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Pressure Induced Quantum Phase Transitions

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 Added by Gillian Gehring
 Publication date 2008
  fields Physics
and research's language is English
 Authors G. A. Gehring




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A quantum critical point is approached by applying pressure in a number of magnetic metals. The observed dependence of Tc on pressure necessarily means that the magnetic energy is coupled to the lattice. A first order phase transition occurs if this coupling exceeds a critical value: this is inevitable if diverges as Tc approaches zero. It is argued that this is the cause of the first order transition that is observed in many systems. Using Landau theory we obtain expressions for the boundaries of the region where phase separation occurs that agree well with experiments done on MnSi and other materials. The theory can be used to obtain very approximate values for the temperature and pressure at the tricritical point in terms of quantities measured at ambient pressure and the measured values of along the second order line. The values of the tricritical temperature for various materials obtained from Landau theory are too low but it is shown that the predicted values will rise if the effects of fluctuations are included.



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Topological insulators (TIs) containing 4f electrons have recently attracted intensive interests due to the possible interplay of their non-trivial topological properties and strong electronic correlations. YbB6 and SmB6 are the prototypical systems with such unusual properties, which may be tuned by external pressure to give rise to new emergent phenomena. Here, we report the first observation, through in-situ high pressure resistance, Hall, X-ray diffraction and X-ray absorption measurements, of two pressure-induced quantum phase transitions (QPTs) in YbB6. Our data revealthat the two insulating phases are separated by a metallic phase due to the pressure-driven valence change of Yb f-orbitals. In combination with previous studies, our results suggest that the two insulating states may be topologically different in nature and originate from the d-p and d-f hybridization, respectively. The tunable topological properties of YbB6 revealed in this study may shed light on the intriguing correlation between the topology and the 4f electrons from the perspective of pressure dependent studies.
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288 - Debalina Banerjee 2021
Pressure induced isostructural insulator to metal transition for SmS is characterised by the presence of an intermediate valence state at higher pressure which cannot be captured by the density functional theory. As a direct outcome of including the charge and spin fluctuations incorporated in dynamical mean field theory, we see the emergence of insulating and metallic phases with increasing pressure as a function of changing valence. This is accompanied by significantly improved predictions of the equilibrium lattice constants and bulk moduli for all Sm-monochalcogenides verifying experiments. Nudged Elastic Band analysis reveals the insulating states to have a finite quasiparticle weight, decreasing as the gap closes rendering the transition to be not Mott-like, and classifies these materials as correlated band insulators. The difference between the discontinuous and continuous natures of these transitions can be attributed to the closeness of the sharply resonant Sm-4f peaks to the fermi level in the predicted metallic states in SmS as compared to SmSe and SmTe.
The relationship is established between the Berry phase and spin crossover in condensed matter physics induced by high pressure. It is shown that the geometric phase has topological origin and can be considered as the order parameter for such transition.
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