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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. The warping of the cylindrical Fermi sheets continuously increases with pressure, which is consistent with 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.

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 mechanical 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. Surprisingly, 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 an intensely field-resilient superconducting state in a striking toroidal shape, persisting 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$_2$.

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.

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.

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$_2$.

We present an extensive powder and single-crystal neutron scattering investigation of the crystallographic structure and magnetic order of the frustrated metallic $f$-electron magnet UNi$_4$B. We carry out a full refinement of the crystallographic structure and conclude that the low-temperature lattice symmetry is orthorhombic (space group Pmm2; cell parameters: $a = 6.963(4)~\r{A}$ , $b = 14.793(9)~\r{A}$ , $c = 17.126(8)~\r{A}$). We determine the magnetically ordered structure, concluding that below $T_\mathrm{N} = 19.5~\mathrm{K}$ the material undergoes a transition into a partially ordered antiferromagnetic state. The magnetic structure is consistent with the existence of toroidal order in this material. We further test the proposal of a second magnetic transition occurring at $330~\mathrm{mK}$, concluding that the thermodynamic anomalies observed at these temperatures do not reflect modifications of the magnetic structure. Our study provides a consistent picture of the interrelationship of structural and magnetic properties in the frustrated magnet UNi$_4$B previously unresolved., Comment: Submitted to Phys. Rev. B

Despite belonging to a well-studied family of transition metal trihalides, VI_3 has received significant attention only recently. As a hard ferromagnetic van der Waals compound with a large coercivity, it attracted much attention because of its potential use in atomically thin spintronic and optoelectronic devices. However, practical exploration of VI_3 is challenging due to its instability under ambient conditions. Here, we present a comprehensive set of optical, x-ray diffraction, magnetization, and ellipsometric measurements of VI3 and demonstrate that, similarly to the related van der Waals ferromagnet CrI_3, the degradation process is accelerated by the presence of moisture. The VI_3 surface was covered by selected media commonly used in physical measurements to test its stability and lower the degradation rate three times or higher, providing practical information for experimentalists interested. The decomposition study at ambient conditions shows that the VI_3 single crystal can be used for most of the bulk, magnetization, and optical measurements without any noticeable change of physical properties, as the significant degradation appears first after ~ 2 hours of exposition as illustrated, e.g., by the evolution of the ferromagnetic T_1 and T_2 transitions. The ellipsometric measurement demonstrates that even the surface remains optically stable for at least 5 minutes., Comment: 14 pages, 6 figures

We present acoustic signatures of the electric quadrupolar degrees of freedom in the honeycomb-layer compound UNi$_4$B. The transverse ultrasonic mode $C_{66}$ shows softening below 30 K both in the paramagnetic phase and antiferromagnetic phases down to $\sim0.33$ K. Furthermore, we traced magnetic field-temperature phase diagrams up to 30 T and observed a highly anisotropic elastic response within the honeycomb layer. These observations strongly suggest that $\Gamma_6$(E$_{\rm 2g}$) electric quadrupolar degrees of freedom in localized $5f^2$ ($J = 4$) states are playing an important role in the magnetic toroidal dipole order and magnetic-field-induced phases of UNi$_4$B, and evidence some of the U ions remain in the paramagnetic state even if the system undergoes magnetic toroidal ordering., Comment: 6 pages, 4 figures + Supplemental Materials (11 pages, 9 figures)

A comprehensive study of magnetocrystalline anisotropy of a layered van-der-Waals ferromagnet VI3 was performed. We measured angular dependences of the torque and magnetization with respect to the direction of the applied magnetic field within the "ac" plane perpendicular to and within the basal ab plane, respectively. A two-fold butterfly-like signal was detected by magnetization in the perpendicular "ac" plane. This signal symmetry remains conserved throughout all magnetic regimes as well as through the known structural transition down to the lowest temperatures. The maximum of the magnetization signal and the resulting magnetization easy axis is significantly tilted from the principal c axis by ~40{\deg}. The close relation of the magnetocrystalline anisotropy to the crystal structure was documented. In contrast, a two-fold-like angular signal was detected in the paramagnetic region within the ab plane in the monoclinic phase, which transforms into a six-fold-like signal below the Curie temperature TC. With further cooling, another six-fold-like signal with an angular shift of ~30{\deg} grows approaching TFM. Below TFM, in the triclinic phase, the original six-fold-like signal vanishes, being replaced by a secondary six-fold-like signal with an angular shift of ~30{\deg}., Comment: 14 pages, 10 figures

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., Comment: Submitted to Phys. Rev. B

