Jens Jensen

Jens Jensen

*This is an algorithmically generated author record based on publicly available information.

Jens Jensen has a total of 202 co-authors. Here are the top co-authors by number of shared publications:


Latest Publications

  • Open Access

    Inducing Ferroquadrupolar Order with Applied Magnetic Field in TbPO 4

    by Paola Caterina Forino, Jens Jensen, Jian Rui Soh, Sonia Francoual, Oksana Zaharko, Steffen Sloth, Alexandra Turrini, Ralf Feyerherm, Karel Prokes, Sofie Holm-Janas, et al.

    In this work, we demonstrate a remarkable field-induced ferroquadrupolar phase emanating from the magnetoelastic coupling. Using synchrotron x-ray and neutron diffraction, heat capacity measurements, as well as mean-field modeling, we show that above the critical magnetic field, ferroquadrupolar O 2 − 2 order is stabilized as the main order parameter, in competition with the field-induced magnetic polarization. This is revealed by a severe lattice distortion and the suppression of antiferromagnetic ordering of the dipole moments. Even in zero magnetic field, the previously reported antiferromagnetic order is a mixed quadrupolar-dipolar phase, further highlighting the crucial role of spin-lattice interactions. This complete understanding of the magnetoelastic phase diagram establishes TbPO 4 as a model system for studying nematic-antiferromagnetic transitions and provides key Hamiltonian parameters for its description.


  • Magnetoelastic phase diagram ofTbNi2B2C

    by Rasmus Toft-Petersen, Thomas Bagger Stibius Jensen, Jens Jensen, Martin von Zimmermann, Steffen Sloth, Frederik Werner Isaksen, Niels Bech Christensen, Yunzhong Chen, Konrad Siemensmeyer, Hazuki Kawano-Furukawa, et al.

    The magnetic phase diagram of the quaternary borocarbide TbNi2B2C is investigated by direct means and by studying magnetically induced modifications of the crystal structure. Detailed superconducting quantum interference device measurements reveal a complex phase diagram with five distinct magnetic phases. The phase boundaries are mapped out comprehensively. Synchrotron hard x-ray measurements in applied magnetic fields are employed to probe the magnetoelastic distortions throughout the phase diagram. The determination of the wave vectors of these field-induced lattice deformations suggests a range of commensurate spin-slip-type magnetic structures at low temperatures with wave vectors of the form (q,0,0) with q=6/11 and 5/9. The proposed magnetic structures yield values of magnetization well in-line with observations. The scattering intensity due to the magnetoelastic deformations exhibits a drastic jump at the phase boundary at 1.3 T and low temperatures.


  • Comprehensive cluster-theory analysis of the magnetic structures and excitations inCoCl2·2H2O

    by Jens Jensen, Jacob Larsen, Ursula B. Hansen

    The magnetic properties of CoCl2·2H2O are analyzed in the mean-field/random-phase approximation using a basis of clusters with four spins along the c-axis chains of Co ions. The model gives a unifying account of the bulk properties, the spin waves, and the higher-order cluster-spin excitations. The theory describes accurately the neutron scattering measurements of the excited doublet of the S=3/2Co++ ions in both the antiferromagnetic and the paramagnetic phases. The theory has been applied by Larsen et al. [ Phys. Rev. B 96, 174424 (2017)] for analyzing the quantum phase transition at a transverse field of 160 kOe and is found to agree closely with their observations.


  • Open Access

    Spin excitations and quantum criticality in the quasi-one-dimensional Ising-like ferromagnetCoCl2·2D2Oin a transverse field

    by J. Larsen, T. K. Schäffer, U. B. Hansen, S. L. Holm, S. R. Ahl, R. Toft-Petersen, J. Taylor, G. Ehlers, J. Jensen, H. M. Rønnow, et al.

    We present experimental evidence for a quantum phase transition in the easy-axis S=3/2 anisotropic quasi-one-dimensional ferromagnet CoCl2·2D2O in a transverse field. Elastic neutron scattering shows that the magnetic order parameter vanishes at a transverse critical field μ0Hc=16.05(4) T, while inelastic neutron scattering shows that the gap in the magnetic excitation spectrum vanishes at the same field value, and reopens for H>Hc. The field dependence of the order parameter and the gap are well described by critical exponents β=0.45±0.09 and zν close to 1/2, implying that the quantum phase transition in CoCl2·2D2O differs significantly from the textbook version of a S=1/2 Ising chain in a transverse field. We attribute the difference to weak but finite three-dimensionality of the magnetic interactions.


  • Open Access

    Field-induced reentrant magnetoelectric phase in LiNiPO4

    by Rasmus Toft-Petersen, Ellen Fogh, Takumi Kihara, Jens Jensen, Katharina Fritsch, Jooseop Lee, Garrett E. Granroth, Matthew B. Stone, David Vaknin, Hiroyuki Nojiri, et al.

    Using pulsed magnetic fields up to 30T we have measured the bulk magnetization and electrical polarization of LiNiPO4 and have studied its magnetic structure by time-of-flight neutron Laue diffraction. Our data establish the existence of a reentrant magnetoelectric phase between 19T and 21T. We show that a magnetized version of the zero field commensurate structure explains the magnetoelectric response quantitatively. The stability of this structure suggests a field-dependent spin anisotropy. Above 21T, a magnetoelectrically inactive, short-wavelength incommensurate structure is identified. Our results demonstrate the combination of pulsed fields with epithermal neutron Laue diffraction as a powerful method to probe even complex phase diagrams in strong magnetic fields.


