arXiv subjects

Jan M. Tomczak

Publications and source records attributed to Jan M. Tomczak.

At least 37 records · Page 2Linked to original sources

arXiv2021

Timescale of local moment screening across and above the Mott transition

A material's phase diagram typically indicates the types of realized long-range orders, corresponding to instabilities in static response functions. In correlated systems, however, key phenomena crucially depend on dynamical processes, too: In a Mott insulator, the electrons' spin moment fluctuates in time, while it is dynamically screened in Kondo systems. Here, we introduce a timescale $t_m$ characteristic for the screening of the local spin moment and demonstrate that it fully characterizes the dynamical mean-field phase diagram of the Hubbard model: The retarded magnetic response delineates the Mott transition and provides a new perspective on its signatures in the supercritical region above. We show that $t_m$ has knowledge of the Widom line and that it can be used to demarcate the Fermi liquid from the bad metal regime. Additionally, it reveals new structures inside the Fermi liquid phase: First, we identify a region with preformed local moments that we suggest to have a thermodynamic signature. Second, approaching the Mott transition from weak coupling, we discover a regime in which the spin dynamics becomes adiabatic, in the sense that it is much slower than valence fluctuations. Our findings provide resolution limits for magnetic measurements and may build a bridge to the relaxation dynamics of non-equilibrium states.

cond-mat.str-el↗

arXiv2021

Correlations turn electronic structure of finite-layer nickelates upside down

Motivated by the recent discovery of superconductivity in the pentalayer nickelate Nd$_6$Ni$_5$O$_{12}$ [Nature Materials 10.1038], we calculate its electronic structure and superconducting critical temperature. We find that electronic correlations are essential for pushing Nd$_6$Ni$_5$O$_{12}$ into the superconducting doping range as they shift the electron pockets above the Fermi energy. As a consequence, Nd$_6$Ni$_5$O$_{12}$ can be described with a single $d_{x^2-y^2}$ orbital per Ni. Instead, for the bilayer nickelate Nd$_3$Ni$_2$O$_6$ we find correlations to drive the system into a three-orbital regime also involving the Ni $d_{xz,yz}$ states. We suggest, however, that single-orbital physics with optimal doping can be restored by substituting 60% of the trivalent Nd or La by tetravalent Zr.

cond-mat.supr-con↗

arXiv2021

Hidden, one-dimensional, strongly nested, and almost half-filled Fermi surface in Ba$_2$CuO$_{3+y}$ superconductors

All previous cuprate superconductors display a set of common features: (i) vicinity to a Cu 3$d^{9}$ configuration; (ii) separated CuO$_2$ planes; (iii) superconductivity for doping $δ\sim$ 0.1$-$0.3. Recently [PNAS {\bf 24}, 12156 (2019)] challenged this picture by discovering "highly overdoped" superconducting Ba$_2$CuO$_{3+y}$. Using density-functional theory + dynamical mean-field theory, we reveal a bilayer structure of Ba$_2$CuO$_{3.2}$ of alternating quasi 2D and quasi 1D character. Correlations tune an inter-layer self-doping leading to an almost half-filled, strongly nested quasi 1D $d_{b^2-c^2}$ band, which is prone to strong antiferromagnetic fluctuations, possibly at the origin of superconductivity in Ba$_2$CuO$_{3+y}$.

cond-mat.supr-con↗

arXiv2021

Designing a mechanically driven spin-crossover molecular switch via organic embedding

Among spin-crossover complexes, Fe-porphyrin (FeP) stands out for molecular spintronic applications: An intricate, yet favourable balance between ligand fields, charge transfer, and the Coulomb interaction makes FeP highly manipulable, while its planar structure facilitates device integration. Here, we theoretically design a mechanical spin-switch device in which external strain triggers the intrinsic magneto-structural coupling of FeP through a purely organic embedding. Exploiting the chemical compatibility and stretchability of graphene nanoribbon electrodes, we overcome common reliability and reproducibility issues of conventional inorganic setups. The competition between the Coulomb interaction and distortion-induced changes in ligand fields requires methodologies beyond the state-of-the-art: Combining density functional theory with many-body techniques, we demonstrate experimentally feasible tensile strain to trigger a low-spin ($S=1$) to high-spin ($S=2$) crossover. Concomitantly, the current through the device toggles by over an order of magnitude, adding a fully planar mechanical current-switch unit to the panoply of molecular spintronics.

