arXiv subjects

Turan Birol

Publications and source records attributed to Turan Birol.

At least 55 records · Page 3Linked to original sources

arXiv2022

Loop currents in $A$V$_3$Sb$_5$ kagome metals: multipolar and toroidal magnetic orders

Experiments in the recently discovered vanadium-based kagome metals have suggested that their charge-ordered state displays not only bond distortions, characteristic of a ``real" charge density-wave (rCDW), but also time-reversal symmetry-breaking, typical of loop currents described by an ``imaginary" charge density-wave (iCDW). Here, we combine density-functional theory, group-theory, and phenomenological modeling to investigate the complex charge-ordered states that arise from interactions between the low-energy van Hove singularities present in the electronic structure of $A$V$_3$Sb$_5$. We find two broad classes of mixed iCDW-rCDW configurations: triple-$\mathbf{Q}$ iCDW, triple-$\mathbf{Q}$ rCDW order, dubbed $3\mathbf{Q}$-$3\mathbf{Q}$, and double-$\mathbf{Q}$ iCDW, single-$\mathbf{Q}$ rCDW order, dubbed $2\mathbf{Q}$-$1\mathbf{Q}$. Moreover, we identify seven different types of iCDW order, stemming from the different vanadium-orbital and kagome-sublattice structures of the two pairs of van Hove singularities present above and below the Fermi level. While the $2\mathbf{Q}$-$1\mathbf{Q}$ states trigger an orthorhombic distortion that breaks the threefold rotational symmetry of the kagome lattice, the $3\mathbf{Q}$-$3\mathbf{Q}$ states induce various types of subsidiary uniform magnetic orders, from conventional ferromagnetism to magnetic octupolar, magnetic toroidal, and even magnetic monopolar order. We show that these exotic orders display unique magneto-striction, magneto-electric, and magneto-electric-striction properties that can be probed experimentally to identify which iCDW state is realized in these compounds. We briefly discuss the impact of an out-of-plane modulation of the charge order and the interplay between these complex charge-ordered states and superconductivity.

cond-mat.str-el↗

arXiv2022

Strain effect on the ground-state structure of Sr2SnO4 Ruddlesden-Popper oxides

Ruddlesden-Popper (RP) oxides (A$_{n+1}$B$_n$O$_{3n+1}$) comprised of perovskite (ABO$_3$)$_n$ slabs can host a wider variety of structural distortions than their perovskite counterparts. This makes accurate structural determination of RP oxides more challenging. In this study, we investigate the structural phase diagram of $n=1$ RP Sr$_2$SnO$_4$, one of alkaline earth stannates that are promising for opto-electronic applications by using group theory-based symmetry analysis and first-principles calculations. We explore the symmetry breaking effects of different dynamical instabilities, predict the energies of phases they lead to, and take into account different (biaxial strain and hydrostatic pressure) boundary conditions. We also address the effect of structural changes on the electronic structure and find that compressive biaxial strain drives Sr$_2$SnO$_4$ into a regime with wider bandgap and lower electron effective mass.

cond-mat.mtrl-sci↗

arXiv2022

Room-Temperature Valence Transition in a Strain-Tuned Perovskite Oxide

Cobalt oxides have long been understood to display intriguing phenomena known as spin-state crossovers, where the cobalt ion spin changes vs. temperature, pressure, etc. A very different situation was recently uncovered in praseodymium-containing cobalt oxides, where a first-order coupled spin-state/structural/metal-insulator transition occurs, driven by a remarkable praseodymium valence transition. Such valence transitions, particularly when triggering spin-state and metal-insulator transitions, offer highly appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr1-xCaxCoO3). Here, we show that in thin films of the complex perovskite (Pr1-yYy)1-xCaxCoO3-δ, heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291 K, i.e., around room temperature. The technological implications of this result are accompanied by fundamental prospects, as complete strain control of the electronic ground state is demonstrated, from ferromagnetic metal under tension to nonmagnetic insulator under compression, thereby exposing a potential novel quantum critical point.