Evolution of magnetism in single crystals of the van der Waals compound VI3 in external pressure up to 7.3 GPa studied by measuring magnetization and ac magnetic susceptibility is reported. Four magnetic phase transitions, at T1 = 54.5 K, T2 = 53 K, TC = 49.5 K, and TFM = 26 K, respectively have been observed at ambient pressure. The first two have been attributed to the onset of ferromagnetism in specific crystal-surface layers. The bulk ferromagnetism is characterized by the magnetic ordering transition at Curie temperature TC and the transition between two different ferromagnetic phases TFM, accompanied by a structure transition from monoclinic to triclinic symmetry upon cooling. The pressure effects on magnetic parameters were studied with three independent techniques. TC was found to be almost unaffected by pressures up to 0.6 GPa whereas TFM increases rapidly with increasing pressure and reaches TC at a triple point at ~ 0.85 GPa. At higher pressures, only one magnetic phase transition is observed moving to higher temperatures with increasing pressure to reach 99 K at 7.3 GPa. In contrast, the low-temperature bulk magnetization is dramatically reduced by applying pressure (by more than 50% at 2.5 GPa) suggesting a possible pressure-induced reduction of vanadium magnetic moment. We discussed these results in light of recent theoretical studies to analyze exchange interactions and provide how to increase the Curie temperature of VI3., Comment: 20 pages, 16 figures

Doping Bi$_2$Se$_3$ by magnetic ions represents an interesting problem since it may break the time reversal symmetry needed to maintain the topological insulator character. Mn dopants in Bi$_2$Se$_3$ represent one of the most studied examples here. However, there is a lot of open questions regarding their magnetic ordering. In the experimental literature different Curie temperatures or no ferromagnetic order at all are reported for comparable Mn concentrations. This suggests that magnetic ordering phenomena are complex and highly susceptible to different growth parameters, which are known to affect material defect concentrations. So far theory focused on Mn dopants in one possible position, and neglected relaxation effects as well as native defects. We have used ab initio methods to calculate the Bi$_2$Se$_3$ electronic structure influenced by magnetic Mn dopants, and exchange interactions between them. We have considered two possible Mn positions, the substitutional and interstitial one, and also native defects. We have found a sizable relaxation of atoms around Mn, which affects significantly magnetic interactions. Surprisingly, very strong interactions correspond to a specific position of Mn atoms separated by van der Waals gap. Based on the calculated data we performed spin dynamics simulations to examine systematically the resulting magnetic order for various defect contents. We have found under which conditions the experimentally measured Curie temperatures ${T_{\rm{C}}}$ can be reproduced, noticing that interstitial Mn atoms appear to be important here. Our theory predicts the change of ${T_{\rm{C}}}$ with a shift of Fermi level, which opens the way to tune the system magnetic properties by selective doping.

Magnetotransport constitutes a useful probe to understand the interplay between electronic band topology and magnetism in spintronics devices based on topological materials. A recent theory of Lu and Shen [Phys. Rev. Lett. 112, 146601 (2014)] on magnetically doped topological insulators predicts that quantum corrections $\Delta\kappa$ to the temperature-dependence of the conductivity can change sign during the Curie transition. This phenomenon has been attributed to a suppression of the Berry phase of the topological surface states at the Fermi level, caused by a magnetic energy gap. Here, we demonstrate experimentally that $\Delta\kappa$ can reverse its sign even when the Berry phase at the Fermi level remains unchanged, provided that the inelastic scattering length decreases with temperature below the Curie transition.

Thermal expansion, magnetostriction and magnetization measurements under magnetic field and hydrostatic pressure were performed on a $\mathrm{UAu_{2}Si_{2}}$ single crystal. They revealed a large anisotropy of magnetoelastic properties manifested by prominent length changes leading to a collapse of the unit-cell volume accompanied by breaking the fourfold symmetry (similar to that in $\mathrm{URu_{2}Si_{2}}$ in the hidden-order state) in the antiferromagnetic state as consequences of strong magnetoelastic coupling. The magnetostriction curves measured at higher temperatures confirm a bulk character of the 50 K weak ferromagnetic phase. The large positive pressure change of the ordering temperature predicted from Ehrenfest relation contradicts the more than an order of magnitude smaller pressure dependence observed by the magnetization and specific heat measured under hydrostatic pressure. A comprehensive magnetic phase diagram of $\mathrm{UAu_{2}Si_{2}}$ in magnetic field applied along the $c$ axis is presented. The ground-state antiferromagnetic phase is suppressed by a field-induced metamagnetic transition that changes its character from the second to the first order at the tricritical point.

