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Abstract
5d iridates have shown vast emergent phenomena due to a strong interplay among their lattice, charge, and spin degrees of freedom, because of which the potential in spintronic application of the thin-film form is highly leveraged. Here we have epitaxially stabilized perovskite SrIr0.8Sn0.2O3 on [001] SrTiO3 substrates through pulsed laser deposition and systematically characterized the structural, electronic, and magnetic properties. Physical property measurements unravel an insulating ground state with a weak ferromagnetism in the compressively strained epitaxial film. The octahedral rotation pattern is identified by synchrotron x-ray diffraction, resolving a mix of a+b−c− and a−b+c− domains. X-ray magnetic resonant scattering directly demonstrates a G-type antiferromagnetic structure of the magnetic order and the spin canting nature of the weak ferromagnetism.
Unexpected, yet useful functionalities emerge when two or more materials merge coherently. Artificial oxide superlattices realize atomic and crystal structures that are not available in nature, thus providing controllable correlated quantum phenomena. This review focuses on 4d and 5d perovskite oxide superlattices, in which the spin–orbit coupling plays a significant role compared with conventional 3d oxide superlattices. Modulations in crystal structures with octahedral distortion, phonon engineering, electronic structures, spin orderings, and dimensionality control are discussed for 4d oxide superlattices. Atomic and magnetic structures, Jeff = 1/2 pseudospin and charge fluctuations, and the integration of topology and correlation are discussed for 5d oxide superlattices. This review provides insights into how correlated quantum phenomena arise from the deliberate design of superlattice structures that give birth to novel functionalities.
Antiferromagnets have been generating intense interest in the spintronics community, owing to their intrinsic appealing properties like zero stray field and ultrafast spin dynamics. While the control of antiferromagnetic (AFM) orders has been realized by various means, applicably appreciated functionalities on the readout side of AFM-based devices are urgently desired. Here, we report the remarkably enhanced anisotropic magnetoresistance (AMR) as giant as ~160% in a simple resistor structure made of AFM Sr2IrO4 without auxiliary reference layer. The underlying mechanism for the giant AMR is an indispensable combination of atomic scale giant-MR-like effect and magnetocrystalline anisotropy energy, which was not accessed earlier. Furthermore, we demonstrate the bistable nonvolatile memory states that can be switched in-situ without the inconvenient heat-assisted procedure, and robustly preserved even at zero magnetic field, due to the modified interlayer coupling by 1% Ga-doping in Sr2IrO4. These findings represent a straightforward step toward the AFM spintronic devices.
Fundamental control of magnetic coupling through heterostructure morphology is a prerequisite for rational engineering of magnetic ground states. We report the tuning of magnetic interactions in superlattices composed of single and bilayers of SrIrO3 inter-spaced with SrTiO3 in analogy to the Ruddlesden-Popper series iridates. Magnetic scattering shows predominately c-axis antiferromagnetic orientation of the magnetic moments for the bilayer, as in Sr3Ir2O7. However, the magnetic excitation gap, measured by resonant inelastic x-ray scattering, is quite different between the two structures, evidencing a significant change in the stability of the competing magnetic phases. In contrast, the single layer iridate hosts a more bulk-like gap. We find these changes are driven by bending of the c-axis Ir-O-Ir bond, which is much weaker in the single layer, and subsequent local environment changes, evidenced through x-ray diffraction and magnetic excitation modeling. Our findings demonstrate how large changes in the magnetic interactions can be tailored and probed in spin-orbit coupled heterostructures by engineering subtle structural modulations.
A fundamental difference between antiferromagnets and ferromagnets is the lack of linear coupling to a uniform magnetic field due to the staggered order parameter. Such coupling is possible via the Dzyaloshinskii-Moriya (DM) interaction but at the expense of reduced antiferromagnetic (AFM) susceptibility due to the canting-induced spin anisotropy. We solve this long-standing problem with a top-down approach that utilizes spin-orbit coupling in the presence of a hidden SU(2) symmetry. We demonstrate giant AFM responses to sub-Tesla external fields by exploiting the extremely strong two-dimensional critical fluctuations preserved under a symmetry-invariant exchange anisotropy, which is built into a square-lattice artificially synthesized as a superlattice of SrIrO3 and SrTiO3. The observed field-induced logarithmic increase of the ordering temperature enables highly efficient control of the AFM order. As antiferromagnets promise to afford switching speed and storage security far beyond ferromagnets, our symmetry-invariant approach unleashes the great potential of functional antiferromagnets.
Here we outline magneto-electric (ME) device concepts based on the voltage control of the interface magnetism of a magneto-electric antiferromagnet gate dielectric formed on a very thin semiconductor channel with large spin orbit coupling. The emphasis of the magneto-electric spin field-effect transistors (ME spin-FETs) is on an anti-ferromagnet spin-orbit read (AFSOR) logic device and a ME spin-FET multiplexer. Both spin-FET schemes exploit the strong spin-orbit coupling in the semiconducting channel materials but remain dependent on the voltage-induced switching of a magneto-electric, so that the switching time is limited only by the switching dynamics of the magneto-electric. The induced exchange field spin-polarizes the channel material, breaking time-reversal symmetry and resulting in the preferential charge transport direction, due to the spin-orbit-driven spin-momentum locking. These devices could provide reliable room-temperature operation with large on/off ratios, well beyond what can be achieved using magnetic tunnel junctions. All of the proposed devices spintronic functionality without the need to switch a ferromagnet, yielding a faster writing speed (~10 ps) at a lower cost in energy (~10 aJ), excellent temperature stability (operational up to 400 K or above), and requiring far fewer device elements (transistor equivalents) than CMOS.
