Magnetic dynamics of bilayer manganite La1.4Sr1.6Mn2O7

U ovom radu je istraživana magnetska dinamika dvoslojnog manganita La1.4Sr1.6Mn2O7 mjerenjem linearne i nelinearne izmjenične susceptibilnosti na temperaturama izmedu 4.2 K i 400 K, te magnetizacije izmedu 320 K i 500 K. Dokazano je postojanje Griffithsove faze na 370 K, dugodosežnog magnetskog uređenja na 91.2 K, te pojava staklastog ponašanja na temperaturama nižima od uređenja, s temperaturom ostakljivanja od 26.4 K. Također, uočeno je pet feromagnetskih prijelaza između 250 K i 320 K, koje pripisujemo kristalnim defektima. Predložen je model prema kojem su defekti zaslužni za metalnu vodljivost duž c kristalnog smjera na temperaturama ispod magnetskog uređenja.The magnetic dynamics of the bilayer manganite La1.4Sr1.6Mn2O7 was investigated by measuring the linear and nonlinear AC susceptibility between 4.2 K and 400 K, and magnetization between 320 K and 500 K. It was proven that a Griffiths-like phase appears at 370 K, and that long range order is set at 91.2 K. Bellow the ordering temperature the system behaves as a reentrant spin glass, with the freezing temperature of 26.4 K. Also, there are five ferromagnetic transitions between 250 K and 320 K, which are attributed to crystal defects. We propose a model according to which these defects cause the metallic conduction in the c crystallographic direction in the magnetically ordered phase

Linear magneto-conductivity as a DC probe of time-reversal symmetry breaking

Several optical experiments have shown that in magnetic materials the principal axes of response tensors can rotate in a magnetic field. Here we offer a microscopic explanation of this effect, and propose a closely related DC transport phenomenon -- an off-diagonal \emph{symmetric} conductivity linear in a magnetic field, which we refer to as linear magneto-conductivity (LMC). Although LMC has the same functional dependence on magnetic field as the Hall effect, its origin is fundamentally different: LMC requires time-reversal symmetry to be broken even before a magnetic field is applied, and is therefore a sensitive probe of magnetism. We demonstrate LMC in three different ways: via a tight-binding toy model, density functional theory calculations on MnPSe$_3$, and a semiclassical calculation. The third approach additionally identifies two distinct mechanisms yielding LMC: momentum-dependent band magnetization and Berry curvature. Finally, we propose an experimental geometry suitable for detecting LMC, and demonstrate its applicability using Landauer-B\"{u}ttiker simulations. Our results emphasize the importance of measuring the full conductivity tensor in magnetic materials, and introduce LMC as a new transport probe of symmetry.Comment: 6+8 pages, 4+3 figure

Elastocaloric signatures of symmetric and antisymmetric strain-tuning of quadrupolar and magnetic phases in DyB2C2

The adiabatic elastocaloric effect measures the temperature change of given systems with strain and probes the entropic landscape in the temperature-strain space. In this study we demonstrate that the DC bias strain-dependence of AC elastocaloric effect can be used to decompose the latter into contributions from symmetric (rotation-symmetry-preserving) and antisymmetric (rotation-symmetry-breaking) strains, using a tetragonal f-electron system DyB2C2--whose antiferroquadrupolar order locally breaks four-fold rotational site symmetries while globally remaining tetragonal--as a showcase example. We capture the strain evolution of the quadrupolar and magnetic phase transitions in the system using both singularities in the elastocaloric coefficient and its jump at the transitions, and the latter we show follows a modified Ehrenfest relation. We find that antisymmetric strain couples to the underlying order parameter in a bi-quadratic manner in the antiferroquadrupolar (AFQ) phase but in a linear-quadratic manner in the canted antiferromagnetic (CAFM) phase; the contrast is attributed to a preserved (broken) tetragonal symmetry in the AFQ (CAFM) phase, respectively. The broken tetragonal symmetry in the CAFM phase is further supported by elastocaloric strain-hysteresis and observation of two sets of domains with mutually perpendicular principal axes in optical birefringence. Additionally, when the quadrupolar moments are ordered in a staggered fashion, we uncover an elastocaloric response that reflects a quadratic increase of entropy with antisymmetric strain, analogous to the role magnetic field plays for Ising antiferromagnets by promoting pseudospin flips. Our results show that AC elastocaloric effect is a compact and incisive thermodynamic probe into the coupling between electronic degrees of freedom and strain, which can potentially be applied to broader classes of quantum materials.Comment: 10 pages, 7 figure