We compare the magnetic and electronic configuration of single Mn atoms in molecular beam epitaxy (MBE) grown Bi$_2$Se$_3$ thin films, focusing on electron paramagnetic (ferromagnetic) resonance (EPR and FMR, respectively) and superconducting quantum interference device (SQUID) techniques. X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) reveal the expected increase of disorder with increasing concentration of magnetic guest atoms, however, Kikuchi patterns show that disorder consists majorly of mum-scale 60deg twin domains in the hexagonal Bi$_2$Se$_3$ structure, which are promoted by the presence of single unclustered Mn impurities. Ferromagnetism below T$_C$ ~ (5.4 +/- 0.3) K can be well described by critical scaling laws M(T) ~ (1-T/T$_C$)$^\beta$ with a critical exponent $\beta$ = (0.34 +/- 0.2)), suggesting 3D Heisenberg class magnetism instead of e.g. 2D-type coupling between Mn-spins in van der Waals gap sites. From EPR hyperfine structure data we determine a Mn$^{2+}$ (d$^5$, S = 5/2) electronic configuration with a g-factor of 2.002 for -1/2 --> +1/2 transitions. In addition, from the strong dependence of the low temperature FMR fields and linewidth on the field strength and orientation with respect to the Bi$_2$Se$_3$ (0001) plane, we derive magnetic anisotropy energies of up to K1 = -3720 erg/cm3 in MBE-grown Mn-doped Bi$_2$Se$_3$, reflecting the first order magneto-crystalline anisotropy of an in-plane magnetic easy plane in a hexagonal (0001) crystal symmetry. Across the ferromagnetic-paramagnetic transition the FMR intensity is suppressed and resonance fields converge the paramagnetic limit of a Mn$^{2+}$ (d$^5$, S = 5/2).

We present a first-principles study of the electronic, magnetic, and transport properties of the topological insulator Bi$_{2}$Te$_{3}$ doped with Mn atoms in substitutional (Mn$_{\rm Bi}$) and interstitial van der Waals gap positions (Mn$_{\rm i}$), which act as acceptors and donors, respectively. The effect of native Bi$_{\rm Te}$- and Te$_{\rm Bi}$-antisite defects and their influence on calculated electronic transport properties is also investigated. We have studied four models representing typical cases, namely (i) Bi$_{2}$Te$_{3}$ with and without native defects, (ii) Mn$_{\rm Bi}$ defects with and without native defects, (iii) the same but for Mn$_{\rm i}$ defects, and (iv) the combined presence of Mn$_{\rm Bi}$ and Mn$_{\rm i}$. It was found that lattice relaxations around Mn$_{\rm Bi}$ defects play an important role for both magnetic and transport properties. The resistivity is strongly influenced by the amount of carriers, their type, and by the relative positions of the Mn-impurity energy levels and the Fermi energy. Our results indicate strategies to tune bulk resistivities, and also help to uncover the location of Mn atoms in measured samples. Calculations indicate that at least two of the considered defects have to be present simultaneously in order to explain the experimental observations, and the role of interstitials may be more important than expected.

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., Comment: 5 pages, 5 figures

We report on magnetization, specific heat and resistivity experiments on single crystals of the novel heavy Fermion compound Ce$_3$PdIn$_{11}$. At ambient pressure the compound exhibits two successive transitions at T$_{1}$ = 1.63 K and T$_{\mathrm {N}}$ = 1.49 K into incommensurate and commensurate local moment antiferromagnetic states, respectively, before becoming superconducting below T$_{\mathrm {c}}$ = 0.42 K. The large values of dB$_{\mathrm {c2}}$/dT and B$_{\mathrm {c2}}$ = 2.8 T imply that heavy quasiparticles form the Cooper pairs. Thus, Ce$_{3}$PdIn$_{11}$ is the first ambient pressure heavy Fermion superconductor where 4f electrons are simultaneously responsible for magnetic order and superconductivity.