Effects of spin-orbit interactions in condensed matter are an important and rapidly evolving topic. Strong competition between spin-orbit, on-site Coulomb and crystalline electric field interactions in iridates drives exotic quantum states that are unique to this group of materials. In particular, the "Jeff = ½" Mott state served as an early signal that the combined effect of strong spin-orbit and Coulomb interactions in iridates has unique, intriguing consequences. In this Key Issues Review, we survey some current experimental studies of iridates. In essence, these materials tend to defy conventional wisdom: absence of conventional correlations between magnetic and insulating states, avoidance of metallization at high pressures, "S-shaped" I-V characteristic, emergence of an odd-parity hidden order, etc. It is particularly intriguing that there exist conspicuous discrepancies between current experimental results and theoretical proposals that address superconducting, topological and quantum spin liquid phases. This class of materials, in which the lattice degrees of freedom play a critical role seldom seen in other materials, evidently presents some profound intellectual challenges that call for more investigations both experimentally and theoretically. Physical properties unique to these materials may help unlock a world of possibilities for functional materials and devices. We emphasize that, given the rapidly developing nature of this field, this Key Issues Review is by no means an exhaustive report of the current state of experimental studies of iridates. 
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Recent years have seen many examples of how the strong spin-orbit coupling (SOC) present in iridates can stabilize new emergent states that are difficult or impossible to realize in more conventional materials. In this review we outline a representative set of studies detailing how heterostructures based on Ruddlesden-Popper (RP) and perovskite iridates can be used to access yet more novel physics. Beginning with a short synopsis of iridate thin film growth, the effects of the heterostructure morphology on the RP iridates including Sr2IrO4 and SrIrO3 are discussed. Example studies explore the effects of epitaxial strain, laser-excitation to access transient states, topological semimetallicity in SrIrO3, 2D magnetism in artificial RP iridates, and interfacial magnetic coupling between iridate and neighboring layers. Taken together, these works show the fantastic potential for controlled engineering of novel quantum phenomena in iridate heterostructures.
We report an experimental investigation of the two-dimensional Jeff = 1/2 antiferromagnetic Mott insulator by varying the interlayer exchange coupling in [(SrIrO3)1, (SrTiO3)m] (m = 1, 2 and 3) superlattices. Although all samples exhibited an insulating ground state with long-range magnetic order, temperature-dependent resistivity measurements showed a stronger insulating behavior in the m = 2 and m = 3 samples than the m = 1 sample which displayed a clear kink at the magnetic transition. This difference indicates that the blocking effect of the excessive SrTiO3 layer enhances the effective electron-electron correlation and strengthens the Mott phase. The significant reduction of the Neel temperature from 150 K for m = 1 to 40 K for m = 2 demonstrates that the long-range order stability in the former is boosted by a substantial interlayer exchange coupling. Resonant x-ray magnetic scattering revealed that the interlayer exchange coupling has a switchable sign, depending on the SrTiO3 layer number m, for maintaining canting-induced weak ferromagnetism. The nearly unaltered transition temperature between the m = 2 and the m = 3 demonstrated that we have realized a two-dimensional Heisenberg antiferromagnet at finite temperatures with diminishing interlayer exchange coupling.
A high-pressure metastable orthorhombic phase of SrIrO3 perovskite has been epitaxially stabilized on several substrates (DyScO3, GdScO3, NdScO3 and SrTiO3) in the form of thin monocrystalline layers with (110) surface orientation. The unit-cell parameters of the pseudomorphic thin SrIrO3 layers depend on the biaxial strain imposed by the various substrates due to the different lattice mismatches of the particular substrate and the bulk orthorhombic SrIrO3 structure. Using X-ray diffractometry, it is shown that both compressive and tensile strain increase the lattice parameters a and b, while the angle γ scales with the applied strain, being smaller or larger than 90° for compressive or tensile strain, respectively, resulting in a small monoclinic distortion of the layer unit cell. Owing to the similarity of the substrate and layer lattices, the diffraction signals from the two structures overlap partially, which complicates structure determination by standard refinement methods using measured integrated intensities. The measured signal is composed of two interfering components corresponding to the waves diffracted by the substrate and by the layer, where the first component is calculated exactly using the known substrate structure, while the second one is determined by the unknown unit-cell parameters of the layer. The unit-cell parameters were refined in order to fit the experimental data with the simulation. The fractional coordinates of the atoms in the unit cell resulting from the fit are similar to those in the bulk structure.
