625 research outputs found
Switchable Hardening of a Ferromagnet at Fixed Temperature
The intended use of a magnetic material, from information storage to power
conversion, depends crucially on its domain structure, traditionally crafted
during materials synthesis. By contrast, we show that an external magnetic
field applied transverse to the preferred magnetization of a model disordered
uniaxial ferromagnet is an isothermal regulator of domain pinning. At elevated
temperatures, near the transition into the paramagnet, modest transverse fields
increase the pinning, stabilize the domain structure, and harden the magnet,
until a point where the field induces quantum tunneling of the domain walls and
softens the magnet. At low temperatures, tunneling completely dominates the
domain dynamics and provides an interpretation of the quantum phase transition
in highly disordered magnets as a localization/delocalization transition for
domain walls. While the energy scales of the rare earth ferromagnet studied
here restrict the effects to cryogenic temperatures, the principles discovered
are general and should be applicable to existing classes of highly anisotropic
ferromagnets with ordering at room temperature or above.Comment: 10 pages, 4 figure
Probing many-body localization in a disordered quantum magnet
Quantum states cohere and interfere. Quantum systems composed of many atoms
arranged imperfectly rarely display these properties. Here we demonstrate an
exception in a disordered quantum magnet that divides itself into nearly
isolated subsystems. We probe these coherent clusters of spins by driving the
system beyond its linear response regime at a single frequency and measuring
the resulting "hole" in the overall linear spectral response. The Fano shape of
the hole encodes the incoherent lifetime as well as coherent mixing of the
localized excitations. For the disordered Ising magnet,
, the quality factor for spectral holes
can be as high as 100,000. We tune the dynamics of the quantum degrees of
freedom by sweeping the Fano mixing parameter through zero via the
amplitude of the ac pump as well as a static external transverse field. The
zero-crossing of is associated with a dissipationless response at the drive
frequency, implying that the off-diagonal matrix element for the two-level
system also undergoes a zero-crossing. The identification of localized
two-level systems in a dense and disordered dipolar-coupled spin system
represents a solid state implementation of many-body localization, pushing the
search forward for qubits emerging from strongly-interacting, disordered,
many-body systems.Comment: 22 pages, 6 figure
X-ray induced persistent photoconductivity in Si-doped AlGaAs
We demonstrate that X-ray irradiation can be used to induce an
insulator-metal transition in Si-doped AlGaAs, a
semiconductor with {\it DX} centers. The excitation mechanism of the {\it DX}
centers into their shallow donor state was revealed by studying the
photoconductance along with fluorescence. The photoconductance as a function of
incident X-ray energy exhibits an edge both at the Ga and As K-edge, implying
that core-hole excitation of Ga and As are efficient primary steps for the
excitation of {\it DX} centers. A high quantum yield () suggests that
the excitation is indirect and nonlocal, due to secondary electrons, holes, and
fluorescence photons.Comment: 3 pages of text, 6 figures. An error in Fig.5 was detected, so we
corrected i
Interacting Dirac Materials
We investigate the extent to which the class of Dirac materials in
two-dimensions provides general statements about the behavior of both fermionic
and bosonic Dirac quasiparticles in the interacting regime. For both
quasiparticle types, we find common features for the interaction induced
renormalization of the conical Dirac spectrum. We perform the perturbative
renormalization analysis and compute the self-energy for both quasiparticle
types with different interactions and collate previous results from the
literature whenever necessary. Guided by the systematic presentation of our
results in Table~\ref{Summary}, we conclude that long-range interactions
generically lead to an increase of the slope of the single-particle Dirac cone,
whereas short-range interactions lead to a decrease. The quasiparticle
statistics does not qualitatively impact the self-energy correction for
long-range repulsion but does affect the behavior of short-range coupled
systems, giving rise to different thermal power-law contributions. The
possibility of a universal description of the Dirac materials based on these
features is also mentioned.Comment: 19 pages and 12 Figures; Contains 6 Appendice
New possibility of the ground state of quarter-filled one-dimensional strongly correlated electronic system interacting with localized spins
We study numerically the ground state properties of the one-dimensional
quarter-filled strongly correlated electronic system interacting
antiferromagnetically with localized spins. It is shown that the
charge-ordered state is significantly stabilized by the introduction of
relatively small coupling with the localized spins. When the coupling becomes
large the spin and charge degrees of freedom behave quite independently and the
ferromagnetism is realized. Moreover, the coexistence of ferromagnetism with
charge order is seen under strong electronic interaction. Our results suggest
that such charge order can be easily controlled by the magnetic field, which
possibly give rise to the giant negative magnetoresistance, and its relation to
phthalocyanine compounds is discussed.Comment: 5pages, 4figure
Suppressed reflectivity due to spin-controlled localization in a magnetic semiconductor
The narrow gap semiconductor FeSi owes its strong paramagnetism to
electron-correlation effects. Partial Co substitution for Fe produces a
spin-polarized doped semiconductor. The spin-polarization causes suppression of
the metallic reflectivity and increased scattering of charge carriers, in
contrast to what happens in other magnetic semiconductors, where magnetic order
reduces the scattering. The loss of metallicity continues progressively even
into the fully polarized state, and entails as much as a 25% reduction in
average mean-free path. We attribute the observed effect to a deepening of the
potential wells presented by the randomly distributed Co atoms to the majority
spin carriers. This mechanism inverts the sequence of steps for dealing with
disorder and interactions from that in the classic Al'tshuler Aronov approach -
where disorder amplifies the Coulomb interaction between carriers - in that
here, the Coulomb interaction leads to spin polarization which in turn
amplifies the disorder-induced scattering.Comment: 6 figures Submitted to PR
Quantum and Classical Glass Transitions in
When performed in the proper low field, low frequency limits, measurements of
the dynamics and the nonlinear susceptibility in the model Ising magnet in
transverse field, , prove the existence
of a spin glass transition for = 0.167 and 0.198. The classical behavior
tracks for the two concentrations, but the behavior in the quantum regime at
large transverse fields differs because of the competing effects of quantum
entanglement and random fields.Comment: 5 pages, 5 figures. Updated figure 3 with corrected calibration
information for thermometr
Quantum critical behavior for a model magnet
The classical, thermally driven transition in the dipolar-coupled Ising ferromagnet LiHoF_4 (T_c=1.53K) can be converted into a quantum transition driven by a transverse magnetic field H_t at T=0. The transverse field, applied perpendicular to the Ising axis, introduces channels for quantum relaxation, thereby depressing T_c. We have determined the phase diagram in the H_t−T plane via magnetic susceptibility measurements. The critical exponent, γ=1, has a mean-field value in both the classical and quantum limits. A solution of the full mean-field Hamiltonian using the known LiHoF_4 crystal-field wave functions, including nuclear hyperfine terms, accurately matches experiment
- …