Jahn-Teller Polarons Research Articles

Dielectric Spectroscopy of Non-Stoichiometric SrMnO3 Thin Films

The dielectric properties of non-stoichiometric SrMnO3 (SMO) thin films grown by molecular beam epitaxy were systematically investigated. Especially, the effects of cation stoichiometry-induced diverse types and densities of defects on the dielectric properties of SMO films were revealed. Two anomalous dielectric relaxation behaviors were observed at different temperatures in both Sr-rich and Mn-rich samples. High-temperature dielectric relaxation, resulting from a short-range Mn-related Jahn–Teller (JT) polaron hopping motion, was reinforced by an enhancement of JT polaron density in the Sr-rich film, which contained abundant SrO Ruddlesden–Popper (R-P) stacking faults. However, an excessive number of disordered Sr vacancy clusters in Mn-rich thin film suppressed the hopping path of JT polarons and enormously weakened this dielectric relaxation. Thus, The Sr-rich film demonstrated a higher dielectric constant and dielectric loss than the Mn-rich film. In addition, low-temperature dielectric relaxation may be attributed to the polarization/charge glass state.

Atomic-layer engineering of nickelates (in search of novel superconductors)

Karl Alex Mueller's intuition was that the nature of mobile charge carriers in manganates and nickelates should be like those in cuprates, which he believed were Jahn-Teller polarons, so he expected them to also host high-temperature superconductivity (HTS). Following his suggestions and persuasion, in the past, we performed several experiments that included atomic-layer-by-layer synthesis of novel, metastable manganates and nickelates. Despite our efforts and several USO ("Unidentified Superconducting Object") sightings, we failed to find any reproducible superconductivity and dropped the project fifteen years ago. Our results are presented here, for the first time — timely, since HTS was very recently discovered in nickelates (under pressure). So, whether his opinions were on the mark or not, Alex did guess it right — again.

Jahn-Teller Polarons in metal oxides and their signature in local lattice distortions

We review evidence of local dynamical lattice distortions in perovskite transition metal oxides, including cuprates and nickelates. These distortions appear as changes of the local symmetry around the metal ions, which are manifest in x-ray absorption spectroscopy and other inelastic scattering techniques. The distortions imply that these materials exhibit inhomogeneous ground states in some regions of their phase diagram. The distortions imply that these materials exhibit inhomogeneous ground states in some regions of their phase diagram. The presence of these distortions is a direct signature of polaronic excitations, as the Jahn-Teller polarons proposed by K.A. Müller, et al. [1] in the case of cuprates and that were previously studied by him in nickelates. [2] In the case of nickelates, we review direct evidence of an anomalous isotopic effect, characteristic of polaronic excitations as suggested by Professor Müller.

Contrasting Pr1–xCaxMnO3 OER Catalysts with Different Valences and Covalences

Perovskite oxides are promising materials for catalyzing the oxygen evolution reaction (OER). However, understanding the properties of active sites that enable high OER activity without corrosive electrode reduction is still elusive. In this work, we use combined electrochemical and environmental transmission electron microscopy (ETEM) studies to compare the OER stability of the (001) surfaces of Pr1–xCaxMnO3 perovskite films with doping of x = 0.1 and x = 0.33. Notably, electrochemical analysis in alkaline conditions, as well as ETEM studies in H2O vapor, shows parallel trends in the stability of both systems: Mn leaching for the Pr1–xCaxMnO3 (PCMO) x = 0.33 system due to oxygen vacancy formation at the surface and higher stability of PCMO x = 0.1, where the oxygen vacancy redox peak is absent in cyclovoltammetry. Electron energy loss spectroscopy reveals the preservation of the Mn valence state in H2O for the PCMO x = 0.1 system, whereas for x = 0.33, Mn is reduced. We interpret this enhanced stability for the low-doped system in terms of a modified Mn 3d–O 2p hybridization, i.e., covalence. For a system with charge localization due to Jahn–Teller polarons, the covalence determines to what degree redox processes of lattice oxygen can arise that can finally lead to corrosive oxygen vacancy formation.