cond-mat.str-el↗

arXiv2020

Pitfalls and solutions for perovskite transparent conductors

Transparent conductors-nearly an oxymoron-are in pressing demand, as ultra-thin-film technologies become ubiquitous commodities. As current solutions rely on non-abundant elements, perovskites such as SrVO3 and SrNbO3 have been suggested as next generation transparent conductors. Our ab-initio calculations and analytical insights show, however, that reducing the plasma frequency below the visible spectrum by strong electronic correlations-a recently proposed strategy-unavoidably comes at a price: an enhanced scattering and thus a substantial optical absorption above the plasma edge. As a way out of this dilemma we identify several perovskite transparent conductors, relying on hole doping, somewhat larger bandwidths and separations to other bands.

cond-mat.mtrl-sci↗

arXiv2020

Resistivity saturation in Kondo insulators

Resistivities of heavy-fermion insulators typically saturate below a characteristic temperature $T^*$. For some, metallic surface states, potentially from a non-trivial bulk topology, are a likely source of residual conduction. Here, we establish an alternative mechanism: At low temperature, in addition to the charge gap, the scattering rate turns into a relevant energy scale, invalidating the semiclassical Boltzmann picture. Finite lifetimes of intrinsic carriers limit conduction, impose the existence of a crossover $T^*$, and control - now on par with the gap - the quantum regime emerging below it. We showcase the mechanism with realistic many-body simulations and elucidate how the saturation regime of the Kondo insulator Ce$_3$Bi$_4$Pt$_3$, for which residual conduction is a bulk property, evolves under external pressure and varying disorder. Using a phenomenological formula we derived for the quantum regime, we also unriddle the ill-understood bulk conductivity of SmB$_6$ - demonstrating that our mechanism is widely applicable to correlated narrow-gap semiconductors.

cond-mat.str-el↗

arXiv2020

Anisotropy of electronic correlations: On the applicability of local theories to layered materials

Besides the chemical constituents, it is the lattice geometry that controls the most important material properties. In many interesting compounds, the arrangement of elements leads to pronounced anisotropies, which reflect into a varying degree of quasi two-dimensionality of their low-energy excitations. Here, we start by classifying important families of correlated materials according to a simple measure for the tetragonal anisotropy of their ab initio electronic (band) structure. Second, we investigate the impact of a progressively large anisotropy in driving the non-locality of many-body effects. To this end, we tune the Hubbard model from isotropic cubic in three dimensions to the two-dimensional limit and analyze it using the dynamical vertex approximation. For sufficiently isotropic hoppings, we find the self-energy to be well separable into a static non-local and a dynamical local contribution. While the latter could potentially be obtained from dynamical mean-field approaches, we find the former to be non-negligible in all cases. Further, by increasing the model-anisotropy, we quantify the degree of quasi two-dimensionality which causes this "space-time separation" to break down. Our systematic analysis improves the general understanding of electronic correlations in anisotropic materials, heterostructures and ultra-thin films, and provides useful guidance for future realistic studies.

cond-mat.str-el↗

arXiv2020

Topotactic hydrogen in nickelate superconductors and akin infinite-layer oxides ABO2

Superconducting nickelates appear to be difficult to synthesize. Since the chemical reduction of ABO3 (A: rare earth; B transition metal) with CaH2 may result in both, ABO2 and ABO2H, we calculate the topotactic H binding energy by density functional theory (DFT). We find intercalating H is energetically favorable for LaNiO2 but not for Sr-doped NdNiO2. This has dramatic consequences for the electronic structure as determined by DFT+dynamical mean field theory: that of 3d9 LaNiO2 is similar to (doped) cuprates, 3d8 LaNiO2H is a two-orbital Mott insulator. Topotactical H might hence explain why some nickelates are superconducting and others are not.

cond-mat.str-el↗

arXiv2019

The AbinitioD$Γ$A Project v1.0: Non-local correlations beyond and susceptibilities within dynamical mean-field theory

The ab initio extension of the dynamical vertex approximation (D$Γ$A) method allows for realistic materials calculations that include non-local correlations beyond $GW$ and dynamical mean-field theory. Here, we discuss the AbinitioD$Γ$A algorithm, its implementation and usage in detail, and make the program package available to the scientific community.