cond-mat.mtrl-sci↗

arXiv2022

Uniaxial strain control of bulk ferromagnetism in rare-earth titanates

The perovskite rare-earth titanates are model Mott insulators with magnetic ground states that are very sensitive to structural distortions. These distortions couple strongly to the orbital degrees of freedom and, in principle, it should be possible to tune the superexchange and the magnetic transition with strain. We investigate the representative system (Y,La,Ca)TiO$_3$, which exhibits low crystallographic symmetry and no structural instabilities. From magnetic susceptibility measurements of the Curie temperature, we demonstrate direct, reversible and continuous control of ferromagnetism by influencing the TiO$_6$ octahedral tilts and rotations with uniaxial strain. The relative change in $T_C$ as a function of strain is well described by ab initio calculations, which provides detailed understanding of the complex interactions among structural, orbital and magnetic properties in rare-earth titanates. The demonstrated manipulation of octahedral distortions opens up far-reaching possibilities for investigations of electron-lattice coupling, competing ground states, and magnetic quantum phase transitions in a wide range of quantum materials.

cond-mat.str-el↗

arXiv2022

Charge density wave order in kagome metal AV$_3$Sb$_5$ (A= Cs, Rb, K)

We employ polarization-resolved electronic Raman spectroscopy and density functional theory to study the primary and secondary order parameters, as well as their interplay, in the charge density wave (CDW) state of the kagome metal AV3Sb5. Previous x-ray diffraction data at 15K established that the CDW order in CsV3Sb5 comprises of a 2x2x4 structure: one layer of inverse-star-of-David and three consecutive layers of star-of-David pattern. We analyze the lattice distortions based the 2x2x4 structure at 15K, and find that U lattice distortion is the primary order parameter while M and L distortions are secondary order parameters for Vanadium displacements. This conclusion is confirmed by the calculation of bare susceptibility that shows a broad peak at around qz=0.25 along the hexagonal Brillouin zone face central line (U-line). We also identify several phonon modes emerging in the CDW state, which are lattice vibration modes related to V and Sb atoms as well as alkali atoms. The detailed temperature evolution of these modes' frequencies, HWHM, and integrated intensities support a phase diagram with two successive structural phase transitions in CsV3Sb5: the first one with a primary order parameter appearing at TS=94K and the second isostructural one appearing at around 70K.

cond-mat.str-el↗

arXiv2022

Strain-tunable metamagnetic critical endpoint in Mott insulating rare-earth titanates

Rare-earth titanates are Mott insulators whose magnetic ground state -- antiferromagnetic (AFM) or ferromagnetic (FM) -- can be tuned by the radius of the rare-earth element. Here, we combine phenomenology and first-principles calculations to shed light on the generic magnetic phase diagram of a chemically substituted titanate on the rare-earth site that interpolates between an AFM and a FM state. Octahedral rotations present in these perovskites cause the AFM order to acquire a small FM component -- and vice-versa -- removing any multicritical point from the phase diagram. However, for a wide parameter range, a first-order metamagnetic transition line terminating at a critical endpoint survives inside the magnetically ordered phase. Like the liquid-gas transition, a Widom line emerges from the endpoint, characterized by enhanced fluctuations. In contrast to metallic FMs, this metamagnetic transition involves two symmetry-equivalent and insulating canted spin states. Moreover, instead of a magnetic field, we show that uniaxial strain can be used to tune this transition to zero temperature, inducing a quantum critical endpoint.

cond-mat.str-el↗

arXiv2022

Charge order breaks time-reversal symmetry in CsV$_3$Sb$_5$

The recently discovered vanadium-based kagome metals $A$V$_{3}$Sb$_{5}$ ($A$~=~K,~Rb,~Cs) exhibit superconductivity at low-temperatures and charge density wave (CDW) order at high-temperatures. A prominent feature of the charge ordered state in this family is that it breaks time-reversal symmetry (TRSB), which is connected to the underlying topological nature of the band structure. In this work, a powerful combination of zero-field and high-field muon-spin rotation/relaxation is used to study the signatures of TRSB of the charge order in CsV$_3$Sb$_5$, as well as its anisotropic character. By tracking the temperature evolution of the in-plane and out-of-plane components of the muon-spin polarization, an enhancement of the internal field width sensed by the muon-spin ensemble was observed below $T_{\rm TRSB}=T_{\rm CDW}\simeq95$~K. Additional increase of the internal field width, accompanied by a change of the local field direction at the muon site from the $ab$-plane to the $c$-axis, was detected below $T^\ast\simeq30$~K. Remarkably, this two-step feature becomes well pronounced when a magnetic field of 8~T is applied along the crystallographic $c-$axis, thus indicating a field-induced enhancement of the electronic response at the CDW transition. These results point to a TRSB in CsV$_3$Sb$_5$ by charge order with an onset of ${\simeq}~95$~K, followed by an enhanced electronic response below ${\simeq}~30$~K. The observed two-step transition is discussed within the framework of different charge-order instabilities, which, in accordance with density functional theory calculations, are nearly degenerate in energy.

