184 research outputs found
Quantum criticality in Yb(Rh0.97Co0.03)2Si2 probed by low-temperature resistivity
Quantum criticality in Yb(Rh0.97Co0.03)2Si2 is investigated by means of
resistivity and magnetoresistance. The partial substitution of Co leads to a
stabilization of the magnetism as expected according to the application of
chemical pressure for Yb systems. However, the signature of the Kondo-breakdown
remains at the same position in the temperature-magnetic field phase diagram
compared to stoichiometric YbRh2Si2. As a consequence, the Kondo-breakdown is
situated within the antiferromagnetic phase. These results fit well within the
global phase diagram under chemical pressure [1].Comment: 4 pages, 4 figures, submitted to ICM/SCES200
Elektrischer Transport und QuantenkritikalitÀt in reinem und substituiertem YbRh2Si2
In der vorliegenden Arbeit wurde der elektrische Transport im Schwere-Fermionen-System YbRh2Si2 sowohl in seiner stöchiometrischen Form als auch mit teilweiser isoelektronischer Substitution von Ir oder Co auf dem Rh-Platz untersucht. In YbRh2Si2 liegt ein quantenkritischer Punkt vor, der zugĂ€nglich ist, indem der antiferromagnetische PhasenĂŒbergang mittels eines kleinen Magnetfelds zum absoluten Nullpunkt der Temperatur unterdrĂŒckt wird.
Die zentralen Messungen des Hallkoeffizienten zeigen einen Ăbergang der in der Extrapolation zu T=0 zu einer DiskontinuitĂ€t wird und somit auf eine Rekonstruktion der FermiflĂ€che am quantenkritischen Punkt schlieĂen lĂ€sst. Dies belegt die unkonventionelle Natur der QuantenkritikalitĂ€t in YbRh2Si2. UnterstĂŒtzt wird dies auf fundamentale Weise durch verknĂŒpfungen mit unkonventionellem Skalierungsverhalten.
In den Proben mit teilweiser Substitution wird der Einfluss einer VerĂ€nderung der Gitterparameter auf die QuantenkritikalitĂ€t mit Hilfe von Widerstandsmessungen untersucht. Dabei zeigt sich, dass der magnetische Ăbergang von der FermiflĂ€chenrekonstruktion separiert wird. FĂŒr Proben mit teilweiser Ir-Substitution, welche negativen DrĂŒcken entspricht, scheint im Zwischenbereich eine neuartige metallische SpinflĂŒssigkeit hervorzutreten.This work investigates the electrical transport of the heavy-fermion compound YbRh2Si2 in its stoichiometric form as well as with slight isoelectronic substitution of Ir or Co on the Rh site. A quantum critical point is present in YbRh2Si2 which is accessed by tuning the transition temperature of the antiferromagnetic order to absolute zero via the application of a small magnetic field.
The central measurements of the Hall coefficient reveal a crossover which sharpens to a discontinuity in the extrapolation to zero temperature implying a reconstruction of the Fermi surface at the quantum critical point. This allows to rule out conventional descriptions of the quantum criticality in YbRh2Si2. A scaling analysis corroborates this on a fundamental basis.
In the samples with partial substitution the effect of unit cell volume change on the quantum criticality was investigated by means of resistivity measurements. Surprisingly, the magnetic transition is separated from the Fermi surface reconstruction. For samples with Ir substitution corresponding to negative chemical pressure, a new metallic spin liquid seems to emerge in the intermediate regime
Fermi-surface collapse and dynamical scaling near a quantum critical point
Quantum criticality arises when a macroscopic phase of matter undergoes a
continuous transformation at zero temperature. While the collective
fluctuations at quantum-critical points are being increasingly recognized as
playing an important role in a wide range of quantum materials, the nature of
the underlying quantum-critical excitations remains poorly understood. Here we
report in-depth measurements of the Hall effect in the heavy-fermion metal
YbRh2Si2, a prototypical system for quantum criticality. We isolate a rapid
crossover of the isothermal Hall coefficient clearly connected to the
quantum-critical point from a smooth background contribution; the latter exists
away from the quantum-critical point and is detectable through our studies only
over a wide range of magnetic field. Importantly, the width of the critical
crossover is proportional to temperature, which violates the predictions of
conventional theory and is instead consistent with an energy over temperature,
E/T, scaling of the quantum-critical single-electron fluctuation spectrum. Our
results provide evidence that the quantum-dynamical scaling and a critical
Kondo breakdown simultaneously operate in the same material. Correspondingly,
we infer that macroscopic scale-invariant fluctuations emerge from the
microscopic many-body excitations associated with a collapsing Fermi-surface.
