UH Physicist John Miller on Collective Quantum Tunneling of CDW Electrons

Q & A with UH Physicist John Miller on Collective Quantum Tunneling of CDW Electrons
Work Published in Physical Review Letters

John H. Miller, Jr., Ph.D., a professor of physics at University of Houston, was lead author on a paper in the January 20 issue of Physical Review Letters. He is also Director of the High Temperature Superconducting Device Applications and Biophysics Lab at the Texas Center for Superconductivity at UH (TcSUH). A condensed matter physicist, Miller specializes in collective electrical transport phenomena such as superconductivity and charge density waves. Recently, his research has branched into biophysics and biomedical applications.

The paper, “Correlated Quantum Transport of Density Wave Electrons,” discusses simulations based on collective quantum tunneling of Charge Density Wave (CDW) electrons. All physical, chemical, and biological phenomena emerge from quantum mechanisms at the microscopic scale. The paper begins to address the question: “To what extent can quantum coherence over long distances, similar to that in a superconductor, occur in other systems of electrons, perhaps at temperatures comparable to that of the human body (98.6°F or 37°C)?” CDWs form in linear chain compounds and are currently the only known systems showing correlated electron current flow at such high temperatures.

TcSUH provided the main source of funding for this research, but support relevant to sensors and related applications was provided by the National Institutes of Health. The crystal growth work was funded by the National Science Foundation and by the Welch Foundation.

Authors on the paper include Arnold M. Guloy, Ph.D., a UH professor of chemistry, also affiliated with TcSUH; graduate student Asanga I. Wijesinghe in the Department of Physics and TcSUH; and Zhongjia Tang, Ph.D., a postdoctoral fellow in the Department of Chemistry.

In a Question and Answer format, Miller shares some insight on the information covered in the paper.

How would you summarize the article?
The electrons in a Charge Density Wave (CDW), like those in a superconductor, can flow through a linear chain compound en masse, in a highly correlated fashion. Our paper discusses simulations based on collective quantum tunneling of CDW electrons, which agree almost precisely with the measured current of flowing CDW electrons. Our paper proposes that the quantum behavior of electrons at the microscopic scale greatly affects the CDW’s behavior at the macroscopic scale. Specifically we propose that, as in a superconductor, the quantum mechanical Schrödinger equation plays the role of the emergent “classical” equation for the CDW electron fluid – a proposal that could ultimately have broad implications.

Would you briefly describe the research and method?
The work was a combination of theory – computer simulations using the Schrödinger equation – and experiment – electrical transport measurements (e.g., current vs. voltage) on needle-like crystals of niobium triselenide and related materials.

Can you describe the lab work involved?
The niobium triselenide crystals were grown in quartz tubes in tube furnaces (Guloy Lab), and the current-voltage and related measurements were done using a closed-cycle cryostat to vary the temperature (Miller Lab).

What are your goals with this research and in this area of study?
One of my long-term goals is to better understand the emergence of classical reality from the underlying quantum behavior at the microscopic level. For example, in the charged superfluid of electron pairs within a superconductor, the emergent “classical” equation of motion is the Schrödinger equation – not Newton’s second law of motion. Since the Schrödinger equation is the hallmark of quantum mechanics, a superconductor can be considered a type of macroscopic quantum phenomenon.

Every electron acts as both a particle and a wave that spreads out over long distances, and every electron is quantum mechanically entangled with every other electron throughout the entire superconductor. In the superconducting cable wound into an MRI magnet, such long-range quantum coherence extends over distances of many miles.

Our proposal that the Schrödinger equation is also the emergent classical equation for the CDW electron fluid could ultimately have broad implications. CDWs form well above room temperature, even temperatures comparable to that of the human body, in some materials. Certain types of CDWs have been detected in networks of actin filaments, which form cellular cytoskeletal structures in humans and other organisms. Thus, this work could ultimately help elucidate the possible role of quantum coherence in biological systems.

What does this research mean for scientists in this field? What are long-term impacts to the public?
The prevailing dogma for three decades is that a CDW classically slides through the crystal according to Aristotle’s velocity-force relation. Our paper challenges this dogma by proposing that the quantum behavior of electrons is manifested in the CDW’s overall behavior. Experiments by a group in Japan, showing oscillations in CDW current vs. magnetic fields applied to ring-shaped crystals of tantalum trisulfide not only support, but demand, a quantum interpretation. These experiments have stimulated our most recent work.

The initial impact will be to improve our understanding of macroscopic quantum phenomena. Longer term, this work could affect fields ranging from cosmology to biology and could lead to practical applications such as magnetic sensors and quantum computing devices.