cond-mat.str-el↗

arXiv2022

Two types of charge order in the superconducting kagome material CsV$_3$Sb$_5$

The kagome metals of the family $A$V$_3$Sb$_5$, featuring a unique structural motif, harbor an array of intriguing phenomena such as chiral charge order and superconductivity. CsV$_3$Sb$_5$ is of particular interest because it displays a double superconducting dome in the region of the temperature-pressure phase diagram where charge order is still present. However, the microscopic origin of such an unusual behavior remains an unsolved issue. Here, to address it, we combine high-pressure, low-temperature muon spin relaxation with first-principles calculations. We observe a pressure-induced threefold enhancement of the superfluid density, which also displays a double peak feature, similar to the superconducting critical temperature. This leads to three distinct regions in the phase diagram, each of which features distinct slopes of the linear relation between superfluid density and the critical temperature. These results are attributed to a possible evolution of the charge order pattern from the superimposed tri-hexagonal Star-of-David phase at low pressures (within the first dome) to the staggered tri-hexagonal phase at intermediate pressures (between the first and second domes). Our findings suggest a change in the nature of the charge ordered state across the phase diagram of CsV$_3$Sb$_5$, with varying degrees of competition with superconductivity.

cond-mat.supr-con↗

arXiv2021

Response to comment on "Spin-lattice coupling and the emergence of the trimerized phase in the $S=1$ kagome antiferromagnet Na$_2$Ti$_3$Cl$_8$", Phys. Rev. Lett. 124, 167203 (2020)

In the context of the $S=1$ kagome antiferromagnet Na$_2$Ti$_3$Cl$_8$, we respond to the comment by Khomskii et al. [D.I. Khomskii, T. Mizokawa and S.V. Streltsov, Phys. Rev. Lett. 127, 049701 (2021)] on previous work by Paul et al. [A. Paul, C.-M. Chung, T. Birol, and H. J. Changlani, Phys. Rev. Lett. 124, 167203 (2020)].

cond-mat.str-el↗

arXiv2021

Revealing the competition between charge-density wave and superconductivity in CsV$_3$Sb$_5$ through uniaxial strain

In this paper we report the impact of uniaxial strain $\varepsilon$ applied along the crystalline $a$ axis on the newly discovered kagome superconductor CsV$_3$Sb$_5$. At ambient conditions, CsV$_3$Sb$_5$ shows a charge-density wave (CDW) transition at $T_{\rm CDW}=94.5$ K and superconducts below $T_C = 3.34$ K. In our study, when the uniaxial strain $\varepsilon$ is varied from $-0.90\%$ to $0.90\%$, $T_C$ monotonically increases by $\sim 33\%$ from 3.0 K to 4.0 K, giving rise to the empirical relation $T_C (\varepsilon)=3.4+0.56\varepsilon+0.12\varepsilon^2$. On the other hand, for $\varepsilon$ changing from $-0.76\%$ to $1.26\%$, $T_{\rm CDW}$ decreases monotonically by $\sim 10\%$ from 97.5 K to 87.5 K with $T_{\rm CDW}(\varepsilon)=94.5-4.72\varepsilon-0.60\varepsilon^2$. The opposite response of $T_C$ and $T_{\rm CDW}$ to the uniaxial strain suggests strong competition between these two orders. Comparison with hydrostatic pressure measurements indicate that it is the change in the $c$-axis that is responsible for these behaviors of the CDW and superconducting transitions, and that the explicit breaking of the sixfold rotational symmetry by strain has a negligible effect. Combined with our first-principles calculations and phenomenological analysis, we conclude that the enhancement in $T_C$ with decreasing $c$ is caused primarily by the suppression of $T_{\rm CDW}$, rather than strain-induced modifications in the bare superconducting parameters. We propose that the sensitivity of $T_{\rm CDW}$ with respect to the changes in the $c$-axis arises from the impact of the latter on the trilinear coupling between the $M_1^+$ and $L_2^-$ phonon modes associated with the CDW. Overall, our work reveals that the $c$-axis lattice parameter, which can be controlled by both pressure and uniaxial strain, is a powerful tuning knob for the phase diagram of CsV$_3$Sb$_5$.

