326 research outputs found
Quantum bicriticality in the heavy-fermion metamagnet YbAgGe
Bicritical points, at which two distinct symmetry-broken phases become
simultaneously unstable, are typical for spin-flop metamagnetism.
Interestingly, the heavy-fermion compound YbAgGe also possesses such a
bicritical point (BCP) with a low temperature T_BCP ~ 0.3 K at a magnetic field
of mu_0 H_BCP ~ 4.5 T. In its vicinity, YbAgGe exhibits anomalous behavior that
we attribute to the influence of a quantum bicritical point (QBCP), that is
close in parameter space yet can be reached by tuning T_BCP further to zero.
Using high-resolution measurements of the magnetocaloric effect, we demonstrate
that the magnetic Grueneisen parameter Gamma_H indeed both changes sign and
diverges as required for quantum criticality. Moreover, Gamma_H displays a
characteristic scaling behavior but only on the low-field side, H < H_BCP,
indicating a pronounced asymmetry with respect to the critical field. We
speculate that the small value of T_BCP is related to the geometric frustration
of the Kondo-lattice of YbAgGe.Comment: submitted to PR
Linearly polarized GHz magnetization dynamics of spin helix modes in the ferrimagnetic insulator CuOSeO
Linear dichroism -- the polarization dependent absorption of electromagnetic
waves -- is routinely exploited in applications as diverse as structure
determination of DNA or polarization filters in optical technologies. Here
filamentary absorbers with a large length-to-width ratio are a prerequisite.
For magnetization dynamics in the few GHz frequency regime strictly linear
dichroism was not observed for more than eight decades. Here, we show that the
bulk chiral magnet CuOSeO exhibits linearly polarized magnetization
dynamics at an unexpectedly small frequency of about 2 GHz. Unlike optical
filters that are assembled from filamentary absorbers, the magnet provides
linear polarization as a bulk material for an extremely wide range of
length-to-width ratios. In addition, the polarization plane of a given mode can
be switched by 90 via a tiny variation in width. Our findings shed a
new light on magnetization dynamics in that ferrimagnetic ordering combined
with anisotropic exchange interaction offers strictly linear polarization and
cross-polarized modes for a broad spectrum of sample shapes. The discovery
allows for novel design rules and optimization of microwave-to-magnon
transduction in emerging microwave technologies.Comment: 20 pages, 4 figure
Efficacy of Online Training for Improving Camp Staff Competency
Preparing competent staff is a critical issue within the camp community. This quasi-experimental study examined the effectiveness of an online course for improving staff competency in camp healthcare practices among college-aged camp staff and a comparison group (N = 55). We hypothesized that working in camp would increase competency test scores due to opportunities for staff to experientially apply knowledge learned online. Hierarchical linear modeling was used to analyse the cross-level effects of a between-individuals factor (assignment to experimental or comparison group) and within-individual effects of time (pre-test, post-test #1, and post-test #2) on online course test scores. At post-test #2, the difference in average test scores between groups was ~30 points, with the treatment group scoring lower on average than the comparison group. Factors that may have influenced these findings are explored, including fatigue and the limited durability of online learning. Recommendations for research and practice are discussed
Emergent Lorentz symmetry with vanishing velocity in a critical two-subband quantum wire
We consider a quantum wire with two subbands of spin-polarized electrons in
the presence of strong interactions. We focus on the quantum phase transition
when the second subband starts to get filled as a function of gate voltage.
