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Register a new userSpin-orbit effects surfacing on manganites
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Added by
Maria Jose Calderon
Publication date
2018
fields
Physics
and research's language is
English
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Spin-orbit coupling in magnetic systems lacking inversion symmetry can give rise to non trivial spin textures. Magnetic thin films and heterostructures are potential candidates for the formation of skyrmions and other non-collinear spin configurations as inversion symmetry is inherently lost at their surfaces and interfaces. However, manganites, in spite of their extraordinarily rich magnetic phase diagram, have not yet been considered of interest within this context as their spin-orbit coupling is assumed to be negligible. We demonstrate here, by means of angular dependent X-ray linear dichroism experiments and theoretical calculations, the existence of a noncollinear antiferromagnetic ordering at the surface of ferromagnetic La$_{2/3}$Sr$_{1/3}$MnO$_3$ thin films whose properties can only be explained by an unexpectedly large enhancement of the spin-orbit interaction. Our results reveal that spin-orbit coupling, usually assumed to be very small on manganites, can be significantly enhanced at surfaces and interfaces adding a new twist to the possible magnetic orders that can arise in electronically reconstructed systems.
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We study a two-dimensional effective orbital superexchange model derived for strongly correlated e_g electrons coupled to t_{2g} core spins in layered manganites. One finds that the ferromagnetic and antiferromagnetic correlations closely compete, and small changes of parameters can switch the type of magnetic order. For the same reason, spin order is easily destroyed with rising temperature, while alternating orbital correlations can persist to temperatures where FM order has already melted. A scenario for the AF phase observed in LaSrMnO_4 is presented.
Oxygen isotope effects on the transport properties have been studied in high-quality epitaxial thin films of La_{0.75}Ca_{0.25}MnO_{3} and Nd_{0.7}Sr_{0.3}MnO_{3}. In the paramagnetic state, the resistivity can be well fitted by rho (T) = (A/sqrt{T})exp(E_{rho}/k_{B}T) with the parameters A and E_{a} depending strongly on the oxygen isotope mass. The resistivity below 80 K almost perfectly follows rho = rho_{o}+ Bomega_{s}/sinh^{2}(hbaromega_{s}/2k_{B}T) with hbaromega_{s}/k_{B} sim 100 K. Both rho_{o} and B increase by about 15(3)% upon raplacing $^{16}$O by $^{18}$O. The results provide quantitative constraints on the basic physics of manganites.
We present a first-principle study of spin-orbit coupling effects on the Fermi surface of Sr2RuO4 and Sr2RhO4. For nearly degenerate bands, spin-orbit coupling leads to a dramatic change of the Fermi surface with respect to non-relativistic calculations; as evidenced by the comparison with experiments on Sr2RhO4, it cannot be disregarded. For Sr2RuO4, the Fermi surface modifications are more subtle but equally dramatic in the detail: spin-orbit coupling induces a strong momentum dependence, normal to the RuO2 planes, for both orbital and spin character of the low-energy electronic states. These findings have profound implications for the understanding of unconventional superconductivity in Sr2RuO4.
Spin-orbit effects in heavy 5$d$ transition metal oxides, in particular, iridates, have received enormous current interest due to the prediction as well as the realization of a plethora of exotic and unconventional magnetic properties. While a bulk of these works are based on tetravalent iridates ($d^5$), where the counter-intuitive insulating state of the rather extended 5$d$ orbitals are explained by invoking strong spin-orbit coupling, the recent quest in iridate research has shifted to the other valencies of Ir, of which pentavalent iridates constitute a notable representative. In contrast to the tetravalent iridates, spin-orbit entangled electrons in $d^4$ systems are expected to be confined to the $J = 0$ singlet state without any resultant moment or magnetic response. However, it has been recently predicted that, magnetism in $d^4$ systems may occur via magnetic condensation of excitations across spin-orbit-coupled states. In reality, the magnetism in Ir$^{5+}$ systems are often quite debatable both from theoretical as well as experimental point of view. Here we provide a comprehensive overview of the spin-orbit coupled $d^4$ model systems and its implications in the studied pentavalent iridates. In particular, we review here the current experimental and theoretical understanding of the double perovskite ($A_2B$YIrO$_6$, $A =$ Sr, Ba, $B =$Y, Sc, Gd), 6H-perovskite (Ba$_3M$Ir$_2$O$_9$, $M =$ Zn, Mg, Sr, Ca), post-perovskite (NaIrO$_3$), and Hexagonal (Sr$_3$MIrO$_6$) iridates, along with a number of open questions that require future investigation.
In this paper, we present a Kane-Mele model in the presence of magnetic field and next nearest neighbors hopping amplitudes for investigations the electronic and optical properties of monolayer Germanene. Specially, we address the dynamical conductivity of the structure as a function of photon frequency and in the presence of magnetic field and spin-orbit coupling at finite temperature. Using linear response theory and Greens function approach, the frequency dependence of optical conductivity has been obtained in the context of Kane-Mele model Hamiltonian. Our results show a finite Drude response at low frequency at non zero value for magnetic field in the presence of spin-orbit coupling. However Drude weight gets remarkable amount in the presence of electron doping. The thermal conductivity and specific heat increase with increasing the temperature at low amounts of temperature due to the increasing of thermal energy of charge carriers and excitation of them to the conduction bands. The results for Seebeck coefficient show the sign of thermopower is negative in the presence of spin-orbit coupling. Also we have studied the temperature dependence of electrical conductivity of Germanene monolayer due to both spin orbit coupling and magnetic field factors in details.
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