Imaging Generalized Wigner Crystal States in a WSe2/WS2 Moiré Superlattice

The Wigner crystal state, first predicted by Eugene Wigner in 19341, has fascinated condensed matter physicists for nearly 90 years2-10. Studies of two-dimensional (2D) electron gases first revealed signatures of the Wigner crystal in electrical transport measurements at high magnetic fields2-4. More recently optical spectroscopy has provided evidence of generalized Wigner crystal states in transition metal dichalcogenide (TMDC) moiré superlattices6-9. Direct observation of the 2D Wigner crystal lattice in real space, however, has remained an outstanding challenge. Scanning tunneling microscopy (STM) in principle has sufficient spatial resolution to image the Wigner crystal, but conventional STM measurements can potentially alter fragile Wigner crystal states in the process of measurement. Here we demonstrate real-space imaging of 2D Wigner crystals in WSe2/WS2 moiré heterostructures using a novel non-invasive STM spectroscopy technique. We employ a graphene sensing layer in close proximity to the WSe2/WS2 moiré superlattice for Wigner crystal imaging, where local STM tunneling current into the graphene sensing layer is modulated by the underlying electron lattice of the Wigner crystal in the WSe2/WS2 heterostructure. Our measurement directly visualizes different lattice configurations associated with Wigner crystal states at fractional electron fillings of n = 1/3, 1/2, and 2/3, where n is the electron number per site. The n=1/3 and n=2/3 Wigner crystals are observed to exhibit a triangle and a honeycomb lattice, respectively, in order to minimize nearest-neighbor occupations. The n = 1/2 state, on the other hand, spontaneously breaks the original C3 symmetry and forms a stripe structure in real space. Our study lays a solid foundation toward the fundamental understanding of rich Wigner crystal states in WSe2/WS2 moiré heterostructures. Furthermore, this new STM technique is generally applicable to imaging novel correlated electron lattices in different van der Waals moiré heterostructures.

On the Logic of Being and Wigner's Astonishment Regarding the Applicability of Mathematics

The Nobel Prize winning Physicist, Eugene Wigner, famously posed a powerful challenge (1960) by asking why is mathematics so effective, especially in the physical sciences. It is possible that the reason for the effectiveness of mathematics is not because mathematics is in any way causative, but instead because mathematics studies the structure of logical possibility and constraint. When plugged into a possible world, mathematics gives us the tools to analyze the logically possible outcomes. Therefore, when a possible world that is expressed mathematically sufficiently aligns with reality, mathematics becomes effective at expressing relationships and outcomes.

Imaging the electronic Wigner crystal in one dimension

The quantum crystal of electrons, predicted more than 80 years ago by Eugene Wigner, remains one of the most elusive states of matter. In this study, we observed the one-dimensional Wigner crystal directly by imaging its charge density in real space. To image, with minimal invasiveness, the many-body electronic density of a carbon nanotube, we used another nanotube as a scanning-charge perturbation. The images we obtained of a few electrons confined in one dimension match the theoretical predictions for strongly interacting crystals. The quantum nature of the crystal emerges in the observed collective tunneling through a potential barrier. These experiments provide the direct evidence for the formation of small Wigner crystals and open the way for studying other fragile interacting states by imaging their many-body density in real space.

Measurement Theory

This chapter reviews measurement theory in quantum mechanics. The measurement prescription in quantum mechanics can be stated in a few lines and has found an enormous range of applications, in all of which it has proved to be consistent with logic and experimental tests. However, the implications seem so bizarre that people such as Albert Einstein and Eugene Wigner have argued that the theory cannot be physically complete as its stands. The chapter then extends the prescription to the case where the state vector is not known. It also discusses some of the “paradoxes” of quantum mechanics. Finally, the chapter presents Bell's theorem, which shows that there cannot be a local underlying deterministic theory for which quantum mechanics plays the role of a statistical approximation.

The Utterly Prosaic Connection between Physics and Mathematics

Eugene Wigner famously argued for the “unreasonable effectiveness of mathematics” as applied to describing physics and other natural sciences in his 1960 essay. That essay has now led to some 58 years of (sometimes anguished) philosophical soul searching—responses range from “So what? Why do you think we developed mathematics in the first place?”, through to extremely speculative ruminations on the existence of the universe (multiverse) as a purely mathematical entity—the Mathematical Universe Hypothesis. In the current essay I will steer an utterly prosaic middle course: Much of the mathematics we develop is informed by physics questions we are trying to solve; and those physics questions for which the most utilitarian mathematics has successfully been developed are typically those where the best physics progress has been made.

New Theories of Nuclear Reactions

Bohr, inspired by Fermi’s discovery of slow neutrons, conceived his theory of the compound nucleus by the end of 1935. He went on to speculate that if the energy of a neutron incident on a nucleus were increased to the fantastically high energy of 1000 million electron volts, the compound nucleus would explode. Using small wooden models Otto Robert Frisch had constructed, Bohr lectured widely on his theory on a trip around the world in the first half of 1937. By then, Russian-born theoretical physicist Gregory Breit and Hungarian-born theoretical physicist Eugene Wigner in Princeton had conceived their fundamentally equivalent theory of neutron+nucleus resonances. Together, their theory and Bohr’s transformed the theory of nuclear reactions. Orso Mario Corbino, Fermi’s mentor, friend, and protector, died on January 23, 1937, at age sixty. Ernest Rutherford, the greatest experimental physicist since Michael Faraday, died on October 19, 1937, at age sixty-six.

Foreword to BIOMATH 2017 Proceedings: Some comments on mathematical modelling and biomathematics.

Both biology and mathematics have existed as well established branches of science for hundreds of years and both, maybe not in a well defined way, have been with the humankind for a couple of thousands of years. Though nature was studied by the ancient civilizations of Mesopotamia, Egypt, the Indian subcontinent and China, the origins of modern biology are typically traced back to the ancient Greece, where Aristotle (384-322 BC) contributed most extensively to its development. Similarly, the ancient Babylonians were able to solve quadratic equation over four millennia ago and we can see the development of mathematical methods in all ancient civilisations, notably in China and on the Indian subcontinent. However, possibly again the Greeks were the first who studied mathematics for its own sake, as a collection of abstract objects and relations between them. Nevertheless, despite the fact that the development of such a mathematics has not required any external stimuli, an amazing feature of the human mind is that a large number of abstract mathematical constructs has proved to be very well suited for describing natural phenomena.This prompted Eugene Wigner to write his famous article The Unreasonable Effectiveness of Mathematics in the Natural Sciences, ...

Just How Unreasonable is the Effectiveness of Mathematics?

Eugene Wigner famously challenged philosophers to account for ‘the unreasonable effectiveness of mathematics’. Mark Steiner responded that mathematics is essentially species specific and thus the strategies involved in its applicability are, at their core, anthropocentric. This chapter tackles Steiner’s claims and suggests that the mystery he sees in mathematics’ applicability can be dispelled by adopting a kind of optimistic attitude with regard to the variety of mathematical structures that are typically made available in any given context. This suggests applying mathematics is simply a matter of finding a structure to fit the phenomena in question. However, as Wilson notes, mathematics is more ‘rigid’ than this attitude assumes and certain ‘special circumstances’ must obtain for it to be brought into contact with physics. We suggest that it is via certain idealizations that these circumstances are constructed and the mystery of the applicability of mathematics is dispelled.