The perovskite SrIrO3 is an exotic narrow-band metal owing to a confluence of the strengths of the spin-orbit coupling (SOC) and the electron-electron correlations. It has been proposed that topological and magnetic insulating phases can be achieved by tuning the SOC, Hubbard interactions, and/or lattice symmetry. Here, we report that the substitution of nonmagnetic, isovalent Sn4+ for Ir4+ in the SrIr1−xSnxO3 perovskites synthesized under high pressure leads to a metal-insulator transition to an antiferromagnetic (AF) phase at TN≥225 K. The continuous change of the cell volume as detected by x-ray diffraction and the λ-shape transition of the specific heat on cooling through TN demonstrate that the metal-insulator transition is of second order. Neutron powder diffraction results indicate that the Sn substitution enlarges an octahedral-site distortion that reduces the SOC relative to the spin-spin exchange interaction and results in the type-G AF spin ordering below TN. Measurement of high-temperature magnetic susceptibility shows the evolution of magnetic coupling in the paramagnetic phase typical of weak itinerant-electron magnetism in the Sn-substituted samples. A reduced structural symmetry in the magnetically ordered phase leads to an electron gap opening at the Brillouin zone boundary below TN in the same way as proposed by Slater.
Perovskite SrIrO3 has long been proposed as an exotic semimetal induced by the interplay between the spin-orbit coupling and electron correlations. However, its low-lying electronic structure is still lacking. We synthesize high-quality perovskite SrIrO3 (100) films by means of oxide molecular beam epitaxy, and then systemically investigate their low energy electronic structure using in-situ angle-resolved photoemission spectroscopy. We find that the hole-like bands around R and the electron-like bands around U(T) intersect the Fermi level simultaneously, providing the direct evidence of the semimetallic ground state in this compound. Comparing with the density functional theory, we discover that the bandwidth of states near Fermi level is extremely small, and there exists a pronounced mixing between the Jeff = 1/2 and Jeff = 3/2 states. Moreover, our data reveal that the predicted Dirac degeneracy protected by the mirror-symmetry, which was theoretically suggested to be the key to realize the non-trivial topological properties, is actually lifted in perovskite SrIrO3 thin films. Our findings pose strong constraints on the current theoretical models for the 5d iridates.
Recently, the effects of spin-orbit coupling (SOC) in correlated materials
have become one of the most actively studied subjects in condensed matter
physics, as correlations and SOC together can lead to the discovery of new
phases. Among candidate materials, iridium oxides (iridates) have been an
excellent playground to uncover such novel phenomena. In this review, we
discuss recent progress in iridates and related materials, focusing on the
basic concepts, relevant microscopic Hamiltonians, and unusual properties of
iridates in perovskite- and honeycomb-based structures. Perspectives on SOC and
correlation physics beyond iridates are also discussed.
High temperature superconductivity in cuprates remains a defining challenge in condensed matter physics. Recently, a new set of related compounds based on Ir rather than Cu has been discovered that may be on the verge of superconductivity themselves or be able to shed new light on the underlying interactions responsible for superconductivity in the cuprates. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
We theoretically study three-dimensional topological semimetals (TSMs) with
nodal lines protected by crystalline symmetries. Compared with TSMs with point
nodes, e.g., Weyl semimetals and Dirac semimetals, where the conduction and the
valence bands touch at discrete points, in these new TSMs the two bands cross
at closed lines in the Brillouin zone. We propose two new classes of symmetry
protected nodal lines in the absence and in the presence of spin-orbital
coupling (SOC), respectively. In the former, we discuss nodal lines that are
protected by the combination of inversion symmetry and time-reversal symmetry;
yet unlike any previously studied nodal lines in the same symmetry class, each
nodal line has a monopole charge and can only be created (annihilated) in
pairs. In the second class, with SOC, we show that a nonsymmorphic symmetry
(screw axis) protects a four-band crossing nodal line in systems having both
inversion and time-reversal symmetries.
The electronic phase diagram of the weak spin-orbit Mott insulator
(Sr(1-x)Lax)3Ir2O7 is determined via an exhaustive experimental study. Upon
doping electrons via La substitution, an immediate collapse in resistivity
occurs along with a narrow regime of nanoscale phase separation comprised of
antiferromagnetic, insulating regions and paramagnetic, metallic puddles
persisting until x~0.04. Continued electron doping results in an abrupt,
first-order phase boundary where the Neel state is suppressed and a homogenous,
correlated, metallic state appears with an enhanced spin susceptibility and
local moments. As the metallic state is stabilized, a weak structural
distortion develops and suggests a competing instability with the parent
spin-orbit Mott state.
Iridates represent a unique material system that possesses both strong spin-orbit coupling and electron correlation. The interplay between spin-orbit coupling and correlation could facilitate the emergence of novel electronic and magnetic states. In this work, we report on a systematic study of magnetism in the double perovskite iridate and its hole-doped compounds ({\mathrm{La}}_{1-{}x}{\mathrm{Sr}}_{x}){}_{2}{\mathrm{ZnIrO}}_{6} via dc magnetization measurement, heat capacity characterization, electron spin resonance spectroscopy, and simple modeling. Undoped undergoes two magnetic transitions, at \sim K and \sim K, respectively. While magnetic hysteresis loops with large remnant moments are observed below these two transition temperatures, the corresponding magnetic states are shown to be canted antiferromagnetic by the linear increase in magnetization in the high field region along with the observation of antiferromagnetic resonance. The nature of the canted antiferromagnetic states with large canting angles can be understood by a simple model that includes both the Heisenberg exchange interaction and the Dzyaloshinskii-Moriya interaction. With the introduction of by doping, the canted antiferromagnetic phases are suppressed, accompanied by an enhancement of electrical conductivity.