Superconductivity and the Jahn–Teller Polaron

In this article, we review the essential properties of high-temperature superconducting cuprates, which are unconventional isotope effects, heterogeneity, and lattice responses. Since their discovery was based on ideas stemming from Jahn–Teller polarons, their special role, together with the Jahn–Teller effect itself, is discussed in greater detail. We conclude that the underlying physics of cuprates cannot stem from purely electronic mechanisms, but that the intricate interaction between lattice and charge is at its origin.

Polaronic Contributions to Friction in a Manganite Thin Film.

Despite the huge importance of friction in regulating movement in all natural and technological processes, the mechanisms underlying dissipation at a sliding contact are still a matter of debate. Attempts to explain the dependence of measured frictional losses at nanoscale contacts on the electronic degrees of freedom of the surrounding materials have so far been controversial. Here, it is proposed that friction can be explained by considering the damping of stick‐slip pulses in a sliding contact. Based on friction force microscopy studies of La(1− x )SrxMnO3 films at the ferromagnetic‐metallic to a paramagnetic‐polaronic conductor phase transition, it is confirmed that the sliding contact generates thermally‐activated slip pulses in the nanoscale contact, and argued that these are damped by direct coupling into the phonon bath. Electron‐phonon coupling leads to the formation of Jahn–Teller polarons and to a clear increase in friction in the high‐temperature phase. There is neither evidence for direct electronic drag on the atomic force microscope tip nor any indication of contributions from electrostatic forces. This intuitive scenario, that friction is governed by the damping of surface vibrational excitations, provides a basis for reconciling controversies in literature studies as well as suggesting possible tactics for controlling friction.

Polaronic Emergent Phases in Manganite-based Heterostructures

Transition metal functional oxides, e.g., perovskite manganites, with strong electron, spin and lattice correlations, are well-known for different phase transitions and field-induced colossal effects at the phase transition. Recently, the interfaces between dissimilar perovskites were shown to be a promising concept for the search of emerging phases with novel functionalities. We demonstrate that the properties of manganite films are effectively controlled by low dimensional emerging phases at intrinsic and extrinsic interfaces and appeared as a result of symmetry breaking. The examples include correlated Jahn–Teller polarons in the phase-separated (La1−yPry)0.7Ca0.3MnO3, electron-rich Jahn–Teller-distorted surface or “dead” layer in La0.7Sr0.3MnO3, electric-field-induced healing of “dead” layer as an origin of resistance switching effect, and high-TC ferromagnetic emerging phase at the SrMnO3/LaMnO3 interface in superlattices. These 2D polaronic phases with short-range electron, spin, and lattice reconstructions could be extremely sensitive to external fields, thus, providing a rational explanation of colossal effects in perovskite manganites.

Magnetic-Field-Induced Suppression of Jahn-Teller Phonon Bands in (La0.6Pr0.4)0.7Ca0.3MnO3: the Mechanism of Colossal Magnetoresistance shown by Raman Spectroscopy

A long-standing issue in the physics of the colossal magnetoresistance is the role of electron-phonon coupling, which manifests itself as Jahn-Teller polarons. The origin and architecture of polarons makes it possible to study their behavior by Raman spectroscopy, which allows to analyze the polaronic behavior in an applied magnetic field. We performed magnetic-field-dependent Raman spectroscopy on thin films of (La0.6Pr0.4)0.7Ca0.3MnO3 in a range of H = 0–50 kOe and compared the obtained Raman spectra with the magnetic field behavior of the electrical resistivity. In the vicinity of the Curie temperature, TC = 197 K, the intensity of the Jahn-Teller stretching mode at 614 cm−1 and of the bending mode at 443 cm−1 was found to be suppressed and enhanced, respectively. This observed behavior has a remarkable similarity with the field and temperature dependence of the colossal magnetoresistance in (La0.6Pr0.4)0.7Ca0.3MnO3. Our work provides direct evidence that the reduction of the amount of Jahn-Teller polarons at the phase transition is the main mechanism underlying the colossal magnetoresistance.

