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, $\mathrm{LiHo_{0.045}Y_{0.955}F_4}$, the quality factor $Q$ 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 $q$ through zero via the amplitude of the ac pump as well as a static external transverse field. The zero-crossing of $q$ 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 Al0.35_{0.35}Ga0.65_{0.65}As

We demonstrate that X-ray irradiation can be used to induce an insulator-metal transition in Si-doped Al$_{0.35}$Ga$_{0.65}$As, 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 ($\gg 1$) 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

Quantum and Classical Glass Transitions in LiHoxY1−xF4Li Ho_x Y_{1-x} F_4

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, $\text{LiHo}_x\text{Y}_{1-x}\text{F}_4$, prove the existence of a spin glass transition for $x$ = 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

Probing many-body localization in a disordered quantum magnet

Excitations in disordered systems are typically categorized as localized or delocalized, depending on whether they entail disturbances extending throughout the system or are confined to small, generally nanometer scale, subsystems. Such categorization is impossible to achieve using traditional spectroscopy where the response to a weak oscillating (ac) electromagnetic probe is measured as a function of frequency. However, the localized excitations can be separated from each other as well as the delocalized continuum by measuring a spectral "hole" in the ordinary response while a large amplitude pump is imposed at a fixed frequency. Localized excitations will result in a very sharp "hole," and any residual couplings to other excitations, both localized and extended, will determine its detailed shape. This technique probes incoherent lifetime effects as well as coherent mixing or quantum interference phenomena, describable in terms of the Fano effect. Here we show that in a disordered Ising magnet, LiHo0.045Y0.955F4, the quality factor Q for spectral holes, the ratio of the drive frequency to their width, can be as high as 100,000. In addition, we can tune the dynamics of the quantum degrees of freedom by sweeping the quantum mixing parameter through zero via the amplitude of the ac pump as well as a static external transverse field. The zero-crossing is associated with a dissipationless response at the drive frequency. The identification of such a point where localized degrees of freedom are minimally mixed with their environment in a dense and disordered dipolar coupled spin system implies control over the bath coupling of qubits emerging from strongly interacting many-body systems

Using thermal boundary conditions to engineer the quantum state of a bulk magnet

The degree of contact between a system and the external environment can alter dramatically its proclivity to quantum mechanical modes of relaxation. We show that controlling the thermal coupling of cubic centimeter-sized crystals of the Ising magnet $LiHo_xY_{1-x}F_4$ to a heat bath can be used to tune the system between a glassy state dominated by thermal excitations over energy barriers and a state with the hallmarks of a quantum spin liquid. Application of a magnetic field transverse to the Ising axis introduces both random magnetic fields and quantum fluctuations, which can retard and speed the annealing process, respectively, thereby providing a mechanism for continuous tuning between the destination states. The non-linear response of the system explicitly demonstrates quantum interference between internal and external relaxation pathways.Comment: 23 pages, 4 figures. Accepted for publication in PNA

Quantum Criticality in an Organic Magnet

Exchange interactions between $S=\frac{1}{2}$ sites in piperazinium hexachlorodicuprate produce a frustrated bilayer magnet with a singlet ground state. We have determined the field-temperature phase diagram by high field magnetization and neutron scattering experiments. There are two quantum critical points: $H_{c1}=7.5$ T separates a quantum paramagnet phase from a three dimensional, antiferromagnetically-ordered state while $H_{c2}=37$ T marks the onset of a fully polarized state. The ordered phase, which we describe as a magnon Bose-Einstein condensate (BEC), is embedded in a quantum critical regime with short range correlations. A low temperature anomaly in the BEC phase boundary indicates that additional low energy features of the material become important near $H_{c1}$.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett. Replaced original text with additional conten