Understanding of metal insulator transitions in a strongly correlated system, driven by Anderson localization (disorder) and/or Mott localization (correlation), is a long standing problem in condensed matter physics. The prevailing fundamental question would be how these two mechanisms contrive to accomplish emergent anomalous behaviors. Here, we have grown high quality perovskite SrIrO3
thin films, containing a strong spin orbit coupled 5d element Ir, on various substrates such as GdScO3 (110), DyScO3 (110), SrTiO3 (001), and NdGaO3 (110) with increasing lattice mismatch, in order to carry out a systematic study on the transport properties. We found that metal insulator transitions can be induced in this system; by either reducing thickness (on best lattice matched substrate) or changing degree of lattice strain (by lattice mismatch between film and substrates) of films. Surprisingly these two pathways seek two distinct types of metal insulator transitions; the former falls into disorder driven Anderson type whereas the latter turns out to be of unconventional Mott-Anderson type with the interplay of disorder and correlation. More interestingly, in the metallic phases of SrIrO3, unusual non-Fermi liquid characteristics emerge in resistivity as Δρ ∝ Tε
with ε evolving from 4/5 to 1 to 3/2 with increasing lattice strain. We discuss theoretical implications of these phenomena to shed light on the metal insulator transitions.
We employ reactive molecular-beam epitaxy to synthesize the metastable
perovskite SrIrO and utilize {\it in situ} angle-resolved photoemission
to reveal its electronic structure as an exotic narrow-band semimetal. We
discover remarkably narrow bands which originate from a confluence of strong
spin-orbit interactions, dimensionality, and both in- and out-of-plane IrO
octahedral rotations. The partial occupation of numerous bands with strongly
mixed orbital characters signals the breakdown of the single-band Mott picture
that characterizes its insulating two-dimensional counterpart,
SrIrO, illustrating the power of structure-property relations for
manipulating the subtle balance between spin-orbit interactions and
electron-electron interactions.
We report point-contact measurements of anisotropic magnetoresistance (AMR)
in a single crystal of antiferromagnetic (AFM) Mott insulator Sr2IrO4. The
point-contact technique is used here as a local probe of magnetotransport
properties on the nanoscale. The measurements at liquid nitrogen temperature
revealed negative magnetoresistances (MRs) (up to 28%) for modest magnetic
fields (250 mT) applied within the IrO2 a-b plane and electric currents flowing
perpendicular to the plane. The angular dependence of MR shows a crossover from
four-fold to two-fold symmetry in response to an increasing magnetic field with
angular variations in resistance from 1-14%. We tentatively attribute the
four-fold symmetry to the crystalline component of AMR and the field-induced
transition to the effects of applied field on the canting of AFM-coupled
moments in Sr2IrO4. The observed AMR is very large compared to the crystalline
AMRs in 3d transition metal alloys/oxides (0.1-0.5%) and can be associated with
the large spin-orbit interactions in this 5d oxide while the transition
provides evidence of correlations between electronic transport, magnetic order
and orbital states. The finding of this work opens an entirely new avenue to
not only gain a new insight into physics associated with spin-orbit coupling
but also better harness the power of spintronics in a more technically
favorable fashion.
Recent studies in devices comprising metal antiferromagnets have demonstrated the feasibility of a novel spintronic concept in which spin-dependent phenomena are governed by an antiferromagnet instead of a ferromagnet. Here we report experimental observation of the anisotropic magnetoresistance in an antiferromagnetic semiconductor Sr2IrO4. Based on ab initio calculations, we associate the origin of the phenomenon with large anisotropies in the relativistic electronic structure. The antiferromagnet film is exchange coupled to a ferromagnet, which allows us to reorient the antiferromagnet spin-axis in applied magnetic fields via the exchange spring effect. We demonstrate that the semiconducting nature of our AFM electrode allows us to perform anisotropic magnetoresistance measurements in the current-perpendicular-to-plane geometry without introducing a tunnel barrier into the stack. Temperature-dependent measurements of the resistance and anisotropic magnetoresistance highlight the large, entangled tunabilities of the ordinary charge and spin-dependent transport in a spintronic device utilizing the antiferromagnet semiconductor.
We have investigated the electronic properties of epitaxial orthorhombic
SrIrO3 thin-films under compressive strain. The metastable, orthorhombic SrIrO3
thin-films are synthesized on various substrates using an epi-stabilization
technique. We have observed that as in-plane lattice compression is increased,
the dc-resistivity (\r{ho}) of the thin films increases by a few orders of
magnitude, and the d\r{ho}/dT changes from positive to negative values.
However, optical absorption spectra show Drude-like, metallic responses without
an optical gap opening for all compressively-strained thin films. Transport
measurements under magnetic fields show negative magneto-resistance at low
temperature for compressively-strained thin-films. Our results suggest that
weak localization is responsible for the strain-induced metal-insulator
transition for the orthorhombic SrIrO3 thin-films.