h/e oscillations in interlayer transport of delafossites

Funding: This project was funded by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant no. 715730, MiTopMat) and also was supported by the Max Planck Society. A.P.M. and R.M. acknowledge support from the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter (EXC 2147). M.D.B., P.M., and V.S. acknowledge studentship funding from the EPSRC under grant no. EP/L015110/1. A.S. was supported by the Israel Science Foundation, the European Research Council (Project LEGOTOP), and the DFG through projectno.CRC-183. M.K. acknowledges support from the SIRIUS irradiation facility through project no. EMIR 2019 18-7099.Microstructures can be carefully designed to reveal the quantum phase of the wave-like nature of electrons in a metal. Here, we report phase-coherent oscillations of out-of-plane magnetoresistance in the layered delafossites PdCoO2 and PtCoO2 The oscillation period is equivalent to that determined by the magnetic flux quantum, h/e, threading an area defined by the atomic interlayer separation and the sample width, where h is Planck's constant and e is the charge of an electron. The phase of the electron wave function appears robust over length scales exceeding 10 micrometers and persisting up to temperatures of T > 50 kelvin. We show that the experimental signal stems from a periodic field modulation of the out-of-plane hopping. These results demonstrate extraordinary single-particle quantum coherence lengths in delafossites.PostprintPeer reviewe

Tuneable electron-magnon coupling of ferromagnetic surface states in PdCoO2

Funding: We gratefully acknowledge support from the European Research Council (through the QUESTDO project, 714193), the Royal Society, the Max Planck Society, and the UKRI Engineering and Physical Sciences Research Council (Grant No. EP/S005005/1). V.S., O.J.C., and L.B. acknowledge the EPSRC for PhD studentship support through Grants EP/L015110/1, EP/K503162/1, and EP/G03673X/1, respectively. I.M. and D.C. acknowledge studentship support from the International Max-Planck Research School for Chemistry and Physics of Quantum Materials.Controlling spin wave excitations in magnetic materials underpins the burgeoning field of magnonics. Yet, little is known about how magnons interact with the conduction electrons of itinerant magnets, or how this interplay can be controlled. Via a surface-sensitive spectroscopic approach, we demonstrate a strong and highly-tuneable electron-magnon coupling at the Pd-terminated surface of the delafossite oxide PdCoO2, where a polar surface charge mediates a Stoner transition to itinerant surface ferromagnetism. We show how the coupling can be enhanced 7-fold with increasing surface disorder, and concomitant charge carrier doping, becoming sufficiently strong to drive the system into a polaronic regime, accompanied by a significant quasiparticle mass enhancement. Our study thus sheds new light on electron-magnon interactions in solid-state materials, and the ways in which these can be controlled.Publisher PDFPeer reviewe

Dual quantum confinement and anisotropic spin splitting in the multi-valley semimetal PtSe2

