Abstract:Bismuth displays puzzling superconducting properties. In its crystalline equilibrium phase, it does not seem to superconduct at accessible low temperatures. However, in the amorphous phase it displays superconductivity at ~ 6 K. Under pressure bismuth has been found to superconduct at Tcs that go from 3.9 K to 8.5 K depending on the phase obtained. So the question is: what electronic or vibrational changes occur that explains this radical transformation in the conducting behavior of this material? In a recent … Show more
“…No sudden changes occur anywhere for the expansions studied, contrary to what was observed in Ref. [19], which may indicate a tendency to possible modifications of the structure. However, a geometry optimization of the expanded crystalline samples is in order to elucidate this point.…”
Section: Results and Analysiscontrasting
confidence: 91%
“…Figure7displays the results for the values of the eDoS at the Fermi level where trends that appear to be systematic are shown. No sudden changes occur anywhere for the expansions studied, contrary to what was observed in Ref [19],. which may indicate a tendency to possible modifications of the structure.…”
Materials may behave in non-expected ways when subject to unexpected conditions. For example, when Bi was turned into an amorphous phase (a-Bi) unexpectedly it became a superconductor at temperatures below 10 K. We provided an explanation as to why a-Bi superconducts and the crystalline (c-Bi) had not been found to do so: we computer calculated their electronic properties and found that a-Bi has a larger electron density of states, eDoS, at the Fermi surface than c-Bi and this explained the phenomenon. We even predicted an upper limit for the superconducting Tc of the crystalline phase, which was experimentally corroborated within the following year. We now decided to investigate what happens to crystalline (Wyckoff structure) and amorphous Bi when pressures below the atmospheric are applied (expansion). Here we show that when expanded, c-Bi becomes more metallic, since the eDoS increases when the volume increases for the Wyckoff structure, while the amorphous eDoS decreases. If the crystalline structure is maintained its Tc would rise under expansion, whereas it would diminish for the a-Bi. Expansion can be obtained in the laboratory by chemically etching Bi-based alloys, a process also known as dealloying, for example.
“…No sudden changes occur anywhere for the expansions studied, contrary to what was observed in Ref. [19], which may indicate a tendency to possible modifications of the structure. However, a geometry optimization of the expanded crystalline samples is in order to elucidate this point.…”
Section: Results and Analysiscontrasting
confidence: 91%
“…Figure7displays the results for the values of the eDoS at the Fermi level where trends that appear to be systematic are shown. No sudden changes occur anywhere for the expansions studied, contrary to what was observed in Ref [19],. which may indicate a tendency to possible modifications of the structure.…”
Materials may behave in non-expected ways when subject to unexpected conditions. For example, when Bi was turned into an amorphous phase (a-Bi) unexpectedly it became a superconductor at temperatures below 10 K. We provided an explanation as to why a-Bi superconducts and the crystalline (c-Bi) had not been found to do so: we computer calculated their electronic properties and found that a-Bi has a larger electron density of states, eDoS, at the Fermi surface than c-Bi and this explained the phenomenon. We even predicted an upper limit for the superconducting Tc of the crystalline phase, which was experimentally corroborated within the following year. We now decided to investigate what happens to crystalline (Wyckoff structure) and amorphous Bi when pressures below the atmospheric are applied (expansion). Here we show that when expanded, c-Bi becomes more metallic, since the eDoS increases when the volume increases for the Wyckoff structure, while the amorphous eDoS decreases. If the crystalline structure is maintained its Tc would rise under expansion, whereas it would diminish for the a-Bi. Expansion can be obtained in the laboratory by chemically etching Bi-based alloys, a process also known as dealloying, for example.
“…The dependence of T c on the parameters is represented in Fig. 3 for a specific value of the pairing potential where the strong dependence of the transition temperature on the factor N(E F ) for a given value of V can be observed 14 . Figure 3 Dependence of the superconducting transition temperature T c on the electronic N(E F ) and vibrational θ D parameters.…”
The first successful theory of superconductivity was the one proposed by Bardeen, Cooper and Schrieffer in 1957. This breakthrough fostered a remarkable growth of the field that propitiated progress and questionings, generating alternative theories to explain specific phenomena. For example, it has been argued that Bismuth, being a semimetal with a low number of carriers, does not comply with the basic hypotheses underlying BCS and therefore a different approach should be considered. Nevertheless, in 2016 based on BCS we put forth a prediction that Bi at ambient pressure becomes a superconductor at 1.3 mK. A year later an experimental group corroborated that in fact Bi is a superconductor with a transition temperature of 0.53 mK, a result that eluded previous work. So, since Bi is superconductive in almost all the different structures and phases, the question is why Bi-IV has been elusive and has not been found yet to superconduct? Here we present a study of the electronic and vibrational properties of Bi-IV and infer its possible superconductivity using a BCS approach. We predict that if the Bi-IV phase structure were cooled down to liquid helium temperatures it would also superconduct at a Tc of 4.25 K.
“…(1), is calculated from the vibrational spectra of each structure. For this we use an expression due to Grimvall, for the Debye frequency ω D 12 that we utilized in our previous work with good results 5–7 . F(ω) is the vDoS of the supercell under consideration, and ω max is the maximum frequency of the corresponding vibrational spectrum.…”
Section: Procedure: the Bcs Approachmentioning
confidence: 99%
“…A year later an experimental group corroborated that in fact Bi is a superconductor with a transition temperature of 0.5 mK, a result that eluded previous work 4 . Analogously we have studied low dimensional structures of bismuth, bilayers or bismuthene, and have predicted that they may become superconductive, with a transition temperature of T c L = 2.61 K, provided the Cooper pairing potential manifests itself in a similar manner as in the bulk 5,6 . Finally, since it is very suggestive that all phases of bismuth under pressure should superconduct we calculated N(E) and F(ω ) for Bi-IV and predicted a transition temperature of 4.25 K 7 , to be corroborated.…”
All solid phases of bismuth under pressure, but one, have been experimentally found to superconduct. From Bi-I to Bi-V, avoiding Bi-IV, they become superconductors and perhaps Bi-IV may also become superconductive. To investigate the influence of the electronic properties
N(E
) and the vibrational properties
F(ω)
on their superconductivity we have
ab initio
calculated them for the corresponding experimental crystalline structures, and using a BCS approach have been able to determine their critical temperatures
T
c
obtaining results close to experiment: For Bi-I (The Wyckoff Phase) we predicted a transition temperature of less than 1.3 mK and a year later a
T
c
of 0.5 mK was measured; for Bi-II
T
c
is 3.9 K measured and 3.6 K calculated; Bi-III has a measured
T
c
of 7 K and 6.5 K calculated for the structure reported by Chen
et al
., and for Bi-V
T
c
~ 8 K measured and 6.8 K calculated. Bi-IV has not been found to be a superconductor, but we have recently predicted a
T
c
of 4.25 K.
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