cond-mat.supr-con↗

arXiv2021

Theory of the charge-density wave in $A$V$_3$Sb$_5$ kagome metals

The family of metallic kagome compounds $A$V$_3$Sb$_5$ ($A$=K, Rb, Cs) was recently discovered to exhibit both superconductivity and charge order. The nature of the charge-density wave (CDW) phase is presently unsettled, which complicates the interpretation of the superconducting ground state. In this paper, we use group-theory and density-functional theory (DFT) to derive and solve a phenomenological Landau model for this CDW state. The DFT results reveal three unstable phonon modes with the same in-plane momentum but different out-of-plane momenta, whose frequencies depend strongly on the electronic temperature. This is indicative of an electronically-driven CDW, stabilized by features of the in-plane electronic dispersion. Motivated by the DFT analysis, we construct a Landau free-energy expansion for coupled CDW order parameters with wave-vectors at the $M$ and $L$ points of the hexagonal Brillouin zone. We find an unusual trilinear term coupling these different order parameters, which can promote the simultaneous condensation of both CDWs even if the two modes are not nearly-degenerate. We classify the different types of coupled multi-$\bf{Q}$ CDW orders, focusing on those that break the sixfold rotational symmetry and lead to a unit-cell doubling along all three crystallographic directions, as suggested by experiments. We determine a region in parameter space, characterized by large nonlinear Landau coefficients, where these phases - dubbed staggered tri-hexagonal and staggered Star-of-David - are the leading instabilities of the system. Finally, we discuss the implications of our results for the kagome metals.

cond-mat.supr-con↗

arXiv2021

Free carrier induced ferroelectricity in layered perovskites

Doping ferroelectrics with carriers is often detrimental to polarization. This makes the design and discovery of metals that undergo a ferroelectric-like transition challenging. In this letter, we show from first principles that the oxygen octahedral rotations in perovskites are often enhanced by electron doping, and this can be used as a means to strengthen the structural polarization in certain hybrid-improper ferroelectrics -- compounds in which the polarization is not stabilized by the long range Coulomb interactions but is instead induced by a trilinear coupling to octahedral rotations. We use this design strategy to predict a cation ordered Ruddlesden-Popper compound that can be driven into a metallic ferroelectric-like phase via electrolyte gating.

cond-mat.mtrl-sci↗

arXiv2021

Chemical bonding and Born charge in 1T-HfS$_2$

We combine infrared absorption and Raman scattering spectroscopies to explore the properties of the heavy transition metal dichalcogenide 1T-HfS$_2$. We employ the LO-TO splitting of the $E_u$ vibrational mode along with a reevaluation of mode mass, unit cell volume, and dielectric constant to reveal the Born effective charge. We find $Z^*_{\rm{B}}$ = 5.3$e$, in excellent agreement with complementary first principles calculations. In addition to resolving controversy over the nature of chemical bonding in this system, we decompose Born charge into polarizability and local charge. We find $α$ = 5.07 Å$^3$ and $Z^{*}$ = 5.2$e$, respectively. Polar displacement-induced charge transfer from sulfur $p$ to hafnium $d$ is responsible for the enhanced Born charge compared to the nominal 4+ in hafnium. 1T-HfS$_2$ is thus an ionic crystal with strong and dynamic covalent effects. Taken together, our work places the vibrational properties of 1T-HfS$_2$ on a firm foundation and opens the door to understanding the properties of tubes and sheets.

cond-mat.mtrl-sci↗

arXiv2021

Robust Gapless Superconductivity in 4Hb-TaS$_2$

The superconducting TMD 4Hb-TaS$_2$ consists of alternating layers of H and T structures, which in their bulk form are metallic and Mott-insulating, respectively. Recently, this compound has been proposed as a candidate chiral superconductor, due to an observed enhancement of the muon spin relaxation at $T_c$. 4Hb-TaS$_2$ also exhibits a puzzling $T$-linear specific heat at low temperatures, which is unlikely to be caused by disorder. Elucidating the origin of this behavior is an essential step in discerning the true nature of the superconducting ground state. Here, we propose a simple model that attributes the $T$-linear specific heat to the emergence of a robust multi-band gapless superconducting state. We show that an extended regime of gapless superconductivity naturally appears when the pair-breaking scattering rate on distinct Fermi-surface pockets differs significantly, and the pairing interaction is predominantly intra-pocket. Using a tight-binding model derived from first-principle calculations, we show that the pair-breaking scattering rate promoted by slow magnetic fluctuations on the T layers, which arise from proximity to a Mott transition, can be significantly different in the various H-layer dominated Fermi pockets depending on their hybridization with T-layer states. Thus, our results suggest that the ground state of 4Hb-TaS$_2$ consists of Fermi pockets displaying gapless superconductivity, which are shunted by superconducting Fermi pockets that are nearly decoupled from the T-layers.

