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ABSTRACT: We report low-energy inelastic neutron scattering data of the paramagnetic
(PM) to hidden-order (HO) phase transition at T0=17.5K in URu2Si2. While
confirming previous results for the HO and PM phases, our data reveal a
pronounced wavevector dependence of quasielastic scattering across the phase
transition. Temperature scans at a small energy transfer of 0.5meV establish an
abrupt step-like suppression of the excitations at Q1=(1.44,0,0) below T0,
whereas excitations at Q0=(1,0,0), associated with large-moment
antiferromagnetism (LMAF) under pressure, are enhanced with a pronounced peak
at T0. This is the behavior expected of a "super-vector" order parameter with
nearly degenerate components for the HO and LMAF leading to nearly isotropic
fluctuations in the combined order-parameter space.
10/2011;
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ABSTRACT: We report for the first time simultaneous microscopic measurements of the lattice constants, the distribution of the lattice constants, and the antiferromagnetic moment in high-purity URu(2)Si(2), combining Larmor and conventional neutron diffraction at low temperatures and pressures up to 18 kbar. Our data demonstrate quantitatively that the small moment in the hidden order (HO) of URu(2)Si(2) is purely parasitic. The excellent experimental conditions we achieve allow us to resolve that the transition line between HO and large-moment antiferromagnetism (LMAF), which stabilizes under pressure, is intrinsically first order and ends in a bicritical point. Therefore, the HO and LMAF must have different symmetry, which supports exotic scenarios of the HO such as orbital currents, helicity order, or multipolar order.
Physical Review Letters 03/2010; 104(10):106406. · 7.37 Impact Factor
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W. J. Duncan,
O. P. Welzel,
D. Moroni-Klementowicz,
C. Albrecht, P. G. Niklowitz,
D. Grüner,
M. Brando,
A. Neubauer,
C. Pfleiderer,
N. Kikugawa,
A. P. Mackenzie,
F. M. Grosche
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ABSTRACT: We examine the low temperature states of two transition metal compounds: (i) NbFe2 is poised on the threshold of ferromagnetism and can be pushed into a spin-aligned state at low temperature by modifying the composition slightly. Stoichiometric NbFe2 has been reported as a rare example of low-temperature spin density wave order in a d-metal system. We have used pressure, field and composition tuning to examine the phase diagram of NbFe2. Near the quantum critical point, we find distinct non-Fermi liquid forms of the resistivity and heat capacity, whereas we observe strong, hysteretic magnetoresistance effects deep in the ordered phase. (ii) Ca3Ru2O7 undergoes first a magnetic transition (TN = 56 K) and then a structural transition (TS = 48 K) on cooling. Most of the Fermi surface is gapped out at low temperature, leading to a very low carrier density and small Fermi surface pockets. Pressure suppresses both TN and TS and, for p > 3.5 GPa, induces a third low temperature state, which is robust up to at least 7.5 GPa.
physica status solidi (b) 02/2010; 247(3):544 - 548. · 1.32 Impact Factor
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R Ritz,
S Mühlbauer,
C Pfleiderer,
T Keller,
J White,
M Laver,
E M Forgan,
R Cubitt,
C Dewhurst, P G Niklowitz,
A Prokofiev,
E Bauer
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ABSTRACT: The non-centrosymmetric heavy fermion compound CePt3Si orders antiferromag-netically at TN = 2.2K, followed by a superconducting transition, where Ts = 0.45 K for high quality samples and Ts = 0.75K for lower quality samples. We have used neutron Larmor diffraction to measure the temperature dependence and distribution of the lattice constants in a single crystal of CePt3Si with Ts = 0.75K. In our study we observe an unusually wide range of lattice parameters for the a-axis and c-axis with Δa/a ≈ Δc/c ≈ 10−3. Small angle neutron scattering suggests an abundance of defects along the lattice planes. When taking into account the pressure dependence of Ts our study attributes the increased value of Ts in low quality samples to an effective negative pressure.