  • Phase diagram of diluted Ising ferromagnetLiHoxY1−xF4

    by P. Babkevich, N. Nikseresht, I. Kovacevic, J. O. Piatek, B. Dalla Piazza, C. Kraemer, K. W. Krämer, K. Prokeš, S. Mat'AŠ, J. Jensen, et al.

    We present a systematic study of the phase diagram of LiHoxY1−xF4 (0.25≤x≤1) Ising ferromagnets obtained from neutron scattering measurements and mean-field calculations. We show that while the thermal phase transition decreases linearly with dilution, as predicted by mean-field theory, the critical transverse field at the quantum critical point is suppressed much faster. This behavior is related to competition between off-diagonal dipolar coupling and quantum fluctuations that are tuned by doping and applied field, respectively. In this paper, we quantify the deviation of the experimental results from mean-field predictions, with the aim that this analysis can be used in future theoretical efforts towards a quantitative description.


  • Open Access

    Low-energy spin-wave excitations in amplitude-modulated magnetic structure of PrNi2Si2

    by J A Blanco, B Fåk, J Jensen, M Rotter, A Hiess, D Schmitt, P Lejay

    Inelastic neutron scattering (INS) experiments and random phase approximation calculations have been used to investigate the low-energy spin-wave excitations in PrNi2Si2. The modulated magnitude of the ordered magnetic moments of Pr 3+ ions implies that the associate, longitudinally polarized magnetic excitations are more intense and dispersive than the usual transverse spin waves. Within the random phase approximation the results are in good overall agreement with the predictions made by the model determined previously from the paramagnetic excitations. The most unusual observation is the well-defined amplitude mode detected close to the magnetic Bragg point existing simultaneously with the phason mode. At low energies, an extra mode is observed to hybridize with the magnetic phasons in the neighborhood of the magnetic Brillouin zone center. A magnetoelastic interaction between the magnetic excitations and the longitudinal phonons is able to explain part of the disturbances, but it is concluded that the extra mode must be of some other, unknown origin.


  • Neutron spectroscopic study of crystal-field excitations and the effect of the crystal field on dipolar magnetism inLiRF4(R=Gd, Ho, Er, Tm, and Yb)

    by P. Babkevich, A. Finco, M. Jeong, B. Dalla Piazza, I. Kovacevic, G. Klughertz, K. W. Krämer, C. Kraemer, D. T. Adroja, E. Goremychkin, et al.

    We present a systematic study of the crystal-field interactions in the LiRF4 (R=Gd, Ho, Er, Tm, and Yb) family of rare-earth magnets. Using detailed inelastic neutron scattering measurements, we have been able to quantify the transition energies and wave functions for each system. This allows us to quantitatively describe the high-temperature susceptibility measurements for the series of materials and make predictions based on a mean-field approach for the low-temperature thermal and quantum phase transitions. We show that coupling between crystal field and phonon states leads to line-shape broadening in LiTmF4 and level splitting in LiYbF4. Furthermore, using high-resolution neutron scattering from LiHoF4, we find anomalous broadening of crystal-field excitations which we attribute to magnetoelastic coupling.


  • Lineshape of the singlet-triplet excitations in the dimer system Sr3Cr2O8to first order in the high-density1/zexpansion

    by J. Jensen, D. L. Quintero-Castro, A. T. M. N. Islam, K. C. Rule, M. Månsson, B. Lake

    The Cr5+ ions in Sr3Cr2O8 constitute a strongly correlated spin-1/2 dimer system. Experiments show that the collective singlet-triplet excitations in this system are well defined in the zero-temperature limit, but, when heated, the inelastic neutron scattering peaks decrease rapidly in intensity and acquire a nonzero line width. When including the fluctuations to leading order in 1/z, where z is the coordination number, the diagrammatic high-density expansion is found to offer an accurate description of the singlet-triplet excitations. The theory explains not only the temperature dependencies of the intensities and line widths, but also that strong correlation causes the lineshapes to become asymmetric at temperatures comparable to the excitation energies.


  • Magnetic ordering in GdNi2B2C revisited by resonant x-ray scattering: Evidence for the double-qmodel

    by P. S. Normile, M. Rotter, C. Detlefs, J. Jensen, P. C. Canfield, J. A. Blanco

    Recent theoretical efforts aimed at understanding the nature of antiferromagnetic ordering in GdNi2B2C predicted double-q ordering. Here we employ resonant elastic x-ray scattering to test this theory against the formerly proposed, single-q ordering scenario. Our study reveals a satellite reflection associated with a mixed-order component propagation wave vector, viz., (qa,2qb,0) with qb=qa≈0.55 reciprocal lattice units, the presence of which is incompatible with single-q ordering but is expected from the double-q model. A (3qa,0,0) wave vector (i.e., third-order) satellite is also observed, again in line with the double-q model. The temperature dependencies of these along with that of a first-order satellite are compared with calculations based on the double-q model and reasonable qualitative agreement is found. By examining the azimuthal dependence of first-order satellite scattering, we show the magnetic order to be, as predicted, elliptically polarized at base temperature and find the temperature dependence of the “out of a-b plane” moment component to be in fairly good agreement with calculation. Our results provide qualitative support for the double-q model and thus in turn corroborate the explanation for the “magnetoelastic paradox” offered by this model.