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1.
J Vis Exp ; (173)2021 07 08.
Artigo em Inglês | MEDLINE | ID: mdl-34309595

RESUMO

Single crystal specimens of the actinide compound uranium ditelluride, UTe2, are of great importance to the study and characterization of its dramatic unconventional superconductivity, believed to entail spin-triplet electron pairing. A variety in the superconducting properties of UTe2 reported in the literature indicates that discrepancies between synthesis methods yield crystals with different superconducting properties, including the absence of superconductivity entirely. This protocol describes a process to synthesize crystals that exhibit superconductivity via chemical vapor transport, which has consistently exhibited a superconducting critical temperature of 1.6 K and a double transition indicative of a multi-component order parameter. This is compared to a second protocol that is used to synthesize crystals via the molten metal flux growth technique, which produces samples that are not bulk superconductors. Differences in the crystal properties are revealed through a comparison of structural, chemical, and electronic property measurements, showing that the most dramatic disparity occurs in the low-temperature electrical resistance of the samples.


Assuntos
Urânio , Elétrons , Supercondutividade , Temperatura
2.
Proc Natl Acad Sci U S A ; 118(27)2021 Jul 06.
Artigo em Inglês | MEDLINE | ID: mdl-34187886

RESUMO

In matter, any spontaneous symmetry breaking induces a phase transition characterized by an order parameter, such as the magnetization vector in ferromagnets, or a macroscopic many-electron wave function in superconductors. Phase transitions with unknown order parameter are rare but extremely appealing, as they may lead to novel physics. An emblematic and still unsolved example is the transition of the heavy fermion compound [Formula: see text] (URS) into the so-called hidden-order (HO) phase when the temperature drops below [Formula: see text] K. Here, we show that the interaction between the heavy fermion and the conduction band states near the Fermi level has a key role in the emergence of the HO phase. Using angle-resolved photoemission spectroscopy, we find that while the Fermi surfaces of the HO and of a neighboring antiferromagnetic (AFM) phase of well-defined order parameter have the same topography, they differ in the size of some, but not all, of their electron pockets. Such a nonrigid change of the electronic structure indicates that a change in the interaction strength between states near the Fermi level is a crucial ingredient for the HO to AFM phase transition.

3.
Proc Natl Acad Sci U S A ; 118(20)2021 05 18.
Artigo em Inglês | MEDLINE | ID: mdl-33975950

RESUMO

Electrical resistivity measurements were performed on single crystals of URu2-x Os x Si2 up to x = 0.28 under hydrostatic pressure up to P = 2 GPa. As the Os concentration, x, is increased, 1) the lattice expands, creating an effective negative chemical pressure P ch(x); 2) the hidden-order (HO) phase is enhanced and the system is driven toward a large-moment antiferromagnetic (LMAFM) phase; and 3) less external pressure P c is required to induce the HO→LMAFM phase transition. We compare the behavior of the T(x, P) phase boundary reported here for the URu2-x Os x Si2 system with previous reports of enhanced HO in URu2Si2 upon tuning with P or similarly in URu2-x Fe x Si2 upon tuning with positive P ch(x). It is noteworthy that pressure, Fe substitution, and Os substitution are the only known perturbations that enhance the HO phase and induce the first-order transition to the LMAFM phase in URu2Si2 We present a scenario in which the application of pressure or the isoelectronic substitution of Fe and Os ions for Ru results in an increase in the hybridization of the U-5f-electron and transition metal d-electron states which leads to electronic instability in the paramagnetic phase and the concurrent formation of HO (and LMAFM) in URu2Si2 Calculations in the tight-binding approximation are included to determine the strength of hybridization between the U-5f-electron states and the d-electron states of Ru and its isoelectronic Fe and Os substituents in URu2Si2.