Polaron freezing and the quantum liquid-crystal phase in the ferromagnetic metallic La0.67Ca0.33MnO3

The remarkable electronic properties of colossal magnetoresistive manganites are widely believed to be caused by the competition between a ferromagnetic metallic state and an antiferromagnetic insulating state with complex spin, charge, and orbital ordering. However, the physics underlying their magnetotransport properties is still not clear, especially the role of correlated Jahn–Teller polarons, which depending on temperature and doping, might form a liquid, glass or stripe polaron state. This question touches one of the most fundamental problems in the physics of doped Mott insulators, i.e. understanding the mechanism that chemical doping makes an insulator becoming superconductive as in the case of cuprates, or exhibiting the colossal magnetoresistance effect, as in the case of manganites. Here, by using 139La NMR and high resolution transmission electron microscopy in the temperature range 3.2–1000 K, we have monitored the formation and evolution of CE-type polarons in optimally doped La0.67Ca0.33MnO3. While NMR experiments show that correlated polarons dominate electron spin dynamics in the ferromagnetic phase, at very low temperatures they appear to form a quantum liquid-crystal like ferromagnetic phase, embedded into a ferromagnetic matrix with 3D polaron correlations. This is evidence that similarly to high Tc cuprates, quantum soft phases underlie the exotic physical properties of colossal magnetoresistive manganites.

Over Half a Century of Research in Oxides

An attempt will be made to recall some of the highlights of the research in oxides achieved by the author and various collaborators over a period of 58 years. These were the observation of the paramagnetic resonance of transition metal ions of high valency like Fe3+, Fe4+, Fe5+ and Co4+ and their photochromic behaviour; then the detection of Mn4+ in BaTiO3 and the proof for the presence of order–disorder character in these ferroelectric transitions, and then the temperature dependence of the rotational order parameter of the structural phase transitions in LaAlO3 and SrTiO3. Near the transition, the observation of critical behaviour compatible with renormalisation group theory and the presence of a Potts transition line, a critical end point and a Lifshitz point under uniaxial stress in SrTiO3 and CaGdF3. This will be followed by the determination of the stabilisation energy of the Jahn–Teller effect with octahedral oxygen neighbours to various ions; leading over to the concept of Jahn–Teller polarons as quasi particles for the generation of superconductivity, and the experimental observation of Jahn–Teller bipolarons and their stabilisation energy determined in one of the HTS copper oxides.

Intrinsic antiferromagnetic coupling underlies colossal magnetoresistance effect: Role of correlated polarons

A commonly believed picture of colossal magnetoresistance (CMR) effect is related to a first-order phase transition and electronic phase separation with coexisting ferromagnetic metallic and antiferromagnetic insulating phases. However, the underlying mechanism, i.e., the characteristic energy scale of the interacting phases and their spatial extent, is still under debate. Here we present experimental evidence on the existence of an effective antiferromagnetic coupling between the ferromagnetic nanodomains in epitaxial thin films of a classical CMR material $({\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Pr}}_{y}){}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ with Pr doping, $y$ = 0.375 and 0.4. This coupling yields to peculiar low-field CMR behavior with magnetic hysteresis and slow resistance relaxation, both induced by the magnetization reversal. The coercive field obeys a square-root temperature dependence for $T\ensuremath{\ll}{T}_{\mathrm{C}}$ and increases anomalously close to the phase transition. We modeled the magnetic structure within the phase-separation scenario as an assembly of single-domain ferromagnetic nanoparticles, antiferromagnetically coupled (pinned) by correlated Jahn-Teller polarons. The concentration of polarons increases drastically close to phase transition as indicated by the third harmonic of the electrical conductivity as well as Raman spectroscopy.

