Sudden adiabaticity signals reentrant bulk superconductivity in UTe2.

There has been a recent surge of interest in UTe2 due to its unconventional magnetic field (H)-reinforced spin-triplet superconducting phases persisting at fields far above the simple Pauli limit for H∥[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∥[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 alternative scenarios. 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; our 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.

Simultaneous measurement of specific heat and thermal conductivity in pulsed magnetic fields.

We report an experimental setup for simultaneously measuring specific heat and thermal conductivity in feedback-controlled pulsed magnetic fields of 50 ms duration at cryogenic temperatures. A stabilized magnetic field pulse obtained by the feedback control, which dramatically improves the thermal stability of the setup and sample, is used in combination with the flash method to obtain absolute values of thermal properties up to 37.2 T in the 22-16 K temperature range. We describe the experimental setup and demonstrate the performance of the present method with measurements on single-crystal samples of the geometrically frustrated quantum spin-dimer system SrCu2(BO3)2. Our proof-of-principle results show excellent agreement with data taken using a standard steady-state method, confirming the validity and convenience of the present approach.

Induced Ferromagnetism in Epitaxial Uranium Dioxide Thin Films.

Actinide materials have various applications that range from nuclear energy to quantum computing. Most current efforts have focused on bulk actinide materials. Tuning functional properties by using strain engineering in epitaxial thin films is largely lacking. Using uranium dioxide (UO2 ) as a model system, in this work, the authors explore strain engineering in actinide epitaxial thin films and investigate the origin of induced ferromagnetism in an antiferromagnet UO2 . It is found that UO2+ x thin films are hypostoichiometric (x<0) with in-plane tensile strain, while they are hyperstoichiometric (x>0) with in-plane compressive strain. Different from strain engineering in non-actinide oxide thin films, the epitaxial strain in UO2 is accommodated by point defects such as vacancies and interstitials due to the low formation energy. Both epitaxial strain and strain relaxation induced point defects such as oxygen/uranium vacancies and oxygen/uranium interstitials can distort magnetic structure and result in magnetic moments. This work reveals the correlation among strain, point defects and ferromagnetism in strain engineered UO2+ x thin films and the results offer new opportunities to understand the influence of coupled order parameters on the emergent properties of many other actinide thin films.

Control of electronic topology in a strongly correlated electron system.

It is becoming increasingly clear that breakthrough in quantum applications necessitates materials innovation. In high demand are conductors with robust topological states that can be manipulated at will. This is what we demonstrate in the present work. We discover that the pronounced topological response of a strongly correlated "Weyl-Kondo" semimetal can be genuinely manipulated-and ultimately fully suppressed-by magnetic fields. We understand this behavior as a Zeeman-driven motion of Weyl nodes in momentum space, up to the point where the nodes meet and annihilate in a topological quantum phase transition. The topologically trivial but correlated background remains unaffected across this transition, as is shown by our investigations up to much larger fields. Our work lays the ground for systematic explorations of electronic topology, and boosts the prospect for topological quantum devices.

Magnetoelastic standing waves induced in UO2 by microsecond magnetic field pulses.

Magnetoelastic dilatometry of the piezomagnetic antiferromagnet UO2 was performed via the fiber Bragg grating method in magnetic fields up to 150 T generated by a single-turn coil setup. We show that in microsecond timescales, pulsed-magnetic fields excite mechanical resonances at temperatures ranging from 10 to 300 K, in the paramagnetic as well as within the robust antiferromagnetic state of the material. These resonances, which are barely attenuated within the 100-µs observation window, are attributed to the strong magnetoelastic coupling in UO2 combined with the high crystalline quality of the single crystal samples. They compare well with mechanical resonances obtained by a resonant ultrasound technique and superimpose on the known nonmonotonic magnetostriction background. A clear phase shift of π in the lattice oscillations is observed in the antiferromagnetic state when the magnetic field overcomes the piezomagnetic switch field H[Formula: see text] T. We present a theoretical argument that explains this unexpected behavior as a result of the reversal of the antiferromagnetic order parameter at Hc.

