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Register a new userExcitonic condensation of strongly correlated electrons: the case of Pr$_{0.5}$Ca$_{0.5}$CoO$_3$
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We use a combination of dynamical mean-field model calculations and LDA+U material specific calculations to investigate the low temperature phase transition in the compounds from the (Pr$_{1-y}$R$_y$)$_x$Ca$_{1-x}$CoO$_3$ (R=Nd, Sm, Eu, Gd, Tb, Y) family (PCCO). The transition, marked by a sharp peak in the specific heat, leads to an exponential increase of dc resistivity and a drop of the magnetic susceptibility, but no order parameter has been identified yet. We show that condensation of spin-triplet, atomic-size excitons provides a consistent explanation of the observed physics. In particular, it explains the exchange splitting on the Pr sites and the simultaneous Pr valence transition. The excitonic condensation in PCCO is an example of a general behavior expected in certain systems in the proximity of a spin-state transition.
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We studied the charge-orbital ordering in the superlattice of charge-ordered insulating Pr$_{0.5}$Ca$_{0.5}$MnO$_3$ and ferromagnetic metallic La$_{0.5}$Sr$_{0.5}$MnO$_3$ by resonant soft x-ray diffraction. A temperature-dependent incommensurability is found in the orbital order. In addition, a large hysteresis is observed that is caused by phase competition between insulating charge ordered and metallic ferromagnetic states. No magnetic phase transitions are observed in contrast to bulk, confirming the unique character of the superlattice. The deviation from the commensurate orbital order can be directly related to the decrease of ordered-layer thickness that leads to a decoupling of the orbital-ordered planes along the c axis.
The idea of exciton condensation in solids was introduced in 1960s with the analogy to superconductivity in mind. While exciton supercurrents have been realized only in artificial quantum-well structures so far, the application of the concept of excitonic condensation to bulk solids leads to a rich spectrum of thermodynamic phases with diverse physical properties. In this review we discuss recent developments in the theory of exciton condensation in systems described by Hubbard-type models. In particular, we focus on the connections to their various strong-coupling limits that have been studied in other contexts, e.g., cold atoms physics. One of our goals is to provide a dictionary which would allow the reader to efficiently combine results obtained in these different fields.
We report low temperature specific heat measurements of Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ ($0.3leq x leq 0.5$) and La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ with and without applied magnetic field. An excess specific heat, $C^{prime}(T)$, of non-magnetic origin associated with charge ordering is found for all the samples. A magnetic field sufficient to induce the transition from the charge-ordered state to the ferromagnetic metallic state does not completely remove the $C^{prime}$ contribution. This suggests that the charge ordering is not completely destroyed by a melting magnetic field. In addition, the specific heat of the Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ compounds exhibit a large contribution linear in temperature ($gamma T$) originating from magnetic and charge disorder.
We report detailed dc magnetization, linear and non-linear ac susceptibility measurements on the hole doped disordered cobaltite La$_{0.5}$Ba$_{0.5}$CoO$_3$. Our results show that the magnetically ordered state of the system consists of coexisting non-ferromagnetic phases along with percolating ferromagnetic-clusters. The percolating ferromagnetic-clusters possibly start a magnetic ordering at the Curie temperature of 201.5(5)~K. The non-ferromagnetic phases mainly consist of antiferromagnetic-clusters with size smaller than the ferromagnetic-clusters. Below Curie temperature the system exhibits an irreversibility in the field cooled and zero field cooled magnetization and frequency dependence in the peak of ac susceptibility. These dynamical features indicate towards the possible coexistence of spin-glass phase along with ferromagnetic-clusters similar to La$_{1-x}$Sr$_{x}$CoO$_3$ (x$geq$0.18), but the absence of field divergence in third harmonic of ac susceptibility and zero field cooled memory clearly rule out any such possibility. We argue that the spin-glass phase in La$_{1-x}$Sr$_{x}$CoO$_3$ (x$geq$0.18) is associated with the presence of incommensurate antiferromagnetic ordering in non-ferromagnetic phases which is absent in La$_{0.5}$Ba$_{0.5}$CoO$_3$. Our analysis show that the observed dynamical features in La$_{0.5}$Ba$_{0.5}$CoO$_3$ are possibly due to progressive thermal blocking of ferromagnetic-clusters which is further confirmed by the Wohlfarths model of superparamagnetism. The frequency dependence of the peak of ac susceptibility obeys the Vogel-Fulcher law with $tau_0approx10^{-9}$s. This together with the existence of an AT line in H-T space indicates the presence of significant inter-cluster interaction among these ferromagnetic-clusters.
Dc magnetic measurements across the charge ordering (CO) transition temperature (T$_{CO}$) in polycrystalline Pr$_{0.5}$Ca$_{0.5}$Mn$_{0.975}$Al$_{0.025}$O$_3$ have been performed under simultaneous influence of external hydrostatic pressure (P) and magnetic field (H). We show the first experimental evidence that the melting of charge order instability obey an interesting scaling function, $delta$T$_{CO}$/P$^alpha$ = $f$(H/P$^beta$) in H-P-T landscape, where $delta$T$_{CO}$ is the suppression of T$_{CO}$ by P and H. Corresponding values of the exponents, $alpha$ = 1.63 and $beta$ = 0.33 have been extracted from data collapsing phenomena. Possible origin of such a scaling behavior has been discussed.
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