arXiv subjects

Joanna Blawat

Publications and source records attributed to Joanna Blawat.

11 recordsLinked to original sources

arXiv2026

Field-rigid Ising antiferromagnetism with giant spin-flip fields in Van der Waals UOTe

Van der Waals antiferromagnets provide a route to thickness-controlled magnetic order, but few combine high-temperature Ising order with conducting, correlated, and topological electronic structure. Here we show that UOTe realizes this combination. Magnetic susceptibility reveals a strongly anisotropic paramagnetic response, while neutron diffraction establishes c-axis antiferromagnetic order below $T_N \simeq 150$ K with an order-parameter exponent $β= 0.14$, close to the two-dimensional Ising value. Torque magnetometry further shows that the ordered state remains well described by a uniaxial antiferromagnet below the high-field transition. Pulsed-field magnetization up to 73 T shows that the ordered state survives to very large fields applied along the c axis before entering a broad metamagnetic regime that begins near 50 T, and remains unsaturated at the highest measured field. Angle-dependent proximity detector oscillator measurements show that the metamagnetic instability is set by the field component along the ordered moment direction, providing direct evidence for Ising-like field rigidity. UOTe therefore establishes a field-rigid Ising antiferromagnet with giant spin-flip fields in a compensated Van der Waals metal, where high-temperature c-axis order, quasi-two-dimensional magnetic criticality, Kondo-associated uranium 5f hybridization, metallic transport, and symmetry-enabled topology coexist in a single material.

cond-mat.str-el↗

arXiv2025

Nature of quantum criticality in the Ising ferromagnet TbV$_6$Sn$_6$

TbV$_6$Sn$_6$ is a topological metal where ferromagnetic Tb ions with strong uniaxial magnetic anisotropy interact with V kagome layers. Inelastic neutron scattering (INS) measurements show that the Tb ions adopt an Ising doublet ground state. Here, we consider whether a transverse magnetic field can drive TbV$_6$Sn$_6$ toward a quantum critical point, providing a rare example of transverse-field Ising criticality in a metallic compound. High-field magnetization measurements reveal a first-order-like spin-reorientation transition at 25.6 T. Our INS-based magnetic model finds that this is caused by an avoided crossing of an excited-state singlet with the ground-state doublet. Surprisingly, our model predicts that quantum critical and tricritical points are accessible within the range of experimentally determined model parameters and may be reached by varying the direction of an applied magnetic field.

cond-mat.str-el↗

arXiv2025

High-Magnetic Field Phases in U$_{1-x}$Th$_x$Te$_2$

At temperatures much lower than its superconducting critical temperature $T_c$ of 2.1 K, the heavy fermion superconductor UTe$_2$ has a remarkable phase diagram of magnetic field $H$ vs. angles $ϕ$ and $θ$ at which $H$ is tilted away from the $b$-axis toward the $a$- and $c$-axes, respectively, in the orthorhombic unit cell. The phase diagram appears to contain three superconducting phases: (1) a low field superconducting phase SC$_{\mathrm{LF}}$ extending over all values of $ϕ$ and $θ$ with an upper critical field $H_{c2}$ with a maximum value of 15 T at $ϕ= θ= 0^\circ$; (2) a high field superconducting phase SC$_{\mathrm{HF}}$ located in a region between $ϕ\approx 7^\circ$ and $θ\approx 4^\circ$ in fields from $H_{c2\mathrm{LF}}$ of the SC$_{\mathrm{LF}}$ phase and the metamagnetic transition at $H_m$ at $\sim 35$ T marking the onset of the magnetic field polarized FP phase: and (3) a SC$_{\mathrm{FP}}$ superconducting phase that resides entirely within the FP phase in a pocket of superconductivity extending from $θ\approx 20^\circ$ to $40^\circ$ in fields from $\sim 40$ T to above 60 T. In this work, we studied the $H$ vs $θ$ phase diagram at a base temperature of $\sim 0.6$ K as a function of Th concentration $x$ in U$_{1-x}$Th$_x$Te$_2$ pseudobinary compounds for $0.5\% \lesssim x \lesssim 4.7\%$. We find that for all values of $x$ within this range, the SC$_{\mathrm{LF}}$ phase is retained with a reduced value of $H_{c2}$ of $\sim 10$ T at $ϕ= θ= 0^\circ$ for $x = 4.7\%$, while the SC$_{\mathrm{HF}}$ phase is suppressed. The SC$_{\mathrm{FP}}$ and FP phases are unaffected to values of $x = 2\%$ but are completely suppressed in the region $x = 2.5$ to $4.7\%$ where the residual resistance ratio RRR has decreased from $\sim 14$ at $x = 1.5\%$ to values of $\sim 3$, indicating a significant increase in disorder.

