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In the current epoch of computing, silicon is crucial. Nevertheless, akin to other semiconductors prevalent in the tech world, minute quantities of diverse elements are frequently incorporated into silicon to modulate its electrical properties, a technique referred to as doping.
At present, researchers have enhanced doping considerably, substituting one out of every eight atoms in germanium — a semiconductor akin to silicon – with the superconducting element gallium, such that the material synthesizes a fresh superconductor suitable for technologies like quantum computation and detection.
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“I surmise there are numerous compelling reasons to be enthusiastic about this,” expressed study co-author Javad Shabani, a physics professor at New York University, in conversation with Live Science.
The premise of doping a semiconductor sufficiently to transform it into a superconductor was initially suggested in 1964 by Marvin Cohen, Emeritus Professor at the University of California, Berkeley, then affiliated with the University of Chicago. This notion was revisited during the 2000s and 2010s, prompting several teams to endeavor to saturate silicon and germanium with superconducting metals to ascertain whether they could materialize the theoretically anticipated novel phase — yet, they encountered obstacles.
“Upon saturation, the lattice is somewhat compromised,” Shabani elucidated, appending that subsequent heating and “annealing” are necessary to conduct further experimentation regarding superconducting attributes, rendering it ambiguous as to whether dopant atoms have merely established an enclave of superconducting material, or whether a distinct superconducting phase has materialized within the bombarded element. He and his team even replicated the experiments themselves. “We simply compounded the enigma,” he conveyed to Live Science.
Layer of hope
Breakthrough materialized when they transitioned to a methodology termed molecular beam epitaxy. Via this approach, they fashioned the germanium crystal layer by layer, exposing the surface to germanium atoms under precise conditions and gallium atom concentrations to ensure the substitution of a gallium atom for a germanium atom in each unit cell of the crystal.
Shabani proposed that they likely weren’t isolated in considering molecular beam epitaxy as a potentially worthwhile endeavor. Nevertheless, prior efforts had been discouraged due to prevalent pessimistic conjectures suggesting that doping to the requisite degrees was not physically attainable contingent on assumptions akin to solubility thresholds. As an illustration, one can progressively dissolve sugar in water up to a threshold, but upon reaching the solubility threshold, the solution becomes saturated, impeding further dissolution and leading to sugar accumulation in solid clumps. Applying analogous rationales to doping may lead to the assumption that beyond a certain limit, the dopant won’t distribute uniformly but rather aggregate.
However, doping via molecular beam epitaxy constitutes an altogether distinct process — the concurrent deposition of both materials — thereby bypassing limitations analogous to a solubility threshold. “We essentially apply a coating,” Shabani commented, affirming the absence of any breached principles.
To validate their findings, Shabani and his team forwarded their samples to counterparts at the University of Queensland in Australia for characterization utilizing their cutting-edge instruments. As underscored by Julian Steele, a researcher at the University of Queensland in Australia involved in the characterization experiments, generally “the requisite precision” to characterize the pertinent superconducting layer embedded within the bulk germanium would be experimentally “intractable.”
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“It represented an auspicious melding of meticulously defined crystal strata and exceptionally precise measurements functioning in concert to generate data with atomic-level resolution,” Steele affirmed in correspondence with Live Science. “The resultant outcome constitutes an indisputably unequivocal depiction of a novel and compelling quantum substance.”
The investigators further observed that the superconducting transition temperature registered at 3.5 Kelvin (marginally above absolute zero) — cryogenically frigid, albeit not as low as the 1 Kelvin prerequisite to attain superconductivity in unadulterated gallium. As Shabani emphasized, a transition temperature even lower than that of the “parent” superconductor, in this instance gallium, would typically be anticipated. This engenders some intriguing questions concerning the prevailing mechanism(s) responsible for superconducting behavior.
“Witnessing sustained research alongside accomplishments in the domain of superconductivity within doped semiconductors, which I pioneered over six decades prior, is immensely gratifying,” Cohen conveyed to Live Science via electronic communication. “I hold the conviction that further insights into superconductivity can be gleaned via research into systems of this typology.”
Scientists produce a material with one in every eight germanium atoms replaced with gallium, so that it superconducts but still interfaces with germanium semiconductors. Building more robust qubits
Peter Jacobson, a University of Queensland researcher also contributing to the characterization experiments, felt particularly impressed by “how clearly the distortion emerged.”
He pointed out that the atom spacing within the plane of each deposited crystal layer remained virtually unaltered from the pristine germanium seed layer, but the spacing perpendicular to this plane experienced a marginal increase, precisely as expected to accommodate the modestly larger gallium atoms. “Observing this behavior with such clarity serves as a potent indicator of the minimal degree of disorder present within these films.”
That diminished disorder bodes favorably for any entity aspiring to “cultivate” alternating strata of semiconducting and superconducting material, an achievement previously unattainable.
This significantly elevates the device density attainable on a wafer, as it enables construction of 3D stacks. Shabani employed the illustration of a Josephson junction — a junction entailing a non-superconducting material nestled between superconducting material on either side. These junctions find application in quantum sensing and as qubits in quantum computing.
“One can integrate 25 million of these onto a solitary wafer,” he stated. He underscores the current size of each Josephson Junction at approximately one millimeter and added: “Each of these could embody a qubit. Potentially a pixel of a sensor, correct?”
The near adherence to conventional crystalline ordering might confer supplementary advantages in shielding against “decoherence” of superconducting qubits. The loss of coherence in qubits incapacitates their ability to concurrently retain multiple values, compelling them to settle for a definite value and, fundamentally, behave as a classical qubit devoid of quantum behavior advantages.
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This represents a vexation in initiatives pertaining to quantum computing, although associations between some of this decoherence and amorphous attributes in utilized materials have been postulated. Additional experiments will be essential for validation, however the augmented crystallinity characterizing these molecular beam epitaxy gallium-doped germanium structures may bolster qubit resilience against decoherence.
The evident prospect lies in capitalizing on pre-existing fabrication techniques for producing germanium and silicon semiconductor computer processors and devices.
“A trillion-dollar silicon germanium infrastructure stands poised to incorporate superconductivity as an emerging asset within its arsenal,” articulated Shabani. “This may substantially expedite solid-state quantum computing — compressing the projected timeline considerably.”
Anna DemmingLive Science Contributor
Anna Demming serves as a freelance science journalist and editor. She possesses a PhD in physics from King’s College London, with a focus on nanophotonics and the interactions between light and minuscule entities. Her editorial journey commenced with Nature Publishing Group in Tokyo during 2006. Subsequently, she functioned as an editor for Physics World and New Scientist. Her contributions as a freelancer extend to publications including The Guardian, New Scientist, Chemistry World, and Physics World, among others. While she is fond of science in general, her particular interests lie in materials science and physics, encompassing quantum physics and condensed matter.
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