Anne Marie de Visser

Direct bulk-sensitive probe of 5f symmetry in URu2Si2

Significance The hidden order problem in URu2Si2 is an unanswered question in the field of strongly correlated electron materials. Although it has been studied for several decades, there is still no consensus about how this new phase forms. Understanding the hidden order phase formation is not only an intellectual problem, it will also advance concepts for designing quantum materials with new exotic properties. Many hidden order scenarios are based on the assumption of certain ground-state symmetries and the present study addresses this aspect. A spectroscopic technique, nonresonant inelastic X-ray scattering, that has become available through the use of high-brilliance synchrotrons, allows us to measure directly in a bulk-sensitive experiment the symmetry of the 5f ground-state wave function in URu2Si2. The second-order phase transition into a hidden order phase in URu2Si2 goes along with an order parameter that is still a mystery, despite 30 years of research. However, it is understood that the symmetry of the order parameter must be related to the symmetry of the low-lying local electronic f-states. Here, we present results of a spectroscopic technique, namely core-level nonresonant inelastic X-ray scattering (NIXS). This method allows for the measurement of local high-multipole excitations and is bulk-sensitive. The observed anisotropy of the scattering function unambiguously shows that the 5f ground-state wave function is composed mainly of the Γ1 with majority Jz = |4⟩ + |−4⟩ and/or Γ2 singlet states. The incomplete dichroism indicates the possibility that quantum states of other irreducible representation are mixed into the ground state.

Evolution of magnetism and its interplay with superconductivity in heavy-fermion UPt3 doped with Pd

The pseudobinary series U(Pt1� xPdx)3 demonstrates a wealth of magnetic and superconducting properties that are exemplary for heavy-fermion physics. In this paper, I present a survey of recent neutron-diffraction and mSR experiments, conducted to study the evolution of magnetism, and its interplay with superconductivity, in UPt3 doped with Pd. The small-moment antiferromagnetic order (SMAF) with TNB6 K reported for pure UPt3, is robust upon alloying till at least x ¼ 0:005: The small ordered moment mðxÞ grows from 0.018mB/U-atom for x ¼ 0:00 to 0.048mB/ U-atom for x ¼ 0:005: TN of the SMAF phase does not vary with Pd content. The increase of mðxÞ correlates with the splitting DTc of the superconducting transition and provides evidence for a Ginzburg–Landau scenario for unconventional superconductivity with magnetism as symmetry-breaking field. The absence of a signal of the SMAF phase in zero-field mSR spectra provides strong evidence for a moment fluctuating at a rate >10 MHz. A second largemoment antiferromagnetic phase (LMAF) is found at higher Pd concentrations. For this phase, at optimum doping (x ¼ 0:05) TN;max ¼ 5:8 K and m ¼ 0:62mB/U-atom. The critical Pd concentration for the emergence of the LMAF phase is xc;afB0:006: At the same Pd content, superconductivity is completely suppressed. The existence of a magnetic quantum critical point in the phase diagram, which coincides with the critical point for superconductivity (xc;af ¼ xc;scE0:006), yields evidence for odd-parity superconductivity mediated by ferromagnetic spinfluctuations. r 2002 Elsevier Science B.V. All rights reserved.

4π-periodic Andreev bound states in a Dirac semimetal

Although signatures of superconductivity in Dirac semimetals have been reported, for instance by applying pressure or using point contacts, our understanding of the topological aspects of Dirac semimetal superconductivity is still developing. Here, we utilize nanoscale phase-sensitive junction technology to induce superconductivity in the Dirac semimetal Bi1−xSbx. Our radiofrequency irradiation experiments then reveal a significant contribution of 4π-periodic Andreev bound states to the supercurrent in Nb–Bi0.97Sb0.03–Nb Josephson junctions. The conditions for a substantial 4π contribution to the supercurrent are favourable because of the Dirac cone’s very broad transmission resonances and a measurement frequency faster than the quasiparticle poisoning rate. In addition, we show that a magnetic field applied in the plane of the junction allows tuning of the Josephson junctions from 0 to π regimes. Our results open the technologically appealing avenue of employing the topological bulk properties of Dirac semimetals for topological superconductivity research and topological quantum computer development.Superconductivity is induced in an accidental Dirac semimetal via the proximity effect.