Journal of Physics Conference Series 02/2010; 200(1):012165.
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ABSTRACT: We investigate the evolution of the electrical resistivity of BaFe(2)As(2) single crystals with pressure. The samples used were from the same batch, grown using a self-flux method, and showed properties that were highly reproducible. Samples were pressurized using three different pressure media: pentane-isopentane (in a piston-cylinder cell), Daphne oil (in an alumina anvil cell) and steatite (in a Bridgman cell). Each pressure medium has its own intrinsic level of hydrostaticity, which dramatically affects the phase diagram. An increasing uniaxial pressure component in this system quickly reduces the spin density wave order and favours the appearance of superconductivity, which is similar to what is seen in SrFe(2)As(2).
Journal of Physics Condensed Matter 02/2010; 22(5):052201. · 2.55 Impact Factor
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ABSTRACT: We report simultaneous measurements of the distribution of lattice constants and the antiferromagnetic moment in high-purity URu2Si2, using both Larmor and conventional neutron diffraction, as a function of temperature and pressure up to 18 kbar. We establish that the tiny moment in the hidden order (HO) state is purely parasitic and quantitatively originates from the distribution of lattice constants. Moreover, the HO and large-moment antiferromagnetism (LMAF) at high pressure are separated by a line of first-order phase transitions, which ends in a bicritical point. Thus the HO and LMAF are coupled non-linearly and must have different symmetry, as expected of the HO being, e.g., incommensurate orbital currents, helicity order, or multipolar order. Comment: 4 pages, 4 figures
09/2009;
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ABSTRACT: Recent small angle neutron scattering suggests that the spin structure in the A phase of MnSi is a so-called triple-Q state, i.e., a superposition of three helices under 120 degrees. Model calculations indicate that this structure in fact is a lattice of so-called Skyrmions, i.e., a lattice of topologically stable knots in the spin structure. We report a distinct additional contribution to the Hall effect in the temperature and magnetic field range of the proposed Skyrmion lattice, where such a contribution is neither seen nor expected for a normal helical state. Our Hall effect measurements constitute a direct observation of a topologically quantized Berry phase that identifies the spin structure seen in neutron scattering as the proposed Skyrmion lattice.
Physical Review Letters 06/2009; 102(18):186602. · 7.37 Impact Factor
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C Pfleiderer,
A Neubauer,
S Mühlbauer,
F Jonietz,
M Janoschek,
S Legl,
R Ritz,
W Münzer,
C Franz, P G Niklowitz,
T Keller,
R Georgii,
P Böni,
B Binz,
A Rosch,
U K Rößler,
A N Bogdanov
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ABSTRACT: Systems lacking inversion symmetry, such as selected three-dimensional compounds, multilayers and surfaces support Dzyaloshinsky-Moriya (DM) spin-orbit interactions. In recent years DM interactions have attracted great interest, because they may stabilize magnetic structures with a unique chirality and non-trivial topology. The inherent coupling between the various properties provided by DM interactions is potentially relevant for a variety of applications including, for instance, multiferroic and spintronic devices. The, perhaps, most extensively studied material in which DM interactions are important is the cubic B20 compound MnSi. We review the magnetic field and pressure dependence of the magnetic properties of MnSi. At ambient pressure this material displays helical order. Under hydrostatic pressure a non-Fermi liquid state emerges, where a partial magnetic order, reminiscent of liquid crystals, is observed in a small pocket. Recent experiments strongly suggest that the non-Fermi liquid state is not due to quantum criticality. Instead it may be the signature of spin textures and spin excitations with a non-trivial topology.
Journal of Physics Condensed Matter 04/2009; 21(16):164215. · 2.55 Impact Factor
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W. J. Duncan,
O. P. Welzel,
D. Moroni-Klementowicz,
C. Albrecht, P G Niklowitz,
D. Grüner,
M. Brando,
A Neubauer,
C. Pfleiderer,
N. Kikugawa,
A. P. Mackenzie,
F. M. Grosche
Physica Status Solidi B - Basic Solid State Physics, v.247, 544-548 (2010).