4.
Nat Commun ; 12(1): 2644, 2021 May 11.
Artigo em Inglês | MEDLINE | ID: mdl-33976162

RESUMO

Chiral superconductors have been proposed as one pathway to realize Majorana normal fluid at its boundary. However, the long-sought 2D and 3D chiral superconductors with edge and surface Majorana normal fluid are yet to be conclusively found. Here, we report evidence for a chiral spin-triplet pairing state of UTe2 with surface normal fluid response. The microwave surface impedance of the UTe2 crystal was measured and converted to complex conductivity, which is sensitive to both normal and superfluid responses. The anomalous residual normal fluid conductivity supports the presence of a significant normal fluid response. The superfluid conductivity follows the temperature behavior predicted for an axial spin-triplet state, which is further narrowed down to a chiral spin-triplet state with evidence of broken time-reversal symmetry. Further analysis excludes trivial origins for the observed normal fluid response. Our findings suggest that UTe2 can be a new platform to study exotic topological excitations in higher dimension.

5.
Proc Natl Acad Sci U S A ; 117(48): 30220-30227, 2020 Dec 01.
Artigo em Inglês | MEDLINE | ID: mdl-33203673

RESUMO

Using inelastic X-ray scattering beyond the dipole limit and hard X-ray photoelectron spectroscopy we establish the dual nature of the U [Formula: see text] electrons in U[Formula: see text] (M = Pd, Ni, Ru, Fe), regardless of their degree of delocalization. We have observed that the compounds have in common a local atomic-like state that is well described by the U [Formula: see text] configuration with the [Formula: see text] and [Formula: see text] quasi-doublet symmetry. The amount of the U 5[Formula: see text] configuration, however, varies considerably across the U[Formula: see text] series, indicating an increase of U 5f itineracy in going from M = Pd to Ni to Ru and to the Fe compound. The identified electronic states explain the formation of the very large ordered magnetic moments in [Formula: see text] and [Formula: see text], the availability of orbital degrees of freedom needed for the hidden order in [Formula: see text] to occur, as well as the appearance of Pauli paramagnetism in [Formula: see text] A unified and systematic picture of the U[Formula: see text] compounds may now be drawn, thereby providing suggestions for additional experiments to induce hidden order and/or superconductivity in U compounds with the tetragonal body-centered [Formula: see text] structure.

6.
Acta Crystallogr B Struct Sci Cryst Eng Mater ; 76(Pt 1): 137-143, 2020 Feb 01.
Artigo em Inglês | MEDLINE | ID: mdl-32831248

RESUMO

The crystal structure of a new superconductor UTe2 has been investigated using single-crystal neutron diffraction for the first time at the low temperature (LT) of 2.7 K, just above the superconducting transition temperature of ∼1.6 K, in order to clarify whether the orthorhombic structure of type Immm (No. 71), reported for the room-temperature (RT) structure persists down to the superconducting phase and can be considered as a parent symmetry for the development of spin-triplet superconductivity. In contrast to the previously reported phase transition at about 100 K [Stöwe (1996). J. Solid State Chem. 127, 202-210], our high-precision LT neutron diffraction data show that the body-centred RT symmetry is indeed maintained down to 2.7 K. No sign of a structural change from RT down to 2.7 K was observed. The most significant change depending on temperature was observed for the U ion position and the U-U distance along the c direction, implying its potential importance as a magnetic interaction path. No magnetic order could be deduced from the neutron diffraction data refinement at 2.7 K, consistent with bulk magnetometry. Assuming normal thermal evolution of the lattice parameters, moderately large linear thermal expansion coefficients of about α = 2.8 (7) × 10-5 K-1 are estimated.

7.
Nature ; 579(7800): 523-527, 2020 03.
Artigo em Inglês | MEDLINE | ID: mdl-32214254

RESUMO

Spin-triplet superconductors are condensates of electron pairs with spin 1 and an odd-parity wavefunction1. An interesting manifestation of triplet pairing is the chiral p-wave state, which is topologically non-trivial and provides a natural platform for realizing Majorana edge modes2,3. However, triplet pairing is rare in solid-state systems and has not been unambiguously identified in any bulk compound so far. Given that pairing is usually mediated by ferromagnetic spin fluctuations, uranium-based heavy-fermion systems containing f-electron elements, which can harbour both strong correlations and magnetism, are considered ideal candidates for realizing spin-triplet superconductivity4. Here we present scanning tunnelling microscopy studies of the recently discovered heavy-fermion superconductor UTe2, which has a superconducting transition temperature of 1.6 kelvin5. We find signatures of coexisting Kondo effect and superconductivity that show competing spatial modulations within one unit cell. Scanning tunnelling spectroscopy at step edges reveals signatures of chiral in-gap states, which have been predicted to exist at the boundaries of topological superconductors. Combined with existing data that indicate triplet pairing in UTe2, the presence of chiral states suggests that UTe2 is a strong candidate for chiral-triplet topological superconductivity.