How does this impact ongoing and further research in this area?
The most direct impact of our paper to others in the field is to understand the quantum nature of CDWs. Recently, a group headed by Prof. Satoshi Tanda of Hokkaido University in Sapporo, Japan, reported oscillations in CDW current vs. magnetic flux in CDW rings. These oscillations show a period proportional to the ratio of two fundamental constants of nature (Planck’s constant of quantum mechanics and the elementary electric charge), providing perhaps the most conclusive evidence to date that CDW transport is a cooperative quantum phenomenon.

This could lead to sensitive magnetic sensors operating at room temperature. Such sensors would have a broad array of applications, ranging from noninvasively measuring fetal heart signals from inside the mother’s womb to magnetotelluric exploration for geothermal energy sources.

What is the overall significance of this research?
Fundamentally, our research could help us better understand how quantum mechanics and its associated phenomena - wave-particle duality, tunneling, entanglement, etc. - extend to the macroscopic scale. Moreover, the possibility of robust quantum effects in CDWs could potentially lead to ways of making quantum computers that are not plagued by the problem of “decoherence,” where quantum effects are destroyed by the clutter of noise due to atoms randomly moving around at high temperatures. Even the universe as a whole appears to behave like an enormous quantum computer – quite unlike the classical bits of information inside an ordinary computer, as portrayed in the “Matrix” movies.

Our results may have relevance to theories of human consciousness, such as that championed by physicist Roger Penrose, Ph.D., at Oxford and neuroscientist/anesthesiologist Stuart Hameroff, M.D., at Arizona Health Sciences Center. The latter noticed that, under general anesthesia, EEG and evoked potential recordings continue unabated, suggesting that action potentials alone are not sufficient to account to our subjective experience of being conscious. They proposed that quantum coherence mediated by microtubules extending throughout the brain’s neuronal networks plays a key role in consciousness experienced by humans and other organisms. Our paper adds support to at least the possibility of large scale quantum coherence at human body temperatures (37ºC or 98.6ºF), since niobium trisulfide undergoes a CDW transition well above those temperatures. Moreover, many biological structures, including actin filaments, microtubules, DNA, etc., have chain-like structures similar to the linear chains of atoms in CDW materials. I suspect, however, that nature is sufficiently complex and subtle that no current theory of human consciousness precisely hits the mark as to what’s really happening.

Finally, superconductivity has reigned as the preeminent macroscopic quantum phenomenon for over a century – culminating in the discovery of high temperature superconductors and the creation of TcSUH here at UH. Our results suggest the possibility of a new type of high temperature macroscopic quantum phenomenon that could potentially lead to applications yet to be imagined.

What are your next steps?
In addition to studying CDW systems, we also plan to study the quantum properties of magnetic flux vortices in high temperature superconductors. We’re currently revising and expanding a paper that proposes quantum mechanisms for the nucleation of flux vortices in HTS thin films and coated conductors used in superconducting cable. We’ve also submitted a proposal to begin addressing the possibility of coherent quantum effects in biological systems. These include the mitochondrial electron transport chain responsible for human metabolism and microtubules, where quantum coherence has been proposed to play a role in human consciousness.

In general, why did you become interested in this topic and how did you get the idea to examine it?
I began working on charge density waves while carrying out my Ph.D. research at the University of Illinois in the 1980s. My co-thesis advisors were John Tucker and John Bardeen, who was the only person in history to receive two Nobel Prizes in Physics. (Bardeen received his first Nobel Prize in 1956 for the co-invention of the transistor and his second Nobel Prize in 1972 for the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity.) Bardeen had proposed a theory of collective tunneling by CDW electrons, and had utilized Tucker’s theory of photon-assisted tunneling to predict measured properties at high frequencies.

During my Ph.D. work, I proposed a slight modification to the theory and carried out alternating current experiments at megahertz frequencies to test the predictions, finding remarkable agreement between the quantum theory and experiment. Some of my Ph.D. work is described in True Genius: The Life and Science of John Bardeen, by Lillian Hoddeson and Vicki Daitch.

I have been interested in macroscopic quantum phenomena since reading about superconductivity. When I was an undergraduate at Northwestern University in the late 1970s, I saw a movie showing the strange properties of superfluid helium. One of the most extraordinary phenomena is “quantum creep,” where superfluid helium in a container literally climbs over the walls of the container to escape.

How long have you been doing this research?
My work on CDWs began around 1981, but my most recent efforts began in 2009, stimulated by the quantum interference effects in CDW rings reported by the Tanda group in Japan.

- Dr. John Miller and Kathy Major, College of Natural Sciences and Mathematics