cond-mat.str-el↗

arXiv2025

Quantum oscillations in the heat capacity of Kondo insulator YbB12

We observe the magnetic quantum oscillation in the heat capacity of the Kondo insulator YbB$_{12}$. The frequency of these oscillations $F = 670$ T, aligns with findings from magnetoresistance and torque magnetometry experiments for $μ_0 H > 35$ T in the Kondo insulating phase. Remarkably, the quantum oscillation amplitudes in the heat capacity are substantial, with $Δ\tilde{C}/T \approx$ 0.5 $\rm{mJ}$ $\rm{mol^{-1}K^{-2}}$ at 0.8 K, accounting for 13$\%$ of the known linear heat capacity coefficient $γ$. Double-peak structures of quantum-oscillation amplitudes due to the distribution function of fermions were identified and used to determine the value of the effective mass from the heat capacity, which agrees well with that from torque magnetometry. These observations support charge-neutral fermions contributing to the quantum oscillations in YbB$_{12}$.

cond-mat.str-el↗

arXiv2024

Deep Learning Based Superconductivity: Prediction and Experimental Tests

The discovery of novel superconducting materials is a longstanding challenge in materials science, with a wealth of potential for applications in energy, transportation, and computing. Recent advances in artificial intelligence (AI) have enabled expediting the search for new materials by efficiently utilizing vast materials databases. In this study, we developed an approach based on deep learning (DL) to predict new superconducting materials. We have synthesized a compound derived from our DL network and confirmed its superconducting properties in agreement with our prediction. Our approach is also compared to previous work based on random forests (RFs). In particular, RFs require knowledge of the chemical properties of the compound, while our neural net inputs depend solely on the chemical composition. With the help of hints from our network, we discover a new ternary compound $\textrm{Mo}_{20} \textrm{Re}_{6} \textrm{Si}_{4}$, which becomes superconducting below 5.4 K. We further discuss the existing limitations and challenges associated with using AI to predict and, along with potential future research directions.

cs.LG↗

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↗

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

Coexistence of bulk-nodal and surface-nodeless Cooper pairings in a superconducting Dirac semimetal

The interplay of nontrivial topology and superconductivity in condensed matter physics gives rise to exotic phenomena. However, materials are extremely rare where it is possible to explore the full details of the superconducting pairing. Here, we investigate the momentum dependence of the superconducting gap distribution in a novel Dirac material PdTe. Using high resolution, low temperature photoemission spectroscopy, we establish it as a spin-orbit coupled Dirac semimetal with the topological Fermi arc crossing the Fermi level on the (010) surface. This spin-textured surface state exhibits a fully gapped superconducting Cooper pairing structure below Tc~4.5K. Moreover, we find a node in the bulk near the Brillouin zone boundary, away from the topological Fermi arc.These observations not only demonstrate the band resolved electronic correlation between topological Fermi arc states and the way it induces Cooper pairing in PdTe, but also provide a rare case where surface and bulk states host a coexistence of nodeless and nodal gap structures enforced by spin-orbit coupling.

cond-mat.supr-con↗

arXiv2022

Unusual electrical and magnetic properties in layered EuZn2As2

Eu-based compounds often exhibit unusual magnetism, which is critical for nontrivial topological properties seen in materials such as EuCd2As2. We investigate the structure and physical properties of EuZn2As2 through measurements of the electrical resistivity, Hall effect, magnetization, and neutron diffraction. Our data show that EuZn2As2 orders antiferromagnetically with an A-type spin configuration below TN = 19 K. Surprisingly, there is strong evidence for dominant ferromagnetic fluctuations above TN, as reflected by positive Curie-Weiss temperature and extremely large negative magnetoresistance (MR) between TN and Tfl » 200 K. Furthermore, the angle dependence of the MRab indicates field-induced spin reorientation from the ab-plane to a direction approximately 45° from the ab plane. Compared to EuCd2As2, the doubled TN and Tfl make EuZn2As2 a better platform for exploring topological properties in both magnetic fluctuation (TN < T < Tfl) and ordered (T < TN) regimes.

cond-mat.mtrl-sci↗

arXiv2021

Determination of the magnetic q vectors in the heavy fermion superconductor Ce3PtIn11

An analysis with transferred hyperfine field has been performed on the spectra of the $^{115}$In nuclear quadrupole resonance experiments of heavy fermion superconductor Ce$_{3}$PtIn$_{11}$ exhibiting co-occurrence of two successive antiferromagnetic orderings ($T_{\rm N1}$ = 2.2 K and $T_{\rm N2}$ = 2.0 K) followed by superconducting transition ($T_{\rm c}$ = 0.32 K). The spectral changes at magnetic transition temperatures $T_{\rm N1}$ and $T_{\rm N2}$ indicate that the Ce(2) site has the dominant magnetic contribution, with a small magnetic moment, but not negligible, at the Ce(1) site. Our analysis using transferred hyperfine field which overcomes the previous simple dipolar model evinces that for $T_{\rm N2} < T < T_{\rm N1 } $, the propagation vectors at the Ce(1) and Ce(2) sublattices are q1 = q2 = (1/2, 1/2, 0 or 1/2), whereas for $T < T_{\rm N2 }$, the propagation vectors are q1 = (1/2,~1/2, 1/6 or 1/3) and q2 = (1/2, 1/2, 1/2 or 0), respectively.

cond-mat.str-el↗