Sr2IrO4 is a prototype of the class of Mott insulators in the strong spin-orbit interaction (SOI) limit described by a Jeff = 1/2 ground state. In Sr2IrO4, the strong SOI is predicted to manifest itself in the locking of the canting of the magnetic moments to the correlated rotation by 11.8(1)° of the oxygen octahedra that characterizes its distorted layered perovskite structure. Using x-ray resonant scattering at the Ir L3 edge we have measured accurately the intensities of Bragg peaks arising from different components of the magnetic structure. From a careful comparison of integrated intensities of peaks due to basal-plane antiferromagnetism, with those due to b-axis ferromagnetism, we deduce a canting of the magnetic moments of 12.2(8)°. We thus confirm that in Sr2IrO4 the magnetic moments rigidly follow the rotation of the oxygen octahedra, indicating that, even in the presence of significant non-cubic structural distortions, it is a close realization of the Jeff = 1/2 state.
The synthesis of some transition metal SrIr1−xMxO3 type oxides is described. These have an orthorhombic perovskite type-structure and are only found when the formal oxidation state of the Ir is approximately equal to +4.5 and for M = Fe, Co, Ni and Zn. Occupancy of the TM eg type orbitals appears to contribute to the break-down of the dimeric motif seen in the 6H–SrIrO3 structure. Variable temperature resistivity measurements show that SrIr0.8Zn0.2O3 is a metal; SrIr0.8Ni0.2O3 an non-metal and SrIr0.8Co0.2O3 undergoes a temperature dependent metal–insulator transition.
We have used epitaxial growth of thin films of magnetite, Fe3O4, on MgO to form domains with unique magnetic properties. The resulting anti-phase domain boundaries consist of strong 180 degree Fe-O-Fe superexchange paths, and result in frustration of the interdomain interactions. Probe-layer Mossbauer spectroscopy and Squid magnetometry have been used to determine the magnetic properties of these nearly perfect layer-by-layer grown films. The potential of these films for inter-domain spin-polarised tunneling will be discussed.
The two-dimensional layered perovskite Sr2IrO4 was proposed to be a spin-orbit Mott insulator, where the effect of Hubbard interaction is amplified on a narrow Jeff=1/2 band due to strong spin-orbit coupling. On the other hand, the three-dimensional orthorhombic perovskite (Pbnm) SrIrO3 remains metallic. To understand the physical origin of the metallic state and possible transitions to insulating phases, we construct a tight-binding model for SrIrO3. The band structure possesses a line node made of Jeff=1/2 bands below the Fermi level. As a consequence, instability toward magnetic ordering is suppressed, and the system remains metallic. This line node, originating from the underlying crystal structure, turns into a pair of three-dimensional nodal points on the introduction of a staggered potential or spin-orbit coupling strength between alternating layers. Increasing this potential beyond a critical strength induces a transition to a strong topological insulator, followed by another transition to a normal band insulator. We propose that materials constructed with alternating Ir- and Rh-oxide layers along the (001) direction, such as Sr2IrRhO6, are candidates for a strong topological insulator.
We report magnetic, electric transport, and calorimetric properties of single-crystal SrIrO3 as a function of temperature T and applied magnetic field H. We find that SrIrO3 is a non-Fermi-liquid metal near a ferromagnetic instability, as characterized by the following properties: (1) small saturation moment and no evidence for long-range order down to 1.7 K, (2) strongly enhanced magnetic susceptibility that diverges as Tγ at low temperatures with 1∕2<γ<1, depending on the applied field, (3) heat capacity C(T,H)∼−T ln T that is readily enhanced in low applied fields, and (4) T3∕2 dependence of electrical resistivity over the range 1.7<T<120 K. The data imply SrIrO3 is a rare example of a stoichiometric oxide compound that exhibits non-Fermi-liquid behavior near a quantum critical point (T=0 and μ0H=0.23 T).
There has been a rapidly growing interest in the interplay between spin-orbit coupling (SOC) and the Hubbard interaction U in correlated materials. A current consensus is that the stronger the SOC, the smaller is the critical interaction Uc required for a spin-orbit Mott insulator, because the atomic SOC splits a band into different total angular momentum bands, narrowing the effective bandwidth. It was further claimed that at large enough SOC, the stronger the SOC, the weaker the Uc, because in general the effective SOC is enhanced with increasing electron-electron interaction strength. Contrary to this expectation, we find that, in orthorhombic perovskite oxides (Pbnm), the stronger the SOC, the bigger the Uc. This originates from a line of Dirac nodes in Jeff=1/2 bands near the Fermi level, inherited from a combination of the lattice structure and a large SOC. Due to this protected line of nodes, there are small hole and electron pockets in SrIrO3, and such a small density of states makes the Hubbard interaction less efficient in building a magnetic insulator. The full phase diagram in U vs SOC is obtained, where nonmagnetic semimetal, magnetic metal, and magnetic insulator are found. Magnetic ordering patterns beyond Uc are also presented. We further discuss implications of our finding in relation to other perovskites such as SrRhO3 and SrRuO3.