Estandarización de un protocolo para diagnóstico de Chagas congénito por detección de ADN de Trypanosoma Cruzi en muestras colectadas en papel filtro / Standardization of a protocol for diagnosis of congenital Chagas by DNA detection of Trypanosoma Cruzi in samples collected on filter paper

En Argentina nacen alrededor de 1.500 niños con la enfermedad de Chagas (EC) congénita por año. Un muy bajo porcentaje de estos recién nacidos son diagnosticados debido a diferentes factores, tales como la falta de métodos diagnósticos sensibles y el déficit en el cumplimiento de las pautas de seguimiento en los niños en riesgo de infección. El diagnóstico temprano es fundamental para garantizar la eficacia del tratamiento. Existe la necesidad de contar con un método de toma de muestra con facilidad de transporte y almacenamiento y un mínimo riesgo biológico. El objetivo del presente estudio fue evaluar la eficacia de la conservación de las muestras de sangre en papel de filtro WHATMAN 903 que permitieran obtener una buena sensibilidad en los resultados de la qPCR para la detección de ADN de T. cruzi en pacientes con Chagas congénito. Para establecer el límite de detección se infectaron artificialmente muestras de sangre negativas con diferentes concentraciones de ADN parasitario. Se llevó a cabo una comparación entre dos kits de extracción comerciales, Qiagen y Roche, encontrando una mayor sensibilidad con el kit deQiagen, los límites de detección fueron LOD95 Roche: 56.87 [36.63-88.29] fg/uL y LOD95 Qiagen: 14.23 [9.30-21.78] fg/uL. Con base en dichos resultados, las muestras de pacientes fueron procesadas con el kit de Qiagen, encontrando una concordancia del 100% entre los resultados obtenidos a partir de sangre entera conservada en guanidina en comparación con las muestras conservadas en papel filtro, demostrando así que el papel de filtro Whatman 903, empleado para la pesquisa neonatal en la República Argentina, es una alternativa válida para ser empleada en el diagnóstico molecular de Chagas congénito. Este trabajo constituye un aporte significativo al diagnóstico temprano y el tratamiento de la enfermedad de Chagas congénita en nuestro país.

Emergent bound states and impurity pairs in chemically doped Shastry-Sutherland system.

Impurities often play a defining role in the ground states of frustrated quantum magnets. Studies of their effects are crucial in understanding of the phase diagram in these materials. SrCu2(BO3)2, an experimental realization of the Shastry-Sutherland (SS) lattice, provides a unique model system for such studies using both experimental and numerical approaches. Here we report effects of impurities on the crystals of bound states, and doping-induced emergent ground states in Mg-doped SrCu2(BO3)2, which remain stable in high magnetic fields. Using four complementary magnetometry techniques and theoretical simulations, a rich impurity-induced phenomenology at high fields is discovered. The results demonstrate a rare example in which even a small doping concentration interacts strongly with both triplets and bound states of triplets, and thus plays a significant role in the magnetization process even at high magnetic fields. Our findings provide insights into the study of impurity effects in geometrically frustrated quantum magnets.

Field Angle Tuned Metamagnetism and Lifschitz Transitions in UPt3.

Strongly correlated electronic systems can harbor a rich variety of quantum spin states. Understanding and controlling such spin states in quantum materials is of great current interest. Focusing on the simple binary system UPt3 with ultrasound (US) as a probe we identify clear signatures in field sweeps demarkating new high field spin phases. Magnetostriction (MS) measurements performed up to 65 T also show signatures at the same fields confirming these phase transitions. At the very lowest temperatures (<200 mK) we also observe magneto-acoustic quantum oscillations which for θ = 90° (B||c-axis) and vicinity abruptly become very strong in the 24.8-30 T range. High resolution magnetization measurements for this same angle reveal a continuous variation of the magnetization implying the subtle nature of the implied transitions. With B rotated away from the c-axis, the US signatures occur at nearly the same field. These transitions merge with the separate sequence of the well known metamagnetic transition which commences at 20 T for θ = 0° but moves to higher fields as 1/cosθ. This merge, suggesting a tricritical behavior, occurs at θ ≈ 51° from the ab-plane. This is an unique off-symmetry angle where the length change along the c-axis is precisely zero due to the anisotropic nature of MS in UPt3 for all magnetic field values.