8.
Phys Rev Lett ; 124(7): 076401, 2020 Feb 21.
Artigo em Inglês | MEDLINE | ID: mdl-32142327

RESUMO

The compound UTe_{2} has recently been shown to realize spin triplet superconductivity from a nonmagnetic normal state. This has sparked intense research activity, including theoretical analyses that suggest the superconducting order parameter to be topologically nontrivial. However, the underlying electronic band structure is a critical factor for these analyses, and remains poorly understood. Here, we present high resolution angle-resolved photoemission measurements covering multiple planes in the 3D Brillouin zone of UTe_{2}, revealing distinct Fermi-level features from two orthogonal quasi-one-dimensional light electron bands and one heavy band. The electronic symmetries are evaluated in comparison with numerical simulations, and the resulting picture is discussed as a platform for unconventional many-body order.

9.
Commun Phys ; 32020.
Artigo em Inglês | MEDLINE | ID: mdl-33655080

RESUMO

Spin-fluctuation-mediated unconventional superconductivity can emerge at the border of magnetism, featuring a superconducting order parameter that changes sign in momentum space. Detection of such a sign-change is experimentally challenging, since most probes are not phase-sensitive. The observation of a spin resonance mode (SRM) from inelastic neutron scattering is often seen as strong phase-sensitive evidence for a sign-changing superconducting order parameter, by assuming the SRM is a spin-excitonic bound state. Here we show that for the heavy fermion superconductor CeCoIn5, its SRM defies expectations for a spin-excitonic bound state, and is not a manifestation of sign-changing superconductivity. Instead, the SRM in CeCoIn5 likely arises from a reduction of damping to a magnon-like mode in the superconducting state, due to its proximity to magnetic quantum criticality. Our findings emphasize the need for more stringent tests of whether SRMs are spin-excitonic, when using their presence to evidence sign-changing superconductivity.

10.
Phys Rev B ; 101(14)2020 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-34131608

RESUMO

Spin triplet superconductivity in the Kondo lattice UTe2 appears to be associated with spin fluctuations originating from incipient ferromagnetic order. Here we show clear evidence of twofold enhancement of superconductivity under pressure, which discontinuously transitions to magnetic order, likely of ferromagnetic nature, at higher pressures. The application of a magnetic field tunes the system back across a first-order phase boundary. Straddling this phase boundary, we find another example of reentrant superconductivity in UTe2. As the superconductivity and magnetism exist on two opposite sides of the first-order phase boundary, our results indicate other microscopic mechanisms may be playing a role in stabilizing spin triplet superconductivity in addition to spin fluctuations associated with magnetism.

11.
Sci Adv ; 5(10): eaaw9061, 2019 10.
Artigo em Inglês | MEDLINE | ID: mdl-31667341

RESUMO

Novel electronic phenomena frequently form in heavy-fermions because of the mutual localized and itinerant nature of f-electrons. On the magnetically ordered side of the heavy-fermion phase diagram, f-moments are expected to be localized and decoupled from the Fermi surface. It remains ambiguous whether Kondo lattice can develop inside the magnetically ordered phase. Using spectroscopic imaging with scanning tunneling microscope, complemented by neutron scattering, x-ray absorption spectroscopy, and dynamical mean field theory, we probe the electronic states in antiferromagnetic USb2. We visualize a large gap in the antiferromagnetic phase within which Kondo hybridization develops below ~80 K. Our calculations indicate the antiferromagnetism and Kondo lattice to reside predominantly on different f-orbitals, promoting orbital selectivity as a new conception into how these phenomena coexist in heavy-fermions. Finally, at 45 K, we find a novel first order-like transition through abrupt emergence of nontrivial 5f-electronic states that may resemble the "hidden-order" phase of URu2Si2.