Using resonant x-ray diffraction, we observe an easy c-axis collinear antiferromagnetic structure for the bilayer Sr3Ir2O7, a significant contrast to the single layer Sr2IrO4 with in-plane canted moments. Based on a microscopic model Hamiltonian, we show that the observed spin-flop transition as a function of number of IrO2 layers is due to strong competition among intra- and interlayer bond-directional pseudodipolar interactions of the spin-orbit entangled J(eff)=1/2 moments. With this we unravel the origin of anisotropic exchange interactions in a Mott insulator in the strong spin-orbit coupling regime, which holds the key to the various types of unconventional magnetism proposed in 5d transition metal oxides.
The orthorhombic Sr Ir O 3 perovskite was synthesized under 5 GPa and 1000 ° C . It is paramagnetic below about 170 K and transfers to an unknown magnetism under higher temperature. A band type metal to insulator transition caused by a pseudogap was observed at about 44 K . Interestingly a positive magnetoresistance, i.e., resistance increased with applying magnetic field, was observed in the orthorhombic Sr Ir O 3 perovskite below about 170 K .
Sr2IrO4 exhibits a novel insulating state driven by spin-orbit interactions.
We report two novel phenomena, namely a large magnetoresistivity in Sr2IrO4
that is extremely sensitive to the orientation of magnetic field but exhibits
no apparent correlation with the magnetization, and a robust metallic state
that is induced by dilute electron (La3+) or hole (K+) doping for Sr2+ ions in
Sr2IrO4. Our structural, transport and magnetic data reveal that a strong
spin-orbit interaction alters the balance between the competing energies so
profoundly that (1) the spin degree of freedom alone is no longer a dominant
force; (2) underlying transport properties delicately hinge on the Ir-O-Ir bond
angle via a strong magnetoelastic coupling; and (3) a highly insulating state
in Sr2IrO4 is proximate to a metallic state, and the transition is governed by
lattice distortions. This work suggests that a novel class of lattice-driven
electronic materials can be developed for applications.
Sr(2)IrO(4) has been suggested as a Mott insulator from a single J(eff)=1/2 band, similar to the cuprates. However, this picture is complicated by the measured large magnetic anisotropy and ferromagnetism. Based on a careful mapping to the J(eff)=1/2 (pseudospin-1/2) space, we propose that the low energy electronic structure of Sr(2)IrO(4) can indeed be described by a SU(2) invariant pseudospin-1/2 Hubbard model very similar to that of the cuprates, but with a twisted coupling to an external magnetic field (a g tensor with a staggered antisymmetric component). This perspective naturally explains the magnetic properties of Sr(2)IrO(4). We also derive several simple facts based on this mapping and the known results about the Hubbard model and the cuprates, which may be tested in future experiments on Sr(2)IrO(4). In particular, we propose that (electron-)doping Sr(2)IrO(4) can potentially realize high-temperature superconductivity.
Recent theory and experiment have revealed that strong spin-orbit coupling
can have dramatic qualitative effects on the band structure of weakly
interacting solids. Indeed, it leads to a distinct phase of matter, the
topological band insulator. In this paper, we consider the combined effects of
spin-orbit coupling and strong electron correlation, and show that the former
has both quantitative and qualitative effects upon the correlation-driven Mott
transition. As a specific example we take Ir-based pyrochlores, where the
subsystem of Ir 5d electrons is known to undergo a Mott transition. At weak
electron-electron interaction, we predict that Ir electrons are in a metallic
phase at weak spin-orbit interaction, and in a topological band insulator phase
at strong spin-orbit interaction. Very generally, we show that with increasing
strength of the electron-electron interaction, the effective spin-orbit
coupling is enhanced, increasing the domain of the topological band insulator.
Furthermore, in our model, we argue that with increasing interactions, the
topological band insulator is transformed into a "topological Mott insulator"
phase, which is characterized by gapless surface spin-only excitations. The
full phase diagram also includes a narrow region of gapless Mott insulator with
a spinon Fermi surface, and a magnetically ordered state at still larger
electron-electron interaction.
We investigated the electronic structures of the 5d Ruddlesden-Popper series Sr n+1Ir nO3n+1 (n=1, 2, and infinity) using optical spectroscopy and first-principles calculations. As 5d orbitals are spatially more extended than 3d or 4d orbitals, it has been widely accepted that correlation effects are minimal in 5d compounds. However, we observed a Mott insulator-metal transition with a change of bandwidth as we increased n. In addition, the artificially synthesized perovskite SrIrO3 showed a very large mass enhancement of about 6, indicating that it was in a correlated metallic state.
We investigated the electronic structure of 5d transition-metal oxide Sr2IrO4 using angle-resolved photoemission, optical conductivity, x-ray absorption measurements, and first-principles band calculations. The system was found to be well described by novel effective total angular momentum Jeff states, in which the relativistic spin-orbit coupling is fully taken into account under a large crystal field. Despite delocalized Ir 5d states, the Jeff states form such narrow bands that even a small correlation energy leads to the Jeff=1/2 Mott ground state with unique electronic and magnetic behaviors, suggesting a new class of Jeff quantum spin driven correlated-electron phenomena.