Measurement of the angle dependence of magnetostriction in pulsed magnetic fields using a piezoelectric strain gauge.

We present a high resolution method for measuring magnetostriction in millisecond pulsed magnetic fields at cryogenic temperatures with a sensitivity of 1.11×10-11/Hz . The sample is bonded to a thin piezoelectric plate such that when the sample's length changes, it strains the piezoelectric and induces a voltage change. This method is more sensitive than a fiber-Bragg grating method. It measures two axes simultaneously instead of one. The gauge is small and versatile, functioning in DC and millisecond pulsed magnetic fields. We demonstrate its use by measuring the magnetostriction of Ca3Co1.03Mn0.97O6 single crystals in pulsed magnetic fields. By comparing our data to new and previously published results from a fiber-Bragg grating magnetostriction setup, we confirm that this method detects magnetostriction effects. We also demonstrate the small size and versatility of this technique by measuring angle dependence with respect to the applied magnetic field in a rotator probe in 65 T millisecond pulsed magnetic fields.

An FBG Optical Approach to Thermal Expansion Measurements under Hydrostatic Pressure.

We report on an optical technique for measuring thermal expansion and magnetostriction at cryogenic temperatures and under applied hydrostatic pressures of 2.0 GPa. Optical fiber Bragg gratings inside a clamp-type pressure chamber are used to measure the strain in a millimeter-sized sample of CeRhIn5. We describe the simultaneous measurement of two Bragg gratings in a single optical fiber using an optical sensing instrument capable of resolving changes in length [dL/L = (L- L0)/L0] on the order of 10-7. Our results demonstrate the possibility of performing high-resolution thermal expansion measurements under hydrostatic pressure, a capability previously hindered by the small working volumes typical of pressure cells.

Fiber Bragg Grating Dilatometry in Extreme Magnetic Field and Cryogenic Conditions.

In this work, we review single mode SiO2 fiber Bragg grating techniques for dilatometry studies of small single-crystalline samples in the extreme environments of very high, continuous, and pulsed magnetic fields of up to 150 T and at cryogenic temperatures down to <1 K. Distinct millimeter-long materials are measured as part of the technique development, including metallic, insulating, and radioactive compounds. Experimental strategies are discussed for the observation and analysis of the related thermal expansion and magnetostriction of materials, which can achieve a strain sensitivity (ΔL/L) as low as a few parts in one hundred million (≈10-8). The impact of experimental artifacts, such as those originating in the temperature dependence of the fiber's index of diffraction, light polarization rotation in magnetic fields, and reduced strain transfer from millimeter-long specimens, is analyzed quantitatively using analytic models available in the literature. We compare the experimental results with model predictions in the small-sample limit, and discuss the uncovered discrepancies.

Missing magnetism in Sr4Ru3O10: Indication for Antisymmetric Exchange Interaction.

Metamagnetism occuring inside a ferromagnetic phase is peculiar. Therefore, Sr4Ru3O10, a T C = 105 K ferromagnet, has attracted much attention in recent years, because it develops a pronounced metamagnetic anomaly below T C for magnetic fields applied in the crystallographic ab-plane. The metamagnetic transition moves to higher fields for lower temperatures and splits into a double anomaly at critical fields H c1 = 2.3 T and H c2 = 2.8 T, respectively. Here, we report a detailed study of the different components of the magnetization vector as a function of temperature, applied magnetic field, and varying angle in Sr4Ru3O10. We discover for the first time a reduction of the magnetic moment in the plane of rotation at the metamagnetic transition. The anomaly shifts to higher fields by rotating the field from H ⊥ c to H || c. We compare our experimental findings with numerical simulations based on spin reorientation models taking into account magnetocrystalline anisotropy, Zeeman effect and antisymmetric exchange interactions. While Magnetocrystalline anisotropy combined with a Zeeman term are sufficient to explain a metamagnetic transition in Sr4Ru3O10, a Dzyaloshinskii-Moriya term is crucial to account for the reduction of the magnetic moment as observed in the experiments.