This insight is expected to be relevant to the unconventional
finite-temperature behavior in a broad range of strongly correlated quantum
systems.Comment: 5 pages, plus supporting materia
Absence of superconducting dome at the charge-density-wave quantum phase transition in 2<i>H</i>âNbSe<sub>2</sub>
Superconductivity is often found in a dome around quantum critical points,
i.e. 2nd-order quantum phase transitions. Here, we show that an enhancement of
superconductivity is avoided at the critical pressure of the
charge-density-wave (CDW) state in NbSe. We present comprehensive
high-pressure Hall effect and magnetic susceptibility measurements of the CDW
and superconducting state in NbSe. Initially, the 2nd-order CDW transition
is suppressed smoothly but it drops to zero abruptly at PCDW = 4.4 GPa thus
indicating a change to 1st order whilstthe superconducting transition
temperature Tc rises continuously up to PCDW but is constant above. The
putative 1st-order nature of the CDW transition is suggested as the cause for
the absence of a superconducting dome at PCDW. Indeed, we show that the
suppression of the superconducting state at low pressures is due to the loss of
density of states inside the CDW phase whilst the initial suppression of the
CDW state is accounted for by the stiffening of the underlying bare phonon
mode.Comment: 16 pages, 5 figures, S
Effects of Ferromagnetic & Carbon-Fibre Z-Pins on the Magnetic Properties of Composites
This paper investigates for the first time the effects of Z-pins on the magnetic properties of composite laminates. In-plane and out-of-plane M-H curves of IM7/8552 laminates with and without Z-pins have been characterised by an MPMS3 SQUID magnetometer. Two kinds of pin materials (T300/BMI composite and ferromagnetic Ni/Fe alloy) have been studied at three different volume fractions (nominally 0.5%, 2% and 4%). The unpinned and carbon-fibre pinned laminates were found to be diamagnetic. The carbon-fibre pin had no significant influence on the global magnetic properties of the laminates. The Ni/Fe alloy pin increased the laminate linear-part magnetic volume susceptibility up to 1.87 and 0.13 for the out-of-plane and in-plane directions, respectively. Numerical modelling has been conducted to support the investigation of the effect of the pin volume fraction on the overall magnetic susceptibility and saturation magnetisation. The laminate out-of-plane susceptibility exhibits a nonlinear behaviour dependent on pin volume fraction, due to interactions between adjacent pins. The saturation magnetisation is proportional to the pin volume fraction and independent of field direction
Electronic Structure of LuRh2Si2: "Small" Fermi Surface Reference to YbRh2Si2
We present band structure calculations and quantum oscillation measurements
on LuRh2Si2, which is an ideal reference to the intensively studied quantum
critical heavy-fermion system YbRh2Si2. Our band structure calculations show a
strong sensitivity of the Fermi surface on the position of the silicon atoms
zSi within the unit cell. Single crystal structure refinement and comparison of
predicted and observed quantum oscillation frequencies and masses yield zSi =
0.379c in good agreement with numerical lattice relaxation. This value of zSi
is suggested for future band structure calculations on LuRh2Si2 and YbRh2Si2.
LuRh2Si2 with a full f electron shell represents the "small" Fermi surface
configuration of YbRh2Si2. Our experimentally and ab initio derived quantum
oscillation frequencies of LuRh2Si2 show strong differences with earlier
measurements on YbRh2Si2. Consequently, our results confirm the contribution of
the f electrons to the Fermi surface of YbRh2Si2 at high magnetic fields. Yet
the limited agreement with refined fully itinerant local density approximation
calculations highlights the need for more elaborated models to describe the
Fermi surface of YbRh2Si2.Comment: 12 pages 10 figure
Existence and stability of skyrmion bags in thin magnetic films
Skyrmion bags are spin textures of any integer topological degree, realized in micromagnetic simulations and experimentally in liquid crystals. They have been proposed as a promising new form of magnetic data storage due to their stability with respect to perturbations and the possibility of encoding different values in topologically distinct magnetization configurations. We simulate skyrmion bags in magnetic thin films having a range of physically realistic material parameters. The results give a range over which stable skyrmion bags may be found in experiment, and we extract a relationship to help guide the production of these potentially useful quasiparticles. Individual magnetic skyrmions are stable, particle-like, spin configurations in the magnetization of chiral magnets.1 Their stability is derived from the skyrmion's topological nature. A single skyrmion in the continuous model is a perfect cover of the two-sphere, illustrated in Fig. 1(c), and hence impossible to unwind. This stability, coupled with the low currents required to move them, makes them seem to be attractive as candidates for future forms of magnetic data storage with read-write capabilities
Mathematical Reflections on Acupoint Combinations in the Traditional Meridian Systems
The meridian system is a systematic order of empirical knowledge functioning as a rational ground for a balanced treatment by combining meridians. In TCM theory, a continuous circulation of Qi through 12 meridians is postulated, described as the Chinese clock (CC). On this basis, combinations of meridians and acupoints had been described in historical writings. The most common is the interior/exterior system beside the neighbouring system, the opposite clock system, and three systems, developed out of the theory of the six stages. All of these represent symmetrical combinations, which were defined by the steps in the CC. We calculated the possible combinations that fit into the systematics of the historical descriptions, leading to 19 systems. Merging the data of the 19 systems, possible steps in the CC clock for balancing a meridian are 1, 2, 3, and 6. Step 4 is not possible. Step 5 is a combinatory possibility but has no widespread tradition except for activating the yin extraordinary vessels. These possibilities can be plotted on the CC as a powerful tool for daily practice. Only two meridians might be excluded as potentially balancing meridians, so it seems almost impossible to define noneffective acupuncture points as controls in clinical trials
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