Researchers at the University of Illinois at Urbana-Champaign have discovered a charge density wave of electrons gaining mass as they interact with the material’s lattice back ions over long distances.
This new research, led by Assistant Professor Fahad Mahmoud (Physics, Materials Research Laboratory) and postdoc Soyeon Kim (current postdoctoral researcher at Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory), is a direct measurement of the Anderson-Higgs mechanism (mass gain) and the first known demonstration of a massive string in charge density wave matter, a prediction made more than 40 years ago.
Their paper, “Observation of a bulky faun in a charge-density wave insulator,” with these results, was recently published in nature materials.
Collective excitations of condensed matter phases often play a fundamental role in the development of fundamental theories for a variety of materials, including superconductors, quantum magnets and charge density waves. In a simple metal, electrons are uniformly distributed through space; The electron density at one point in space is equal to the density of another point in space. However, in certain metals, the electron density develops a sinusoidal (wave) pattern (charge density wave). Mahmoud explains that given that a charge-density wave is frozen in space, if the wave is disturbed, it will “ring” (i.e. a collective excitation is generated for it). It can resonate with a change in the amplitude of the wave pattern, or the charge density wave can slide back and forth (phase shift). The subsequent collective excitation is called a phason and is like sound waves in a substance – it has negligible mass.
More than 40 years ago, researchers predicted that if a phaseon interacted strongly with the back lattice of ions over long distances (long-range Coulomb interactions), it would try to pull in heavy ions as they moved. As a result, the phazon will require much more energy to move it—the phazon is said to be “gaining mass.” This mass gain of the car is thought to occur due to the same mechanism by which all the fundamental massive particles in the universe gain mass (a phenomenon known as the Anderson-Higgs mechanism). Direct observation of this collective gain has remained elusive, mainly due to the lack of long-range Coulomb interactions in most charge density wave materials.
The material used in this study was tantalum selenium iodide (TaSe).4)2I) is a very good insulator at low temperatures and is one of the best known insulators for charge density waves. Because of this, long-range Coulomb interactions are likely to be present in the system and those interactions could give mass to massless excitations. Theoretically, if matter is heated, it will become less insulating, the Coulomb interactions will weaken, and the massive phaseon should become massless.
Mahmoud, Kim and their assistants were able to investigate the electric charge density wave in TaSe4)2I by developing a nonlinear optical technique known as time-domain (THz) emission spectroscopy at low temperatures (below 10 K, -442 degrees Fahrenheit). Using this technique, an ultrafast infrared laser pulse, lasting less than 150 fs (1 fs is a millionth of a billionth of a second), was shone on TaSe4)2I am testing, which generates the collective excitation of the system. What they discovered is a massive phaseon that radiates in the THz region of frequencies, with an extremely narrow bandwidth. When they heated the material, the massive phaseon became massless (it stopped radiating), matching old theoretical predictions.
while (TaSe4)2I lead hosts a huge car, which can be a very difficult material to work with because it grows as very thin needles which makes aligning the sample very difficult. Kim described the process as “like trying to highlight the side of a chopstick.” One of the collaborators on this research, Daniel Shoemaker (Associate Professor, MatSE, UIUC) was able to grow (TaSe4)2I crystals are of considerably large width, which enabled the application of THz emission spectroscopy to this material.
Patrick Lee, the William and Emma Rogers Professor of Physics at MIT and one of the pioneers of the theory that predicts the massive lady responsible: density waves, commented. “It speaks to the power of modern nonlinear optical techniques and the creativity of experimentalists. The method is generic, and we may see applications to other collective modes as well.”
At the applied level, generating narrowband radiation in the region of THz frequencies can be very challenging. However, due to the strikingly narrow bandwidth of THz radiation generated by the massive phaseon in (TaSe4)2The possibility of developing it (and other materials) as a THz emitter is very promising. The frequency and intensity of the THz emission can be controlled by changing the sample properties, applying external magnetic fields or stress.
Mahmoud summarizes, “This is the first known demonstration of a bulky Fason in charge density wave matter and resolves the long-standing question of whether charge density wave gains mass by coupling with long-range Coulomb interactions. This is a key result that will have a profound impact on the field of highly correlated materials.” , and in understanding the interplay of interactions, wave-density ordering, and superconductivity in materials.”
Other contributors to this work include Yinchuan Lv (graduate student, Physics, UIUC), Xiao-Qi Sun (postdoc, physics, UIUC), Chengxi Zhao (graduate student, MatSE/MRL, UIUC), Nina Belinsky ( Graduate Student, Physics/MRL, UIUC), Azel Murzabekova (Graduate Student, Physics/MRL, UIUC), Kejian Qu (Graduate Student, Physics/MRL, UIUC), Ryan A. Duncan (Postdoc, Stanford Institute PULSE/Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory), Quynh LD Nguyen (Project Scientist, Stanford PULSE/Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory), Mariano Trigo (Principal Scientist, Stanford PULSE/Stanford Science Institute Materials and Energy, SLAC National Accelerator Laboratory), and Barry Bradlin (Assistant Professor, Physics, UIUC).
