Current Research

The general theme for my research is the examination of many body systems in the vicinity of zero temperature (T=0) quantum phase-transitions. More often than not, phase-transitions take place at finite temperatures as the thermal energy becomes less than a characteristic energy scale of the system. It then becomes favorable to change the symmetry and nature of the material from say a liquid to a solid as when water freezes. However several examples of quantum phase-transitions between `disordered' and `ordered' phases that occur at T=0 have recently emerged, and deserve attention because of the rich and unexplored properties of quantum disordered phases.

QUANTUM DISORDERED PHASES IN ONE DIMENSION: The prime example of a T=0 quantum disordered phase is the antiferromagnetic integer spin chain alluded to earlier. Even as the thermal energy becomes insignificant compared to the energy scale for interactions between spins there is no phase transition. Instead the system settles into a unique quantum disordered phase, the analogue of a liquid at zero temperature. However, modification of crystalline anisotropy or inter chain coupling has been predicted to induce a T=0 phase transition to a long range ordered phase, the analogue of a solid. We will explore the properties of this unique phase transition for the first time. I am in particular intrigued by studying the fate of the unperturbed spin fluctuation mode which we discovered in the quantum disordered phase: Will it survive as a well defined spin wave mode, or will the appearance of long range order quite counter intuitively induce dissipation ? Experience from the long range ordered half integer spin chain, which has strong dissipation of spin fluctuations, would in fact suggest the latter unusual scenario. We will induce the transition by the application of pressure or large magnetic fields to well defined model systems, and study the ordered phase and the critical properties of the phase transition using magnetic neutron scattering.

It may be possible to drive the half integer spin chain from order to disorder at T=0 by introducing antiferromagnetic exchange coupling to next nearest neighbors. We are in the process of identifying S=1/2 chains with appreciable next nearest neighbor interactions which may serve as model systems in which to study spin correlations in a half integer quantum spin liquid. Investigating such novel quantum disordered phases is important because it extends our very limited experience with these unique states of matter. It may also be possible to explore the critical properties of the phase transition from disorder back to order at zero temperature by the application of pressure.

QUANTUM DISORDERED PHASES IN TWO DIMENSIONS: It remains unclear whether quantum disordered phases exist in simple 2 dimensional spin systems. The most promising candidate system so far is the Kagom\'{e} antiferromagnet which is a periodically diluted version of the triangular lattice antiferromagnet. This lattice does not lend itself to conventional antiferromagnetic ordering and may in fact have a disordered ground state for sufficiently small spin quantum number. I pioneered the study of the Kagom\'{e} antiferromagnet in 1990 setting off a large theoretical effort to understand this system. \newpage I both plan to perform more detailed studies of model systems which are currently available and hunt down better model systems which can be grown in single crystalline form. A dedicated effort along these lines could lead to an experimental identification and characterization of the first two dimensional quantum spin liquid.

ZERO TEMPERATURE PHASE TRANSITIONS IN METALS: Perhaps the most common quantum disordered phase is the conventional metallic phase itself. Coherence such as that achieved in the superconducting phase is absent or does not appear until the thermal energy is far below the characteristic strength of electron-electron interactions. Metals that are close to but fail to achieve a coherent ground state at T=0 deserve our attention because they can display unusual and unexplained properties.

HEAVY FERMION SYSTEMS : In the so called heavy Fermion systems almost localized 4f or 5f electrons are close to achieving antiferromagnetic order but remain paramagnetic because conduction electrons screen their magnetic moment. The proximity to an antiferromagnetically ordered phase however results in a metal with unusually strong low energy spin fluctuations and conduction electrons behaving as though they where 1000 times heavier than normal. In preparing an invited review article covering neutron scattering from heavy Fermion systems, it became obvious to me that most previous experiments in this field, ours included, have taken the form of brief surveys of `remarkable' and unusual properties. Now is the time then to settle down with a well defined system and obtain an extensive characterization of the quintessential spin fluctuations which will warrant and provoke more detailed theories than currently available. In analogy with my suggestions for the low dimensional magnets I also believe we will learn more about the origin of the heavy Fermion state by driving the material from that unusual phase into more common metallic ground states. The application of high pressure has already been shown to induce a transition from heavy Fermion behavior to more conventional metallic behavior. On the other hand changing the electron density by doping can induce the transition to metallic antiferromagnetism. By inelastic magnetic neutron scattering experiments I plan to provide the important microscopic characterization of the evolution from the heavy Fermion phase into the more common metallic phases from which it results.

TRANSITION-METAL OXIDES : Perhaps the most dramatic of all T=0 phase transitions is the metal-insulator transition which changes the resistivity of a material from 0 to infinity as, for example, the electron density is varied. Since the phase diagram of all high temperature oxide superconductors contains an insulating phase it has become important to understand under what circumstances this transition takes place. My recent discoveries in $\rm V_2O_3$ have put me in a position to produce real progress in this respect and maybe to understand what prevents a strongly correlated metallic oxide such as $\rm V_2O_3$ from superconducting. We have nearly completed a careful characterization of the incommensurate spin fluctuations in the metallic phase which appears to be a classical example of a metal close to a Fermi surface nesting instability. It is truly exciting now to discover what happens to the helical spin density wave when the Fermi surface completely vanishes at the transition to the insulating phase. Will the insulating gap show up in the spin fluctuation spectrum ? Will the wavelength of the spin density wave change abruptly? Exploring such unknown territory while aware of the progress in our understanding of condensed matter which will result is what motivates me to strive for perfection and excellence in my experimental research.

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