Pressure-enhanced ferromagnetism and metallicity in La1.24Sr1.76Mn2O7 bilayered manganite system

We report the low temperature magnetization, electrical resistance, and magnetoresistance of La1.24Sr1.76Mn2O7 bilayer manganite system under hydrostatic pressure. At ambient pressure, the compound shows a sharp ferromagnetic transition (TC) accompanied by a metal–insulator transition (TMI) at 130 K. We observe that the TC and TMI increase with hydrostatic pressure at a rate of dTC/dP = 2.08 K/kbar and dTMI/dP = 2 K/kbar, respectively. Also, we observe an appreciable increase of magnetic moments at low temperatures with increasing pressure. The high temperature regime of temperature dependence of resistivity curves was fitted with the Emin–Holstein’s polaron hopping model and the calculated activation energy values suggest that the applied pressure weakens the formation of Jahn–Teller polarons. The magnetoresistance ratio (MRR) was measured at TC and at 4.2 K upon an external magnetic field of 5 T. The observed MRR at TC is about 210 % and the applied pressure increases the MRR significantly. These results can be interpreted by the pressure-enhanced overlap between the orbitals of Mn–O–Mn, which facilitates the charge transfer and hence enhances the ferromagnetism and metallicity.

Jahn-Teller assisted polaronic hole hopping as a charge transport mechanism in CuO nanograins

Impedance spectroscopy has been employed to investigate the dielectric and electric transport phenomena in sol-gel synthesized CuO nanograins. Semiconducting features of the grains and grain boundaries have been endorsed to the thermal activation of the localized charge carriers. On cooling below 303 K, a transition from Jahn-Teller polaron hopping mechanism to the Mott’s variable range hopping mechanism has been observed owing to random potential fluctuations among localized sites. Activation energies for conduction and relaxation processes at grain boundaries provide strong signatures for the involvement of Jahn-Teller adiabatic small polarons as a charge transport mechanism in CuO nanograins.

Electron doping in the cubic perovskiteSrMnO3: Isotropic metal versus chainlike ordering of Jahn-Teller polarons

Single crystals of electron-doped SrMnO3 with a cubic perovskite structure have been systematically investigated as the most canonical (orbital-degenerate) double-exchange system, whose ground states have been still theoretically controversial. With only 1-2% electron doping by Ce substitution for Sr, a G-type antiferromagnetic metal with a tiny spin canting in a cubic lattice shows up as the ground state, where the Jahn-Teller polarons with heavy mass are likely to form. Further electron doping above 4%, however, replaces this isotropic metal with an insulator with tetragonal lattice distortion, accompanied by a quasi-one-dimensional 3z^2-r^2 orbital ordering with the C-type antiferromagnetism. The self-organization of such dilute polarons may reflect the critical role of the cooperative Jahn-Teller effect that is most effective in the originally cubic system.

Polaronic pseudogap in the metallic phase ofLa0.625Ca0.375MnO3 thin films

The electronic density of states (DOS) ofLa0.625Ca0.375MnO3 (LCMO) strain-free epitaxial thin films with an insulator–metal transition temperature (TIM) of 250 K was probed using variable-temperature scanning tunneling microscopyand spectroscopy. We find a depression in the DOS with a finite zero biasconductance (ZBC) signifying a pseudogap in the 78–310 K temperature range.With cooling, the ZBC is found to increase, indicating an increased DOS nearEF. We interpret the pseudogap as a signature of Jahn–Teller polarons while the ZBC change,in agreement with the bulk insulator–metal transition, optical Drude peak andphotoemission experiments, indicates the presence of free carriers at the Fermi energy inthe metallic phase. The free carriers are discussed in terms of correlated polaronic states.

Ordering of polarons in the charge-disordered phase ofPr0.7Ca0.3MnO3

${\text{Pr}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{MnO}}_{3}$ (PCMO, $x=0.3$) ceramic is a member of the manganite family that exhibits a colossal magnetoresistance. We report a comprehensive study of perovskite manganite PCMO $(x=0.3)$ ceramic with EPR, dielectric spectroscopy, and ac electrical conductivity. Activated-type temperature dependence of EPR linewidth is a strong signature for a hopping of Jahn-Teller polarons above $\ensuremath{\approx}150\text{ }\text{K}$. Analysis of the electrical conductivity and dielectric constant data suggests the ordering of polarons in the charge-disordered state of PCMO $(x=0.3)$ below 60 K. This provides a new understanding of the phase diagrams in PCMO systems.