This review describes a new paradigm of electronics based on the spin degree of freedom of the electron. Either adding the
spin degree of freedom to conventional charge-based electronic devices or using the spin alone has the potential advantages
of nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities
compared with conventional semiconductor devices. To successfully incorporate spins into existing semiconductor technology,
one has to resolve technical issues such as efficient injection, transport, control and manipulation, and detection of spin
polarization as well as spin-polarized currents. Recent advances in new materials engineering hold the promise of realizing
spintronic devices in the near future. We review the current state of the spin-based devices, efforts in new materials fabrication,
issues in spin transport, and optical spin manipulation.
Over the past few years, Sr2IrO4, a single-layer member of the Ruddlesden–Popper series iridates, has received much attention as a close analog of cuprate high-temperature superconductors. Although there is not yet firm evidence for superconductivity, a remarkable range of cuprate phenomenology has been reproduced in electron- and hole-doped iridates including pseudogaps, Fermi arcs, and d-wave gaps. Furthermore, many symmetry-breaking orders reminiscent of those decorating the cuprate phase diagram have been reported using various experimental probes. We discuss how the electronic structures of Sr2IrO4 through strong spin-orbit coupling leads to the low-energy physics that had long been unique to cuprates, what the similarities and differences between cuprates and iridates are, and how these advance the field of high-temperature superconductivity by isolating essential ingredients of superconductivity from a rich array of phenomena that surround it. Finally, we comment on the prospect of finding a new high-temperature superconductor based on the iridate series. Expected final online publication date for the Annual Review of Condensed Matter Physics Volume 10 is March 10, 2019. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
The novel electronic state of the canted antiferromagnetic (AFM) insulator strontium iridate (Sr2IrO4) is well described by the spin–orbit‐entangled isospin Jeff = 1/2, but the role of isospin in transport phenomena remains poorly understood. In this study, antiferromagnet‐based spintronic functionality is demonstrated by combining the unique characteristics of the isospin state in Sr2IrO4. Based on magnetic and transport measurements, a large and highly anisotropic magnetoresistance (AMR) is obtained by manipulating the AFM isospin domains. First‐principles calculations suggest that electrons whose isospin directions are strongly coupled to the in‐plane net magnetic moment encounter an isospin mismatch when moving across the AFM domain boundaries, which generates a high resistance state. By rotating a magnetic field that aligns in‐plane net moments and removes domain boundaries, the macroscopically ordered isospins govern dynamic transport through the system, which leads to the extremely angle‐sensitive AMR. As this work establishes a link between isospins and magnetotransport in strongly spin–orbit‐coupled AFM Sr2IrO4, the peculiar AMR effect provides a beneficial foundation for fundamental and applied research on AFM spintronics. Antiferromagnet‐based spintronic functionality is demonstrated by combining the unique characteristics of the isospin state in layered perovskite strontium iridate (Sr2IrO4). Different from conventional electronic transport, each charge carrier in Sr2IrO4 contains a well‐defined isospin whose direction is controlled by the net magnetic moment. Consequently, the isospin mismatch across the domain boundaries acts as an essential mechanism for the large and highly anisotropic magnetoresistance.
We investigated magnetotransport properties and charge dynamics of strain-free perovskite SrIrO3. Both the longitudinal and transverse magnetoresistivity (MR) are significantly enhanced with decreasing temperature, in accord with the evolution of the Dirac semimetallic state. The electron correlation effect in the Dirac state shows up as a dramatic change in charge dynamics with temperature and as an enhanced paramagnetic susceptibility. We propose that the field-induced topological transition of the Dirac node coupled to the enhanced paramagnetism causes the unique MR of correlated Dirac electrons.
Measuring how the magnetic correlations throughout the Brillouin zone evolve in a Mott insulator as charges are introduced dramatically improved our understanding of the pseudogap, non-Fermi liquids and high TC superconductivity. Recently, photoexcitation has been used to induce similarly exotic states transiently. However, understanding how these states emerge has been limited because of a lack of available probes of magnetic correlations in the time domain, which hinders further investigation of how light can be used to control the properties of solids. Here we implement magnetic resonant inelastic X-ray scattering at a free electron laser, and directly determine the magnetization dynamics after photo-doping the Mott insulator Sr2IrO4. We find that the non-equilibrium state ~2ps after the excitation has strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. The magnetism recovers its two-dimensional (2D) in-plane Neel correlations on a timescale of a few ps, while the three-dimensional (3D) long-range magnetic order restores over a far longer, fluence-dependent timescale of a few hundred ps. The dramatic difference in these two timescales, implies that characterizing the dimensionality of magnetic correlations will be vital in our efforts to understand ultrafast magnetic dynamics.
By using a combination of heteroepitaxial growth, structure refinement based on synchrotron x-ray diffraction, and first-principles calculations, we show that the symmetry-protected Dirac line nodes in the topological semimetallic perovskite SrIrO3 can be lifted simply by applying epitaxial constraints. In particular, the Dirac gap opens without breaking the Pbnm mirror symmetry. In virtue of a symmetry-breaking analysis, we demonstrate that the original symmetry protection is related to the n-glide operation, which can be selectively broken by different heteroepitaxial structures. This symmetry protection renders the nodal line a nonsymmorphic Dirac semimetallic state. The results highlight the vital role of crystal symmetry in spin-orbit-coupled correlated oxides and provide a foundation for experimental realization of topological insulators in iridate-based heterostructures.