Composition induced metal-insulator quantum phase transition in the Heusler type Fe2VAl

We report the magnetism and transport properties of the Heusler compound Fe2+x V1-x Al at  -0.10  ⩽  x  ⩽  0.20 under pressure and a magnetic field. A metal-insulator quantum phase transition occurred at x  ≈  -0.05. Application of pressure or a magnetic field facilitated the emergence of finite zero-temperature conductivity σ 0 around the critical point, which scaled approximately according to the power law (P  -  P c ) (γ) . At x  ⩽  -0.05, a localized paramagnetic spin appeared, whereas above the ferromagnetic quantum critical point at x  ≈  0.05, itinerant ferromagnetism was established. At the quantum critical points at x  =  -0.05 and 0.05, the resistivity and specific heat exhibited singularities characteristic of a Griffiths phase appearing as an inhomogeneous electronic state.

Evolution of the spin-split quantum Hall states with magnetic field tilt in the InAs-based double quantum wells

Development of quantum Hall peculiarities due to mobility gap between spin-split magnetic levels with addition of the parallel magnetic field component B|| is analyzed in double quantum wells (DQW) created in InGaAs/GaAs and InAs/AlSb heterosystems chosen due to their relatively large bulk g-factors. In InGaAs/GaAs DQWs, the nonmonotonous behavior of these peculiarities is observed and explained within single-electron approach in terms of competition between enhanced spin splitting and localization of electrons in the layers of DQW with increased B||. In InAs/AlSb DQW, the tunneling connection between the layers is very weak due to high barrier, nevertheless the collective odd-numbered peculiarities are revealed that exist due to spontaneous interlayer phase coherence. B|| destroys these states that is manifested, in particular, in the suppression of the peculiarity for filling factor v = 3.

Magnetic-field-induced quantum Hall—insulator transition and persistent photoconductivity in InAs/GaAs quantum dot layers

We have investigated the temperature dependence of resistance in the temperature range T=0.07–300K and in magnetic field up to 35T in InAs/GaAs quantum dot layers. In samples with relatively high carrier concentration quantum Hall effect—insulator transition was observed in high magnetic fields. Two-dimensional Mott variable range hopping conductivity has been observed at low temperatures in samples with low carrier concentration. The length of localization correlates very well with the quantum dot cluster size obtained by atomic force microscope. In all samples a positive persistent photoconductivity was observed.

Transport and Thermal Properties of Some Selected Heavy-Fermion Materials: Probing the Electronic Instability

In the past decade it has been recognized that the deloacalization of 4f or 5f electrons in cerium or uranium intermetallics occasionally gives rise to the low-temperature formation of a strongly correlated electron band close to the Fermi level. The unprecedented strong renormalization is reflected in a Fermi-liquid quasiparticle mass of the order of 100 times the free electron mass. Concurrently, the Fermi-liquid temperature is strongly renormalized: T F ~ 100 K. The enormous effective mass is built up by a variety of competing electron-electron interactions, of which the on-site Kondo effect and the inter-site Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions are thought to play the leading parts. The stabilization of the heavy-fermion state is attended by striking anomalies in the thermal and transport properties. As to the thermal properties, spin degrees of freedom turn up predominantly in the low-temperature entropy and enhance the electronic specific heat accordingly, while the strains are tightly coupled to the heavy-electron bands via anomalously large Gruneisen parameters giving rise to large coefficients of thermal expansion. As far as the transport properties are concerned, a rapid variation of the scattering processes takes place at low temperatures, as a consequence of the competing on-site and inter-site interactions. This is manifested in the electrical resistivity as a crossover from the Kondo-esque increase to the Fermi-liquid-like decrease in the coherent state, at lowering the temperature. At studying the physics of correlated electrons in heavy-fermion materials, the influence of clean external parameters, e.g. pressure and magnetic field, has received a wide attention. The pressure effects on the heavy-fermion bands are unusually large because of the strong hybridization. However, the influence of a magnetic field is relatively small, so that very strong magnetic fields are needed in order to investigate the heavy-fermion state.

Interaction between counter-propagating quantum Hall edge channels in the 3D topological insulator BiSbTeSe2

The quantum Hall effect is studied in the topological insulator BiSbTeSe2. By employing top- and back-gate electric fields at high magnetic field, the Landau levels of the Dirac cones in the top and bottom topological surface states can be tuned independently. When one surface is tuned to the electron-doped side of the Dirac cone and the other surface to the hole-doped side, the quantum Hall edge channels are counter-propagating. The opposite edge mode direction, combined with the opposite helicities of top and bottom surfaces, allows for scattering between these counter-propagating edge modes. The total Hall conductance is expected to be integer valued only when the scattering is strong. For weaker interaction, a noninteger quantum Hall effect is expected and indications for this effect are measured.