J. E. Lorenzo

Synchrotron X-ray powder diffraction studies in pulsed magnetic fields

X-ray powder diffraction experiments under pulsed magnetic fields were carried out at the DUBBLE beamline (BM26B) at the ESRF. A mobile generator delivered 110kJ to the magnet coil, which was sufficient to generate peak fields of 30T. A liquid He flow cryostat allowed us to vary the sample temperature accurately between 8 and 300K. Powder diffraction patterns of several samples were recorded using 21keV monochromatic x-rays and an on-line image plate detector. Here we present the first results on the suppression of the Jahn-Teller structural distortion in TbVO4 by magnetic field. These data clearly demonstrate the feasibility of x-ray powder diffraction experiments under pulsed magnetic fields with relatively inexpensive instrumentation.

A 30 T pulsed magnet with conical bore for synchrotron powder diffraction

We report on the design, construction, and operation of a horizontal field, 30 T magnet system with a conical bore optimized for synchrotron x-ray powder diffraction. The magnet offers ±31° optical access downstream of the sample, which allows to measure a sufficiently large number of Debye rings for an accurate crystal structure analysis. Combined with a 290 kJ generator, magnetic field pulses of 60 ms length were generated in the magnet, with a rise time of 4.1 ms and a repetition rate of 6 pulses/h at 30 T. The coil is mounted inside a liquid nitrogen bath. A liquid helium flow cryostat reaches into the coil and allows sample temperature between 5 and 250 K. The setup was used on the European Synchrotron Radiation Facility beamlines ID20 and ID06.

Direct observation of the high magnetic field effect on the Jahn-Teller state in TbVO4

We report the first direct observation of the influence of high magnetic fields on the Jahn-Teller (JT) transition in TbVO(4). Contrary to spectroscopic and magnetic methods, x-ray diffraction directly measures the JT distortion; the splitting between the (311)/(131) and (202)/(022) pairs of Bragg reflections is proportional to the order parameter. Our experimental results are compared to mean-field calculations, taking into account all possible orientations of the grains relative to the applied field, and qualitative agreement is obtained.

Effect of dimensionality on sliding charge density waves: The quasi-two-dimensional TbTe 3 system probed by coherent x-ray diffraction

We report on sliding Charge Density Wave (CDW) in the quasi two-dimensional TbTe3 system probed by coherent x-ray diffraction combined with in-situ transport measurements. We show that the non-Ohmic conductivity in TbTe3 is made possible thanks to a strong distortion of the CDW. Our diffraction experiment versus current shows first that the CDW remains undeformed below the threshold current IS and then suddenly rotates and reorders by motion above threshold. Contrary to quasi-one dimensional systems, the CDW in TbTe3 does not display any phase shifts below IS and tolerates only slow spatial variations of the phase above. This is a first observation of CDW behavior in the bulk in a quasi-two dimensional system allowing collective transport of charges at room temperature. Interaction between pairs of quasiparticles often leads to broken-symmetry ground states in solids. Typical examples are the formation of Cooper pairs in supercon-ductors, charge-density waves (CDWs) and spin-density waves driven by electron-phonon or electron-electron interactions[1]. The CDW ground state is characterized by a spatial modulation η cos(2k F x + φ) of the electron density and a concomitant periodic lattice distortion with the same 2k F wave vector leading to a gap opening in the electron spectrum. The first CDW systems were discovered in the beginning of the 70s in two-dimensional transition metal dichalcogenides MX 2 [2]. CDW state was then discovered in quasi-one dimensional systems like NbSe 3 , TaS 3 , the blue bronze K 0.3 MoO 3 and in organic compounds like TTF-TNCQ. However, the most remarkable property of a CDW has been discovered a few years later in quasi one-dimensional systems: a CDW may slide carrying correlated charges[3]. The sliding mode is achieved when an electric field applied to the sample is larger than a threshold value, manifesting then collective Frohlich-type transport. This sliding phenomenon is clearly observed by transport measurements. The differential resistance remains constant up to a threshold current and then decreases for larger currents in addition to the generation of an ac voltage, the frequency of which increases with the applied current[3]. In spite of numerous studies, the physical mechanism leading to the sliding phenomenon is still far to be fully understood. One of the difficulties comes from the fact that the sliding mode displays two different aspects. On the one hand, the CDW is a classical state, similar to an elastic object in presence of disorder[4], displaying creep, memory effects and hysteresis[5, 6]. On the other hand, a CDW is a macroscopic quantum state[7], carrying charges by tunneling through disorder[8] and displaying Aharonov-Bohm effects[9] over microscopic distances[10]. Recently a new class of quasi-two dimensional CDW compounds, rare-earth tritellurides RTe 3 , have raised an intense research activity thanks to their peculiar properties[11-13]. RTe 3 structures are orthorhombic (Cmcm) but the a and c lattice parameters lying in the Te planes are almost equal (c-a=0.002A002A with a=4.307A307A for TbTe 3 at T=300K) and the double Te-layers are linked together by a c-glide plane. The almost square Te sheets lead to nearly isotropic properties in the (a,c) plane. The resistance measured along a and c differs by only 10% at 300K in TbTe 3 [14] and the Fermi surface displays an almost square-closed shape in the (a*,c*) plane[15]. These quasi-two dimensional systems exhibit a unidirectional CDW wave vector along c* (2k F ∼ 2/7 c* in TbTe 3) and a surprisingly large Peierls transition temperature, around 300 K, through the whole R-series and above for lighter rare-earth elements. The stabilization of the CDW in TbTe 3 over the almost square underlying atomic lattice is reminiscent of copper-oxide planes in high temperature superconductors in which a CDW state was also recently observed[16]. However, the most surprising property of TbTe 3 is its ability to displays non-linear transport[17] despite the two-dimensional character of the atomic structure. The aim of the present work is to show that, despite similar resistivity curves, the depining process in quasi one and two-dimensional systems are quite different. For that purpose, coherent x-ray diffraction has been used to study the behavior of the 2k F satellite reflection upon application of an external current. As the sliding state of a CDW mainly involves fluctuations of the CDW phase to overcome pinning centers , coherent x-ray diffraction is a suitable technique thanks to its high sensitivity to the phase of any modulation. The extreme case of a single phase shift, such