We report on the discovery and investigation of a new 218 heavy fermion compound. Crystals have been synthesized from In-flux. Structurally, Ce2PtIn8 is located between the cubic CeIn3 and the more two-dimensional CeTIn5 (T = transition metal) type of compounds. The weak anisotropy of the paramagnetic susceptibility suggests rather 3D magnetic correlations. Specific heat, electrical resistivity and magnetization measurements revealed that Ce2PtIn8 orders antiferromagnetically below Tn = 2.1 K. An order-to-order transition is observed at Tm = 2 K. Similarities in the H - T phase diagram to other CenTmIn3n+2m (T = Rh, Pt) compounds point to a pressure-induced quantum phase transition (QPT) which, according to the tentative location of Ce2PtIn8 in the recent proposed global phase diagram for QPT, would be of spin density wave type., Comment: recent work on Ce3PdIn11 (arXiv:1403.4363) strongly suggest that the observed magnetic transitions in Ce2PtIn8 likely arise from Ce3PtIn11 inclusions. For this reason the paper has been withdrawn until further studies confirm data presented

We report a detailed study of the polymorphic transitions in ternary stannide CeRuSn on high quality single crystals through a combination of X-ray diffraction experiments conducted at 300, 275 and 120 K, and measurements of the thermal expansion, magnetization, and resistivity, along main crystallographic axes. In addition, the transition was followed as a function of pressure up to 0.8 GPa. The present X-ray diffraction data show that the room temperature polymorph consists of the lattice doubled along the c axis with respect to the CeCoAl-type structure consistent with previous reports. Upon cooling, the compound undergoes two successive transitions, first to a quintuple (290 K) and than to a triple CeCoAl superstructure at 225 K. The transitions are accompanied by a tremendous volume change due to a strong shrinking of the lattice along the c axis, which is clearly observed in thermal expansion. We advance arguments that the volume collapse originates from an increasing number of crystallographically inequivalent Ce sites and the change of ratio between the short and long Ce-Ru bonds. The observed properties of the polymorphic transition in CeRuSn are reminiscent of the transition in elementary Cerium, suggesting that similar physics, i.e., a Kondo influenced transition and strong lattice vibrations might be the driving forces.

In this paper, specific features of Sm magnetism in an intermetallic compound have been studied. For this purpose, a high-quality single crystal of SmPd2Al3 was grown and subjected to detailed measurements of specific heat, magnetization, ac susceptibility, and electrical resistivity with respect to temperature and a magnetic field applied along the principal crystallographic directions. SmPd2Al3 magnetism was found to be strongly anisotropic with the easy-magnetization direction along the c axis where the main magnetic features are concentrated. The a-axis response remains weak, paramagneticlike, even in the magnetically ordered state. Ferromagnetism with TC=12.4 K has been indicated by all the measured physical properties. At lower temperatures, three successive order-order phase transitions have been observed on the temperature dependence of the specific heat as three anomalies: at 3.4, 3.9, and 4.4 K, respectively. The low-temperature magnetization data can be understood within a scenario that considers the antiferromagnetic ground state as being gradually destroyed through a series of four metamagnetic transitions at 0.03, 0.35, 0.5, and 0.75 T, as detected in the 1.8 K magnetization data. The experimental data are discussed together with the results of electronic-structure and crystal-field calculations from first principles, which were performed as an important part of the study for comprehension and explanation of the observed behavior of the SmPd2Al3 compound.

Comment to D. Kaczorowski et al., Phys. Rev. Lett. 103, 027003 (2009)., Comment: submited to PRL 24Aug09, accepted

UCoGe is reported as a weak ferromagnetic (FM) two-band superconductor (SC) with the critical temperature, $T_{\mathrm{SC}}\sim$ 0.7 K and the Curie temperature, $T_{\mathrm{C}}\sim$ 3 K at ambient pressure. We report exotic attributes of the SC and FM state in moderate magnetic fields. The observed phenomena clearly demonstrate that the SC regime is superior to the paramagnetic state in the vicinity of $T_{\mathrm{SC}}$. Above $T_{\mathrm{SC}}$, the zero-field state is characterized by a regime with strong FM spin fluctuations, suggesting proximity of the FM quantum critical point (FM-QCP). In addition, we observed that the robust SC regime develops independently on the existence or lack of the long-range FM ordering., Comment: 4 pages, 4 figures

We present results of measurements of electrical, magnetic and thermal properties of EuB$_{5.99}$C$_{0.01}$. The observed anomalously large negative magnetoresistance as above, so below the Curie temperature of ferromagnetic ordering $T_C$ is attributed to fluctuations in carbon concentration. Below $T_C$ the carbon richer regions give rise to helimagnetic domains, which are responsible for an additional scattering term in the resistivity, which can be suppressed by a magnetic field. Above $T_C$ these regions prevent the process of percolation of magnetic polarons (MPs), acting as "spacers" between MPs. We propose that such "spacers", being in fact volumes incompatible with existence of MPs, may be responsible for the decrease of the percolation temperature and for the additional (magneto)resistivity increase in systems with MPs., Comment: 4 pages, 5 figures, Submitted for publication