cond-mat.supr-con↗

arXiv2021

Direct observation and consequences of dopant segregation inside and outside dislocation cores in perovskite BaSnO3

Distinct dopant behaviors inside and outside dislocation cores are identified by atomic-resolution electron microscopy in perovskite BaSnO3 with considerable consequences on local atomic and electronic structures. Driven by elastic strain, when A-site designated La dopants segregate near a dislocation core, the dopant atoms accumulate at the Ba sites in compressively strained regions. This triggers formation of Ba-vacancies adjacent to the core atomic sites resulting in reconstruction of the core. Notwithstanding the presence of extremely large tensile strain fields, when La atoms segregate inside the dislocation core, they become B-site dopants, replacing Sn atoms and compensating the positive charge of the core oxygen vacancies. Electron energy-loss spectroscopy shows that the local electronic structure of these dislocations changes dramatically due to the segregation of the dopants inside and around the core ranging from formation of strong La-O hybridized electronic states near the conduction band minimum to insulator-to-metal transition.

cond-mat.mtrl-sci↗

arXiv2021

First-principles characterization of the magnetic properties of Cu$_2$(OH)$_3$Br

Low dimensional spin-1/2 transition metal oxides and oxyhalides continue to be at the forefront of research investigating nonclassical phases such as quantum spin liquids. In this study, we examine the magnetic properties of the oxyhalide $\text{Cu}_2\text{(OH)}_3\text{Br}$ in the botallackite structure using first-principles density functional theory, linear spin-wave theory, and exact diagonalization calculations. This quasi-2D system consists of $\text{Cu}^{2+}$ $\mathrm{S} = 1/2$ moments arranged on a distorted triangular lattice. Our exact diagonalization calculations, which rely on a first-principles-based magnetic model, generate spectral functions consistent with inelastic neutron scattering (INS) data. By performing computational experiments to disentangle the chemical and steric effects of the halide ions, we find that the dominant effect of the halogen ions is steric in the $\text{Cu}_2\text{(OH)}_3\text{X}$ series of compounds.

cond-mat.mtrl-sci↗

arXiv2020

Suppressing The Ferroelectric Switching Barrier in Hybrid Improper Ferroelectrics

Integration of ferroelectric materials into novel technological applications requires low coercive field materials, and consequently, design strategies to reduce the ferroelectric switching barriers. In this first principles study, we show that biaxial strain, which has a strong effect on the ferroelectric ground states, can also be used to tune the switching barrier of hybrid improper ferroelectric Ruddlesden-Popper oxides. We identify the region of the strain -- tolerance factor phase diagram where this intrinsic barrier is suppressed, and show that it can be explained in relation to strain induced phase transitions to nonpolar phases.

cond-mat.mtrl-sci↗

arXiv2020

Electronic correlations in the semiconducting half-Heusler compound FeVSb

Electronic correlations are crucial to the low energy physics of metallic systems with localized $d$ and $f$ states; however, their effect on band insulators and semiconductors is typically negligible. Here, we measure the electronic structure of the half-Heusler compound FeVSb, a band insulator with filled shell configuration of 18 valence electrons per formula unit ($s^2 p^6 d^{10}$). Angle-resolved photoemission spectroscopy (ARPES) reveals a mass renormalization of $m^{*}/m_{bare}= 1.4$, where $m^{*}$ is the measured effective mass and $m_{bare}$ is the mass from density functional theory (DFT) calculations with no added on-site Coulomb repulsion. Our measurements are in quantitative agreement with dynamical mean field theory (DMFT) calculations, highlighting the many-body origin of the mass renormalization. This mass renormalization lies in dramatic contrast to other filled shell intermetallics, including the thermoelectric materials CoTiSb and NiTiSn; and has a similar origin to that in FeSi, where Hund's coupling induced fluctuations across the gap can explain a dynamical self-energy and correlations. Our work calls for a re-thinking of the role of correlations and Hund's coupling in intermetallic band insulators.

cond-mat.mtrl-sci↗