral anomaly behavior of a Weyl semimetal. Furthermore, we find the large negative LMR of the lightly hole-doped sample extends to a wide temperature range above the Néel temperature (T_N) where the magnetoconductivity is proportional to B^2. This fact, together with the short-range intralayer ferromagnetic correlation revealed in isothermal magnetization measurements, suggests the possible presence of the Weyl state above T_N. These results demonstrate that in the c-axis magnetotransport of Mn(Bi1-xSbx)2Te4, the spin scattering is dominant in the lightly electron-doped sample but overwhelmed by the chiral anomaly effect in the lightly hole-doped sample due to the presence of the Weyl state. These findings extend the understanding of the transport properties of Mn(Bi1-xSbx)2Te4.

cond-mat.mtrl-sci↗
arXiv2023

Hole doping in compositionally complex correlated oxide enables tunable exchange biasing

Magnetic interfaces and the phenomena arising from them drive both the design of modern spintronics and fundamental research. Recently, it was revealed that through designing magnetic frustration in configurationally complex entropy stabilized oxides, exchange bias can occur in structurally single crystal films. This eliminates the need for complex heterostructures and nanocomposites in the design and control of magnetic response phenomena. In this work, we demonstrate through hole doping of a high entropy perovskite oxide that tuning of magnetic responses can be achieved. With detailed magnetometry, we show magnetic coupling exhibiting a variety of magnetic responses including exchange bias and antiferromagnetic spin reversal in the entropy stabilized ABO3 perovskite oxide La1-xSrx(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 family. We find that manipulation of the A-site charge state can be used to balance magnetic phase compositions and coupling responses. This allows for the creation of highly tunable exchange bias responses. In the low Sr doping regime, a spin frustrated region arising at the antiferromagnetic phase boundary is shown to directly couple to the antiferromagnetic moments of the film and emerges as the dominant mechanism, leading to a vertical shift of magnetization loops in response to field biasing. At higher concentrations, direct coupling of antiferromagnetic and ferromagnetic regions is observed. This tunability of magnetic coupling is discussed within the context of these three competing magnetic phases, revealing critical features in designing exchange bias through exploiting spin frustration and disorder in high entropy oxides.

cond-mat.str-el↗
arXiv2023

Surprisingly large anomalous Hall effect and giant negative magnetoresistance in half-topological semimetals

Large intrinsic anomalous Hall effect (AHE) due to the Berry curvature in magnetic topological semimetals is attracting enormous interest due to its fundamental importance and technological relevance. Mechanisms resulting in large intrinsic AHE include diverging Berry curvature in Weyl semimetals, anticrossing nodal rings or points of non-trivial bands, and noncollinear spin structures. Here we show that a half-topological semimetal (HTS) state near a topological critical point can provide a new mechanism for driving an exceptionally large AHE. We reveal this through a systematic experimental and theoretical study of the antiferromagnetic (AFM) half-Heusler compound TbPdBi. We not only observed an unusual AHE with a surprisingly large anomalous Hall angle ΘH (tan ΘH ~ 2, the largest among the antiferromagnets) in its field-driven ferromagnetic (FM) phase, but also found a distinct Hall resistivity peak in the canted AFM phase within a low field range, where its isothermal magnetization is nearly linearly dependent on the field. Moreover, we observed a nearly isotropic, giant negative magnetoresistance with a magnitude of ~98%. Our in-depth theoretical modelling demonstrates that these exotic transport properties originate from the HTS state. A minimal Berry curvature cancellation between the trivial spin-up and nontrivial spin-down bands results not only in an extremely large AHE, but it also enhances the spin polarization of the spin-down bands substantially and thus leads to a giant negative magnetoresistance. Our study advances the understanding of the interplay between band topology and magnetism and offers new clues for materials design for spintronics and other applications.

cond-mat.mes-hall↗
arXiv2022

Strongly correlated itinerant magnetism on the boundary of superconductivity in a magnetic transition metal dichalcogenide

Metallic ferromagnets with strongly interacting electrons often exhibit remarkable electronic phases such as ferromagnetic superconductivity, complex spin textures, and nontrivial topology. In this report, we discuss the synthesis of a layered magnetic metal NiTa$_4$Se$_8$ (or Ni$_{1/4}$TaSe$_{2}$) with a Curie temperature of 58 Kelvin. Magnetization data and \textit{ab initio} calculations indicate that the nickel atoms host uniaxial ferromagnetic order of about 0.7$μ_{B}$ per atom, while an even smaller moment is generated in the itinerant tantalum conduction electrons. Strong correlations are evident in flat bands near the Fermi level, a high heat capacity coefficient, and a high Kadowaki-Woods ratio. When the system is diluted of magnetic ions, the samples become superconducting below about 2 Kelvin. Remarkably, electron and hole Fermi surfaces are associated with opposite spin polarization. We discuss the implications of this feature on the superconductivity that emerges near itinerant ferromagnetism in this material, including the possibility of spin-polarized superconductivity.

cond-mat.str-el↗