12.
Science ; 365(6454): 684-687, 2019 08 16.
Artigo em Inglês | MEDLINE | ID: mdl-31416960

RESUMO

Spin-triplet superconductors potentially host topological excitations that are of interest for quantum information processing. We report the discovery of spin-triplet superconductivity in UTe2, featuring a transition temperature of 1.6 kelvin and a very large and anisotropic upper critical field exceeding 40 teslas. This superconducting phase stability suggests that UTe2 is related to ferromagnetic superconductors such as UGe2, URhGe, and UCoGe. However, the lack of magnetic order and the observation of quantum critical scaling place UTe2 at the paramagnetic end of this ferromagnetic superconductor series. A large intrinsic zero-temperature reservoir of ungapped fermions indicates a highly unconventional type of superconducting pairing.

13.
Nat Commun ; 10(1): 644, 2019 02 07.
Artigo em Inglês | MEDLINE | ID: mdl-30733441

RESUMO

Uranium compounds can manifest a wide range of fascinating many-body phenomena, and are often thought to be poised at a crossover between localized and itinerant regimes for 5f electrons. The antiferromagnetic dipnictide USb2 has been of recent interest due to the discovery of rich proximate phase diagrams and unusual quantum coherence phenomena. Here, linear-dichroic X-ray absorption and elastic neutron scattering are used to characterize electronic symmetries on uranium in USb2 and isostructural UBi2. Of these two materials, only USb2 is found to enable strong Hund's rule alignment of local magnetic degrees of freedom, and to undergo distinctive changes in local atomic multiplet symmetry across the magnetic phase transition. Theoretical analysis reveals that these and other anomalous properties of the material may be understood by attributing it as the first known high temperature realization of a singlet ground state magnet, in which magnetism occurs through a process that resembles exciton condensation.

14.
Nat Phys ; 15(12)2019.
Artigo em Inglês | MEDLINE | ID: mdl-34131432

RESUMO

Applied magnetic fields underlie exotic quantum states, such as the fractional quantum Hall effect1 and Bose-Einstein condensation of spin excitations2. Superconductivity, however, is inherently antagonistic towards magnetic fields. Only in rare cases3-5 can these effects be mitigated over limited fields, leading to re-entrant superconductivity. Here, we report the coexistence of multiple high-field re-entrant superconducting phases in the spin-triplet superconductor UTe2 (ref. 6). We observe superconductivity in the highest magnetic field range identified for any re-entrant superconductor, beyond 65 T. Although the stability of superconductivity in these high magnetic fields challenges current theoretical models, these extreme properties seem to reflect a new kind of exotic superconductivity rooted in magnetic fluctuations7 and boosted by a quantum dimensional crossover8.

15.
Phys Rev B ; 100(22)2019 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-34136735

RESUMO

Low-temperature electrical and thermal transport, magnetic penetration depth, and heat capacity measurements were performed on single crystals of the actinide superconductor UTe2 to determine the structure of the superconducting energy gap. Heat transport measurements performed with currents directed along both crystallographic a and b axes reveal a vanishingly small residual fermionic component of the thermal conductivity. The magnetic field dependence of the residual term follows a rapid, quasilinear increase consistent with the presence of nodal quasiparticles, rising toward the a-axis upper critical field where the Wiedemann-Franz law is recovered. Together with a quadratic temperature dependence of the magnetic penetration depth up to T/T c = 0.3, these measurements provide evidence for an unconventional spin-triplet superconducting order parameter with point nodes. Millikelvin specific heat measurements performed on the same crystals used for thermal transport reveal an upturn below 300 mK that is well described by a divergent quantum-critical contribution to the density of states (DOS). Modeling this contribution with a T -1/3 power law allows restoration of the full entropy balance in the superconducting state and a resultant cubic power law for the electronic DOS below T c , consistent with the point-node gap structure determined by thermal conductivity and penetration depth measurements.

16.
Proc Natl Acad Sci U S A ; 115(34): 8558-8562, 2018 08 21.
Artigo em Inglês | MEDLINE | ID: mdl-30082391

RESUMO

We report anomalous physical properties of high-quality single-crystalline FeSi over a wide temperature range of 1.8-400 K. The electrical resistivity ρ(T) can be described by activated behavior with an energy gap Δ = 57 meV between 150 and 67 K, below which the estimated energy gap is significantly smaller. The magneto-resistivity and Hall coefficient change sign in the vicinity of 67 K, suggesting a change of dominant charge carriers. At ∼19 K, ρ(T) undergoes a cross-over from semiconducting to metallic behavior which is very robust against external magnetic fields. The low-temperature metallic conductivity depends strongly on the width/thickness of the sample. In addition, no indication of a bulk-phase transition or onset of magnetic order is found down to 2 K from specific heat and magnetic susceptibility measurements. The measurements are consistent with one another and point to complex electronic transport behavior that apparently involves a conducting surface state in FeSi at low temperatures, suggesting the possibility that FeSi is a 3D topological insulator.