Ultrafast polaron dynamics in La0.7Ca0.3MnO3 thin films

In this work, we use ultrafast optical pump-optical probe (OPOP) spectroscopy to probe the polaron dynamics in La0.7Ca0.3MnO3 (LCMO) thin films. The temporal evolution in transient reflectivity change δR/R exhibits two relaxing components: a fast component with a time constant of subpicosecond and a slow component with time constant ranging from tens of ps to hundreds of ps. The amplitude of the fast component exhibits the similar temperature depence with that of the resistivity, and the neutron scattering intensity due to nanoscale correlated polarons. The results strongly suggest that the fast photoinduced reflectivity change may have been due to the photoexcitation and trapping process of correlated Jahn-Teller (T-J) polarons in the paramagnetic (PM) and ferromagnetic (FM) phases.

Ultrafast optical probes of polaron dynamics in La0.7Ca0.3MnO3 thin films

In this work, we use ultrafast optical pump-optical probe spectroscopy to probe the polaron dynamics in La0.7Ca0.3MnO3 (LCMO) thin films. The temporal evolution in transient reflectivity change ΔR/R exhibits two relaxing components: a fast component with a time constant of subpicosecond and a slow component with time constant ranging from tens of picoseconds to hundreds of picoseconds. The amplitude of the fast component, though has been seldom discussed before, exhibits the similar temperature dependence with that of the resistivity and the neutron scattering intensity due to nanoscale correlated polarons. The results strongly suggest that the fast photoinduced reflectivity change may have been due to the photoexcitation and trapping process of correlated Jahn–Teller polarons in the paramagnetic and ferromagnetic phases and, thus, implies the presence of electronic inhomogeneity in LCMO manganites.

Jahn-Teller spin polarons in perovskite cobaltites

The interplay of the electron transfer integral and lattice elastic energy in ${A}_{1\ensuremath{-}x}{A}_{x}^{\ensuremath{'}}{\text{CoO}}_{3}$ is investigated by neutron diffraction and the pair density function analysis. By effectively widening the bandwidth with Ca, Sr, and Ba substitutions at the $A$ site, evidence for the stability of the intermediate-spin (IS) configuration through the formation of static local Jahn-Teller (JT) distortions is provided. When the critical radius exceeds $\ensuremath{\sim}1.22\text{ }\text{\AA{}}$, spin activated JT polarons are formed in the absence of long-range magnetic ordering. JT polarons are prevalent in crystals with Sr and Ba but not in those with Ca. In the metallic and magnetically ordered phase, the static signature of the distortions is absent, suggesting that the IS state may be dynamically shared between ${\text{Co}}^{3+}$ and ${\text{Co}}^{4+}$ ions.

Electrical and thermal properties of Pr2/3(Ba1−xCsx)1/3MnO3 manganites

Electrical and thermal properties of Pr2/3(Ba1−x Cs x )1/3MnO3 (0 ≤ x ≤ 0.25) manganite perovskites are reported here. Two insulator-metal (I-M) transitions (T P1 &T P2) are observed in the electrical resistivity (ρ) of the pristine Pr2/3Ba1/3MnO3 (PBMO) sample, and they are systematically shifted to lower temperatures with increasing Cs substitution. An upturn in ρ is noticed below 50 K in these perovskites, presumably due to the combined effect of weak localization, electron-electron and electron-phonon scattering. It is found that the absolute value of room-temperature thermoelectric power (TEP) gradually decreases with increasing Cs content, implying the annihilation of the charge carriers with doping. An analysis of the electrical resistivity and thermoelectric power data indicates that the paramagnetic insulating state above T P1 is governed by the small polaron hopping due to a non-adiabatic process. It is argued that the electron-magnon scattering processes are responsible for low temperature metallic behavior of TEP. A distinct specific heat peak below T P1 is observed, attributed to the magnetic ordering, and its broadening with Cs-doping corresponds to the increase of magnetic inhomogeneity. Further, the temperature variation of thermal conductivity and the low temperature plateau in κ has been associated with the delocalization of Jahn-Teller polarons and transition from Umklapp scattering to a defect-limited scattering, respectively.