Upper Critical Field and Kondo Effects in Fe(Te0.9Se0.1) Thin Films by Pulsed Field Measurements.

The transition temperatures of epitaxial films of Fe(Te0:9Se0:1) are remarkably insensitive to applied magnetic field, leading to predictions of upper critical fields Bc2(T = 0) in excess of 100 T. Using pulsed magnetic fields, we find Bc2(0) to be on the order of 45 T, similar to values in bulk material and still in excess of the paramagnetic limit. The same films show strong magnetoresistance in fields above Bc2(T), consistent with the observed Kondo minimum seen above Tc. Fits to the temperature dependence in the context of the WHH model, using the experimental value of the Maki parameter, require an effective spin-orbit relaxation parameter of order unity. We suggest that Kondo localization plays a similar role to spin-orbit pair breaking in making WHH fits to the data.

Coherent pulse interrogation system for fiber Bragg grating sensing of strain and pressure in dynamic extremes of materials.

A 100 MHz fiber Bragg grating (FBG) interrogation system is described and applied to strain and pressure sensing. The approach relies on coherent pulse illumination of the FBG sensor with a broadband short pulse from a femtosecond modelocked erbium fiber laser. After interrogation of the FBG sensor, a long multi-kilometer run of single mode fiber is used for chromatic dispersion to temporally stretch the spectral components of the reflected pulse from the FBG sensor. Dynamic strain or pressure induced spectral shifts in the FBG sensor are detected as a pulsed time domain waveform shift after encoding by the chromatic dispersive line. Signals are recorded using a single 35 GHz photodetector and a 50 G Samples per second, 25 GHz bandwidth, digitizing oscilloscope. Application of this approach to high-speed strain sensing in magnetic materials in pulsed magnetic fields to ~150 T is demonstrated. The FBG wavelength shifts are used to study magnetic field driven magnetostriction effects in LaCoO3. A sub-microsecond temporal shift in the FBG sensor wavelength attached to the sample under first order phase change appears as a fractional length change (strain: ΔL/L<10-4) in the material. A second application used FBG sensing of pressure dynamics to nearly 2 GPa in the thermal ignition of the high explosive PBX-9501 is also demonstrated. Both applications demonstrate the use of this FBG interrogation system in dynamical extreme conditions that would otherwise not be possible using traditional FBG interrogation approaches that are deemed too slow to resolve such events.

Magnetic nanopantograph in the SrCu2(BO3)2 Shastry-Sutherland lattice.

Magnetic materials having competing, i.e., frustrated, interactions can display magnetism prolific in intricate structures, discrete jumps, plateaus, and exotic spin states with increasing applied magnetic fields. When the associated elastic energy cost is not too expensive, this high potential can be enhanced by the existence of an omnipresent magnetoelastic coupling. Here we report experimental and theoretical evidence of a nonnegligible magnetoelastic coupling in one of these fascinating materials, SrCu2(BO3)2 (SCBO). First, using pulsed-field transversal and longitudinal magnetostriction measurements we show that its physical dimensions, indeed, mimic closely its unusually rich field-induced magnetism. Second, using density functional-based calculations we find that the driving force behind the magnetoelastic coupling is the CuOCu superexchange angle that, due to the orthogonal Cu(2+) dimers acting as pantographs, can shrink significantly (0.44%) with minute (0.01%) variations in the lattice parameters. With this original approach we also find a reduction of ∼ 10% in the intradimer exchange integral J, enough to make predictions for the highly magnetized states and the effects of applied pressure on SCBO.