17.
Proc Natl Acad Sci U S A ; 114(37): 9826-9831, 2017 09 12.
Artigo em Inglês | MEDLINE | ID: mdl-28847927

RESUMO

Electrical transport measurements were performed on URu2 - x Fe x Si2 single-crystal specimens in high magnetic fields up to 45 T (DC fields) and 60 T (pulsed fields). We observed a systematic evolution of the critical fields for both the hidden-order (HO) and large-moment antiferromagnetic (LMAFM) phases and established the 3D phase diagram of T-H-x In the HO phase, H/H0 scales with T/T0 and collapses onto a single curve. However, in the LMAFM phase, this single scaling relation is not satisfied. Within a certain range of x values, the HO phase reenters after the LMAFM phase is suppressed by the magnetic field, similar to the behavior observed for URu2Si2 within a certain range of pressures.

18.
Proc Natl Acad Sci U S A ; 113(47): 13348-13353, 2016 11 22.
Artigo em Inglês | MEDLINE | ID: mdl-27830647

RESUMO

Thermal expansion, electrical resistivity, magnetization, and specific heat measurements were performed on URu2-xFexSi2 single crystals for various values of Fe concentration x in both the hidden-order (HO) and large-moment antiferromagnetic (LMAFM) regions of the phase diagram. Our results show that the paramagnetic (PM) to HO and LMAFM phase transitions are manifested differently in the thermal expansion coefficient. The uniaxial pressure derivatives of the HO/LMAFM transition temperature T0 change dramatically when crossing from the HO to the LMAFM phase. The energy gap also changes consistently when crossing the phase boundary. In addition, for Fe concentrations at xc ≈ 0.1, we observe two features in the thermal expansion upon cooling, one that appears to be associated with the transition from the PM to the HO phase and another one at lower temperature that may be due to the transition from the HO to the LMAFM phase.

19.
Zhongguo Zhen Jiu ; 34(8): 813-6, 2014 Aug.
Artigo em Chinês | MEDLINE | ID: mdl-25335268

RESUMO

ZHU Lian is a deceased famous acupuncture and Moxibustion specialist, the first director and the founder of institute of Acupuncture-Moxibustion of China Academy of Chinese Medical Sciences. This article discusses the thought and idea of education and teaching of acupuncture-moxibustion from the following three aspects: diversified education and training mode, teaching idea of new acupuncture-moxibustion with a lot of characteristics, and the founding of professional acupuncture-moxibustion college. All above have both distinct characteristics of the times and positively enlightening significance of reality.


Assuntos
Acupuntura/educação , Acupuntura/história , Terapia por Acupuntura/história , China , Educação Médica/história , História do Século XX , Humanos , Ensino/história
20.
Nat Commun ; 5: 3229, 2014.
Artigo em Inglês | MEDLINE | ID: mdl-24499997

RESUMO

Many of the iron pnictides have strongly anisotropic normal-state characteristics, important for the exotic magnetic and superconducting behaviour these materials exhibit. Yet, the origin of the observed anisotropy is unclear. Electronically driven nematicity has been suggested, but distinguishing this as an independent degree of freedom from magnetic and structural orders is difficult, as these couple together to break the same tetragonal symmetry. Here we use time-resolved polarimetry to reveal critical nematic fluctuations in unstrained Ba(Fe1-xCox)2As2. The femtosecond anisotropic response, which arises from the two-fold in-plane anisotropy of the complex refractive index, displays a characteristic two-step recovery absent in the isotropic response. The fast recovery appears only in the magnetically ordered state, whereas the slow one persists in the paramagnetic phase with a critical divergence approaching the structural transition temperature. The dynamics also reveal a gigantic magnetoelastic coupling that far exceeds electron-spin and electron-phonon couplings, opposite to conventional magnetic metals.

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