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C. Pfleiderer,
P. Böni,
C. Franz,
T. Keller,
A Neubauer, P G Niklowitz,
P. Schmakat,
M Schulz,
Y. -K. Huang,
J. A. Mydosh,
M. Vojta,
W. Duncan,
F. M. Grosche,
M. Brando,
M. Deppe,
C. Geibel,
F. Steglich,
A. Krimmel,
A. Loidl
Journal of Low Temperature Physics, v.161, 167-181 (2010).
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ABSTRACT: We report the transverse magnetoresistance ρxx and Hall effect ρxy in MnSi between room temperature and 2.5 K for magnetic field up to 9 T. Magnetic field suppresses the resistivity at all temperatures and fields, characteristic of scattering by an abundance of soft magnetic fluctuations that are quenched by magnetic field. Anomalous contributions to the Hall effect for temperatures below the onset of helimagnetic order, , and magnetic field are proportional to with a proportionality constant SH that is essentially temperature independent and M the uniform magnetization. Above the helimagnetic transition temperature this description fails. We propose that additional effects have to be considered for a comprehensive account of the Hall effect and magnetoresistance in MnSi.
Physica B Condensed Matter 404(19):3163-3166. · 1.06 Impact Factor
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P. G. Niklowitz
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ABSTRACT: We report low-temperature and high-pressure measurements of the electrical resistivity ρ(T) of the antiferromagnetic compound NiS2 in its high-pressure metallic state. The form of ρ(T,p) suggests the presence of a quantum phase transition at a critical pressure pc=76±5 kbar. Near pc, the temperature variation of ρ(T) is similar to that observed in NiS2−xSex near the critical composition x=1, where metallic antiferromagnetism is suppressed at ambient pressure. In both cases, ρ(T) varies approximately as T1.5 over a wide range below 100 K. This lets us assume that the high-pressure metallic phase of stoichiometric NiS2 also develops itinerant antiferromagnetism, which becomes suppressed at pc. However, on closer analysis, the resistivity exponent in NiS2 exhibits an undulating variation with temperature not seen in NiSSe (x=1). This difference in behavior may be due to the effects of spin-fluctuation scattering of charge carriers on cold and hot spots of the Fermi surface in the presence of quenched disorder, which is higher in NiSSe than in stoichiometric NiS2.
Phys. Rev. B. 77(11).
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ABSTRACT: Following the border of antiferromagnetism (AF) to zero temperature is a promising route to unconventional metallic and superconducting phases. Many interesting examples of antiferromagnetic quantum phase transitions can only be reached by pressure tuning. The range of quantitative experimental probes, which can be realised in a high-pressure environment is limited. However, advances have recently been made in neutron scattering, where elliptically shaped neutron guides now increase the beam intensity directed to mm size sample for high pressure studies. This has been demonstrated on the simple antiferromagnet NiS2. Neutron scattering also allows highly accurate measurements of the lattice constant via the Larmor diffraction technique, which proved extremely useful in studying the high-pressure phase diagram of the itinerant helimagnet MnSi. We now combined Larmor diffraction with conventional diffraction measurements to investigate the pressure–temperature phase diagram of URu2Si2 up to 20 kbar. URu2Si2 offers a further spectacular example for the presence of unconventional phases in the vicinity of antiferromagnetism. In this compound, antiferromagnetism is replaced below approximately 5 kbar by the mysterious “hidden order” (HO) and unconventional superconductivity. Our measurements allow the observation of magnetic order and changes in the a- and c-axis lattice constants across the phase transitions in the same experiment. The results contain clear indications of a first-order transition and strong differences between the AF phase and the HO phase in the coupling to the lattice.
Physica B: Condensed Matter.