Performing a one-loop renormalization group (RG) analysis of the effective
Hamiltonian, we identify the critical fixed-point theory as a conformal field
theory having an enhanced SU(2) symmetry and central charge 3/2. While the
fixed point is Lorentz invariant, the effective 'speed of light' nevertheless
vanishes at low energies due to marginally irrelevant operators leading to a
diverging critical specific heat coefficient.Comment: 4 pages, 3 figures, minor changes, published versio
Multiscale quantum criticality: Pomeranchuk instability in isotropic metals
As a paradigmatic example of multi-scale quantum criticality, we consider the
Pomeranchuk instability of an isotropic Fermi liquid in two spatial dimensions,
d=2. The corresponding Ginzburg-Landau theory for the quadrupolar fluctuations
of the Fermi surface consists of two coupled modes, critical at the same point,
and characterized by different dynamical exponents: one being ballistic with
dynamical exponent z=2 and the other one is Landau-damped with z=3, thus giving
rise to multiple dynamical scales. We find that at temperature T=0, the
ballistic mode governs the low-energy structure of the theory as it possesses
the smaller effective dimension d+z. Its self-interaction leads to logarithmic
singularities, which we treat with the help of the renormalization group. At
finite temperature, the coexistence of two different dynamical scales gives
rise to a modified quantum-to-classical crossover. It extends over a
parametrically large regime with intricate interactions of quantum and
classical fluctuations leading to a universal T-dependence of the correlation
length independent of the interaction amplitude. The multiple scales are also
reflected in the phase diagram and in the critical thermodynamics. In
particular, we find that the latter cannot be interpreted in terms of only a
single dynamical exponent: whereas, e.g., the critical specific heat is
determined by the z=3 mode, the critical compressibility is found to be
dominated by the z=2 fluctuations.Comment: 15 pages, 6 figures; (v2) RG implementation with arbitrary dynamical
exponent z, discussion on fixed-points adde
Dimensional crossover in quantum critical metallic magnets
Nearly magnetic metals often have layered lattice structures, consisting of
coupled planes. In such a situation, physical properties will display, upon
decreasing temperature or energy, a dimensional crossover from two-dimensional
(2d) to three-dimensional (3d) behavior, which is particularly interesting near
quantum criticality. Here we study this crossover in thermodynamics using a
suitably generalized Landau-Ginzburg-Wilson approach to the critical behavior,
combined with renormalization group techniques. We focus on two experimentally
relevant cases: the crossover from a 2d to a 3d antiferromagnet, and the
crossover from a 2d ferromagnet to a 3d antiferromagnet. We discuss the
location of phase boundary and crossover lines and determine the crossover
functions for important thermodynamic quantities. As naive scaling does not
apply at and above the upper critical dimension, two crossover scales arise
which can be associated with separate dimensional crossovers of classical and
quantum fluctuations, respectively. In particular, we find an intermediate
regime with novel power laws where the quantum fluctuations still have a 2d and
the classical fluctuations already have a 3d character. For the
ferromagnet-to-antiferromagnet crossover, the mismatch of the dynamical
exponents between the 2d and 3d regimes leads to an even richer crossover
structure, with an interesting 2d non-critical regime sandwiched between two
critical regimes. For all cases, we find that thermal expansion and
compressibility are particularly sensitive probes of the dimensional crossover.
Finally, we relate our results to experiments on the quantum critical
heavy-fermion metals CeCu(6-x)Au(x), YbRh(2)Si(2) and CeCoIn(5).Comment: 18 pages, 8 figures, published versio
Scattering of high-energy magnons off a magnetic skyrmion
We discuss the scattering of high-energy magnons off a single magnetic skyrmion within the field-polarized
ground state of a two-dimensional chiral magnet. For wavevectors larger than the inverse skyrmion radius,krs>>1 the magnon scattering is dominated by an emerging magnetic field whose flux density is essentially determined
by the topological charge density of the skyrmion texture. This leads to skew and rainbow scattering
characterized by an asymmetric and oscillating differential cross section. We demonstrate that the transversal
momentum transfer to the skyrmion is universal due to the quantization of the total emerging flux while the longitudinal
momentum transfer is negligible in the high-energy limit. This results in a magnon-driven skyrmion
motion approximately antiparallel to the incoming magnon current and a universal relation between current and
skyrmion-velocity
Low spin wave damping in the insulating chiral magnet CuOSeO
Chiral magnets with topologically nontrivial spin order such as Skyrmions
have generated enormous interest in both fundamental and applied sciences. We
report broadband microwave spectroscopy performed on the insulating chiral
ferrimagnet CuOSeO. For the damping of magnetization dynamics we
find a remarkably small Gilbert damping parameter of about at
5 K. This value is only a factor of 4 larger than the one reported for the best
insulating ferrimagnet yttrium iron garnet. We detect a series of sharp
resonances and attribute them to confined spin waves in the mm-sized samples.
Considering the small damping, insulating chiral magnets turn out to be
promising candidates when exploring non-collinear spin structures for high
frequency applications.Comment: 5 pages, 5 figures, and supplementary materia
Periodicity and Chaos Amidst Twisting and Folding in Two-Dimensional Maps
We study the dynamics of three planar, noninvertible maps which rotate and fold the plane. Two maps are inspired by real-world applications whereas the third map is constructed to serve as a toy model for the other two maps. The dynamics of the three maps are remarkably similar. A stable fixed point bifurcates through a Hopf-Neimark-Sacker which leads to a countably infinite set of resonance tongues in the parameter plane of the map. Within a resonance tongue a periodic point can bifurcate through a period-doubling cascade. At the end of the cascade we detect Henon-like attractors which are conjectured to be the closure of the unstable manifold of a saddle periodic point. These attractors have a folded structure which can be explained by means of the concept of critical lines. We also detect snap-back repellers which can either coexist with Henon-like attractors or which can be formed when the saddle-point of a Henon-like attractor becomes a source
- …