40-tesla pulsed-field cryomagnet for single crystal neutron diffraction

We present the first long-duration and high duty cycle 40-T pulsed-field cryomagnet addressed to single crystal neutron diffraction experiments at temperatures down to 2 K. The magnet produces a horizontal field in a bi-conical geometry, ±15° and ±30° upstream and downstream of the sample, respectively. Using a 1.15 MJ mobile generator, magnetic field pulses of 100 ms length are generated in the magnet, with a rise time of 23 ms and a repetition rate of 6-7 pulses per hour at 40 T. The setup was validated for neutron diffraction on the CEA-CRG three-axis spectrometer IN22 at the Institut Laue Langevin.

Synchrotron X-ray Powder Diffraction and Absorption Spectroscopy in Pulsed Magnetic Fields with Milliseconds Duration

X‐ray Powder Diffraction and X‐ray Absorption Spectroscopy experiments (WAS) and X‐ray magnetic circular dichroism (XMCD) experiments were carried out at the ESRF DUBBLE beam line (BM26) and at the energy dispersive beam line (ID24), respectively. A mobile pulse generator, developed at the LNCMP, delivered 110kJ to the load coil, which was sufficient to generate peak fields of 30T with a rise time of about 5 ms. A liquid He flow cryostat allowed us to vary the sample temperature accurately between 4.2K and 300K.Powder diffraction patterns of TbVO4 were recorded in a broad temperature range using 21 keV monochromatic X‐rays and using an on‐line image plate detector. We observed the suppression of the Jahn‐Teller structural distortion in TbVO4 due to the high magnetic pulsed field.XAS spectra could be measured and finite XMCD signals, directly proportional to the magnetic moment on the Gd absorber atom, were measured in thin Gd foils. Thanks to its element and orbital selectivity, XMCD proofs to be very use...

Pulsed magnetic field synchrotron X-ray powder diffraction of the Jahn-Teller distortion in TbVO4

X-ray powder diffraction experiments under pulsed magnetic fields were carried out at the DUBBLE beam line at the ESRF. A mobile generator delivered 110 kJ to the load coil, which was sufficient to generate peak fields of 30 T. A liquid He flow cryostat allowed us to vary the sample temperature accurately between 8 K and 300 K. Powder diffraction patterns of TbVO4 were recorded in a broad temperature range using 21 keV monochromatic X-rays and an on-line image plate detector. We present results on the suppression of the Jahn-Teller structural distortion in TbVO4by to the magnetic field.

Synchrotron X‐ray Powder Diffraction Studies in Pulsed Magnetic Fields

X‐ray powder diffraction experiments under pulsed magnetic fields were carried out at the DUBBLE beamline (BM26B) at the ESRF. A mobile generator delivered 110kJ to the magnet coil, which was sufficient to generate peak fields of 30T. A liquid He flow cryostat allowed us to vary the sample temperature accurately between 8K and 300K.