The control of magnetism by electric fields is an important goal for the future development of low-power spintronics. Various approaches have been proposed on the basis of either single-phase multiferroic materials or hybrid structures in which a ferromagnet is influenced by the electric field applied to an adjacent insulator (usually having a ferroelectric, piezoelectric, or multiferroic character). The electric field effect on magnetism can be driven by purely electronic or electrostatic effects or can occur through strain coupling. Here we review progress in the electrical control of magnetic properties (anisotropy, spin order, ordering temperature, domain structure) and its application to prototype spintronic devices (spin valves, magnetic tunnel junctions). We tentatively identify the main outstanding difficulties and give perspectives for spintronics and other fields.
The structure of the atmospheric pressure form of SrIrO3 is shown to be a monoclinic distortion of the hexagonal BaTiO3 structure (, , , β = 93.26°). The cell dimensions have been studied to 1000°C and the coefficients of thermal expansion given. The structure transforms at 40 kbar and 1000°C to an orthorhombic perovskite (, , ) with a 3% decrease in volume. This high pressure phase only retransforms slowly at atmospheric pressure and 1200°C and exhibits metallic conductivity and Pauli paramagnetism.
The post-perovskite (pPv) is the high-pressure phase of some highly distorted perovskites. The pPv phase of MgSiO3 stabilized under 125 GPa and 2000 K cannot be quenched to ambient pressure. In contrast, the pPv CaIrO3 can be synthesized under a modest pressure or even at ambient pressure. However, the pPv CaIrO3 has not been fully characterized. We report here systematic structural studies, measurements of transport and magnetic properties including critical phenomena, specific heat, and thermal conductivity in a series of samples Ca1-xSrxIrO3 synthesized under high pressure. The Ca1-xSrxIrO3 samples exhibit an evolution from the pPv phase to the perovskite phase. We have also prepared the perovskite (Pv phase) CaIrO3 with the wet chemical method. Rietveld refinements of the pPv and Pv phase CaIrO3 have been made based on high-resolution synchrotron diffraction. In comparison with effects of the chemical substitution on the crystal structure and physical properties, we have studied the structure and magnetic properties of the pPv CaIrO3 under hydrostatic pressure. Results have been discussed in the context of orbital ordering biased on the intrinsic structural distortion and the strong spin-orbit coupling that is much enhanced in these 5d oxides with the pPv structure.
We discuss phenomena arising from the combined influence of electron
correlation and spin-orbit coupling, with an emphasis on emergent quantum
phases and transitions in heavy transition metal compounds with 4d and 5d
elements. A common theme is the influence of spin-orbital entanglement produced
by spin-orbit coupling, which influences the electronic and magnetic structure.
In the weak-to-intermediate correlation regime, we show how non-trivial
band-like topology leads to a plethora of phases related to topological
insulators. We expound these ideas using the example of pyrochlore iridates,
showing how many novel phases such as the Weyl semi-metal, axion insulator,
topological Mott insulator, and topological insulators may arise in this
context. In the strong correlation regime, we argue that spin-orbital
entanglement fully or partially removes orbital degeneracy, reducing or
avoiding the normally ubiquitous Jahn-Teller effect. As we illustrate for the
honeycomb lattice iridates and double perovskites, this leads to enhanced
quantum fluctuations of the spin-orbital entangled states and the chance to
promote exotic quantum spin liquid and multipolar ordered ground states.
Connections to experiments, materials, and future directions are discussed.
We report a single-crystal neutron diffraction study of the layered . This work unambiguously determines the magnetic structure of the
system and reveals that the spin orientation rigidly tracks the staggered
rotation of the octahedra in . The long-range
antiferromagnetic order has a canted spin configuration with an ordered moment
of 0.208(3) /Ir site within the basal plane; a detailed examination of
the spin canting yields 0.202(3) and 0.049(2) /site for the a axis and
the b axis, respectively. It is intriguing that forbidden nuclear reflections
of space group are also observed in a wide temperature range from 4
K to 600 K, which suggests a reduced crystal structure symmetry. This
neutron-scattering work provides a direct, well-refined experimental
characterization of the magnetic and crystal structures that are crucial to the
understanding of the unconventional magnetism exhibited in this unusual
magnetic insulator.
Measurement of the quantum-mechanical phase in quantum matter provides the most direct manifestation of the underlying abstract
physics. We used resonant x-ray scattering to probe the relative phases of constituent atomic orbitals in an electronic wave
function, which uncovers the unconventional Mott insulating state induced by relativistic spin-orbit coupling in the layered
5d transition metal oxide Sr2IrO4. A selection rule based on intra-atomic interference effects establishes a complex spin-orbital state represented by an effective
total angular momentum = 1/2 quantum number, the phase of which can lead to a quantum topological state of matter.
We study the magnetic interactions in Mott-Hubbard systems with partially filled t_{2g} levels and with strong spin-orbit coupling. The latter entangles the spin and orbital spaces, and leads to a rich variety of the low energy Hamiltonians that extrapolate from the Heisenberg to a quantum compass model depending on the lattice geometry. This gives way to "engineer" in such Mott insulators an exactly solvable spin model by Kitaev relevant for quantum computation. We, finally, explain "weak" ferromagnetism, with an anomalously large ferromagnetic moment, in Sr2IrO4.