The authors gratefully acknowledge support from the Leverhulme Trust (Grant No. RL-2016-006), the Royal Society, the European Research Council (Grant No. ERC-714193QUESTDO) CREST, JST (No. JPMJCR16F1), and the International Max-Planck Partnership for Measurement and Observation at the Quantum Limit. OJC, VS, and LB acknowledge EPSRC for PhD studentship support through grant Nos. EP/K503162/1, EP/L015110/1 and EP/G03673X/1. IM acknowledges PhD studentship support from the IMPRS for the Chemistry and Physics of Quantum Materials.We investigate the electronic structure of a two-dimensional electron gas created at the surface of the multivalley semimetal 1T−PtSe2. Using angle-resolved photoemission and first-principles-based surface space-charge calculations, we show how the induced quantum well sub-band states form multiple Fermi surfaces, which exhibit highly anisotropic Rashba-like spin splittings. We further show how the presence of both electronlike and holelike bulk carriers causes the near-surface band bending potential to develop an unusual nonmonotonic form, with spatially segregated electron accumulation and hole accumulation regions, which in turn amplifies the induced spin splitting. Our results thus demonstrate the novel environment that semimetals provide for tailoring electrostatically induced potential profiles and their corresponding quantum sub-band states.PostprintPeer reviewe

Fermiology and superconductivity of topological surface states in PdTe2

We gratefully acknowledge support from the Leverhulme Trust, the Engineering and Physical Sciences Research Council, UK (Grant Nos. EP/M023427/1 and EP/I031014/1), the Royal Society. JC, MJN, LB, VS, and JMR acknowledge EPSRC for PhD studentship support through grant Nos. EP/K503162/1, EP/G03673X/1, EP/L505079/1, and EP/L015110/1.We study the low-energy surface electronic structure of the transition-metal dichalcogenide superconductor PdTe2 by spin- and angle-resolved photoemission, scanning tunneling microscopy, and density-functional theory-based supercell calculations. Comparing PdTe2 with its sister compound PtSe2, we demonstrate how enhanced interlayer hopping in the Te-based material drives a band inversion within the antibonding p -orbital manifold well above the Fermi level. We show how this mediates spin-polarized topological surface states which form rich multivalley Fermi surfaces with complex spin textures. Scanning tunneling spectroscopy reveals type-II superconductivity at the surface, and moreover shows no evidence for an unconventional component of its superconducting order parameter, despite the presence of topological surface states.PostprintPeer reviewe

Spin-carrier coupling induced ferromagnetism and giant resistivity peak in EuCd2_2P2_2

EuCd$_2$P$_2$ is notable for its unconventional transport: upon cooling the metallic resistivity changes slope and begins to increase, ultimately 100-fold, before returning to its metallic value. Surprisingly, this giant peak occurs at 18K, well above the N\'{e}el temperature ($T_N$) of 11.5K. Using a suite of sensitive probes of magnetism, including resonant x-ray scattering and magneto-optical polarimetry, we have discovered that ferromagnetic order onsets above $T_N$ in the temperature range of the resistivity peak. The observation of inverted hysteresis in this regime shows that ferromagnetism is promoted by coupling of localized spins and itinerant carriers. The resulting carrier localization is confirmed by optical conductivity measurements

Hidden kagome-lattice picture and origin of high conductivity in delafossite PtCoO2

We study the electronic structure of delafossite PtCoO2 to elucidate its extremely small resistivity and high mobility. The band exhibits steep dispersion near the Fermi level despite the fact that itis formed mainly by Pt d orbitals that are typically localized. We propose a picture based on two hidden kagome-lattice-like electronic structure: one originating from Pt s + px/py orbitals, and the other from Pt d3z^2-r^2 + dxy/dx^2y^2 orbitals, each placed on the bonds of the triangular lattice. In particular, we find that the underlying Pt s + px/py bands actually determine the steepness of the original dispersion, so that the large Fermi velocity can be attributed to the large width of the Pt s + px/py band. In addition, the kagome-like electronic structure gives rise to "orbital-momentum locking" on the Fermi surface, which reduces the electron scattering by impurities. We conclude that the combination of the large Fermi velocity and the orbital-momentum locking is likely to be the origin of the extremely small resistivity in PtCoO2.PostprintPeer reviewe