Fermi surface reconstruction and multiple quantum phase transitions in the antiferromagnet CeRhIn5.

Conventional, thermally driven continuous phase transitions are described by universal critical behavior that is independent of the specific microscopic details of a material. However, many current studies focus on materials that exhibit quantum-driven continuous phase transitions (quantum critical points, or QCPs) at absolute zero temperature. The classification of such QCPs and the question of whether they show universal behavior remain open issues. Here we report measurements of heat capacity and de Haas-van Alphen (dHvA) oscillations at low temperatures across a field-induced antiferromagnetic QCP (Bc0 ≈ 50 T) in the heavy-fermion metal CeRhIn5. A sharp, magnetic-field-induced change in Fermi surface is detected both in the dHvA effect and Hall resistivity at B0* ≈ 30 T, well inside the antiferromagnetic phase. Comparisons with band-structure calculations and properties of isostructural CeCoIn5 suggest that the Fermi-surface change at B0* is associated with a localized-to-itinerant transition of the Ce-4f electrons in CeRhIn5. Taken in conjunction with pressure experiments, our results demonstrate that at least two distinct classes of QCP are observable in CeRhIn5, a significant step toward the derivation of a universal phase diagram for QCPs.

Anisotropic cascade of field-induced phase transitions in the frustrated spin-ladder system BiCu2PO6.

BiCu(2)PO(6) is a frustrated two-leg spin-ladder compound with a spin gap that can be closed with a magnetic field of approximately 20 T. This quantum phase transition and its related phase diagram as a function of magnetic field and temperature (H, T) are investigated up to 60 T by means of specific heat, magnetocaloric effect, magnetization, and magnetostriction measurements. In contrast to other gapped quantum magnets, BiCu(2)PO(6) undergoes a series of unexpected first- and second-order phase transitions when an external magnetic field is applied along the crystallographic c axis. The application of a magnetic field along the b axis induces two second-order phase transitions. We propose that the anisotropy and complex phase diagram result from the interplay between strong geometrical frustration and spin-orbit interaction necessary for the description of this fascinating magnetic system.

AC measurement of heat capacity and magnetocaloric effect for pulsed magnetic fields.

A new calorimeter for measurements of the heat capacity and magnetocaloric effect of small samples in pulsed magnetic fields is discussed for the exploration of thermal and thermodynamic properties at temperatures down to 2 K. We tested the method up to µ(0)H=50 T, but it could be extended to higher fields. For these measurements we used carefully calibrated bare-chip Cernox(®) and RuO(2) thermometers, and we present a comparison of their performances. The monotonic temperature and magnetic field dependences of the magnetoresistance of RuO(2) allow thermometry with a precision as good as ±4 mK at T=2 K. To test the performance of our calorimeter, heat capacity and magnetocaloric effect for the spin-dimer compound Sr(3)Cr(2)O(8) and the triangular lattice antiferromagnet RbFe(MoO(4))(2) are presented.

Electron-spin domains: magnetic enhancement of superconductivity.

An inhomogeneous superconducting state, not yet conclusively identified, was predicted by Fulde and Ferrell and Larkin and Ovchinnikov (FFLO) to arise in superconductors with strong Pauli limiting, a consequence of the electrons' Zeeman (spin) energy in a magnetic field. Radovan et al. propose that the observed cascades of steps in magnetization of the heavy fermion superconductor CeCoIn5, within the recently discovered second low-temperature state, are due to transitions between Landau-level (LL) states with different m-quanta vortices, expected under certain conditions when the magnetic field is swept within the FFLO state. The authors then conclude that the observed steps in magnetization constitute a proof that the low-temperature state in CeCoIn5 is indeed an FFLO state. We argue that this interpretation of the observed steps in magnetization cannot be supported on either quantitative or qualitative grounds.