cond-mat.str-el↗

arXiv2019

Isoelectronic tuning of heavy fermion systems: Proposal to synthesize Ce3Sb4Pd3

The study of (quantum) phase transitions in heavy-fermion compounds relies on a detailed understanding of the microscopic control parameters that induce them. While the influence of external pressure is rather straight forward, atomic substitutions are more involved. Nonetheless, replacing an elemental constituent of a compound with an isovalent atom is---effects of disorder aside---often viewed as merely affecting the lattice constant. Based on this picture of chemical pressure, the unit-cell volume is identified as an empirical proxy for the Kondo coupling. Here instead, we propose an "orbital scenario" in which the coupling in complex systems can be tuned by isoelectronic substitutions with little or no effect onto cohesive properties. Starting with the Kondo insulator Ce$_3$Bi$_4$Pt$_3$, we consider---within band-theory---isoelectronic substitutions of the pnictogen (Bi$\rightarrow$Sb) and/or the precious metal (Pt$\rightarrow$Pd). We show for the isovolume series Ce$_3$Bi$_4$(Pt$_{1-x}$Pd$_x$)$_3$ that the Kondo coupling is in fact substantially modified by the different radial extent of the $5d$ (Pt) and $4d$ (Pd) orbitals, while spin-orbit coupling mediated changes are minute. Combining experimental Kondo temperatures with simulated hybridization functions, we also predict effective masses $m^*$, finding excellent agreement with many-body results for Ce$_3$Bi$_4$Pt$_3$. Our analysis motivates studying the so-far unknown Kondo insulator Ce$_3$Sb$_4$Pd$_3$, for which we predict $m^*/m_{band}=\mathcal{O}(10)$.

cond-mat.str-el↗

arXiv2019

Realistic many-body theory of Kondo insulators: Renormalizations and fluctuations in Ce$_3$Bi$_4$Pt$_3$

Our theoretical understanding of heavy-fermion compounds mainly derives from iconic models, such as those due to Kondo or Anderson. While providing invaluable qualitative insight, detailed comparisons to experiments are encumbered by the materials' complexity, including the spin-orbit coupling, crystal fields, and ligand hybridizations. Here, we study the paradigmatic Kondo insulator Ce$_3$Bi$_4$Pt$_3$ with a first principles dynamical mean-field method that includes these complications. We find that salient signatures of many-body effects in this material---large effective masses, the insulator-to-metal crossover, the concomitant emergence of Curie-Weiss behaviour and notable transfers of optical spectral weight---are captured quantitatively. With this validation, we elucidate the fabric of the many-body state. In particular, we extent the phenomenology of the Kondo crossover to time-scales of fluctuations: We evidence that spin and charge degrees of freedom each realize two regimes in which fluctuations adhere to vastly different decay laws. We find these regimes to be separated by a {\it common} temperature $T^{max}_χ$, linked to the onset of Kondo screening. Interestingly, below (above) $T^{max}_χ$, valence fluctuations become faster (slower) than the dynamical screening of the local moments. Overall, however, spin and charge fluctuations occur on comparable time-scales of $\mathcal{O}(0.5-12\hbox{ fs})$, placing them on the brink of detection for modern time-resolved probes.

cond-mat.str-el↗

arXiv2018

Strain-induced tuning of the electronic Coulomb interaction in 3d transition metal oxide perovskites

Epitaxial strain offers an effective route to tune the physical parameters in transition metal oxides. So far, most studies have focused on the effects of strain on the bandwidths and crystal field splitting, but recent experimental and theoretical works have shown that also the effective Coulomb interaction changes upon structural modifications. This effect is expected to be of paramount importance in current material engineering studies based on epitaxy-based material synthesization. Here, we perform constrained random phase approximation calculations for prototypical oxides with a different occupation of the d shell, LaTiO3 (d1), LaVO3 (d2), and LaCrO3 (d3), and systematically study the evolution of the effective Coulomb interactions (Hubbard U and Hund's J) when applying epitaxial strain. Surprisingly, we find that the response upon strain is strongly dependent on the material. For LaTiO3, the interaction parameters are determined by the degree of localization of the orbitals, and grow with increasing tensile strain. In contrast, LaCrO3 shows the opposite trends: the interactions parameters shrink upon tensile strain. This is caused by the enhanced screening due to the larger electron filling. LaVO3 shows an intermediate behavior.

cond-mat.str-el↗

arXiv2018

Thermoelectricity in correlated narrow-gap semiconductors

We review many-body effects, their microscopic origin, as well as their impact onto thermoelectricity in correlated narrow-gap semiconductors. Members of this class---such as FeSi and FeSb$_2$---display an unusual temperature dependence in various observables: insulating with large thermopowers at low temperatures, they turn bad metals at temperatures much smaller than the size of their gaps. This insulator-to-metal crossover is accompanied by spectral weight-transfers over large energies in the optical conductivity and by a gradual transition from activated to Curie-Weiss-like behaviour in the magnetic susceptibility. We show a retrospective of the understanding of these phenomena, discuss the relation to heavy-fermion Kondo insulators---such as Ce$_3$Bi$_4$Pt$_3$ for which we present new results---and propose a general classification of paramagnetic insulators. From the latter FeSi emerges as an orbital-selective Kondo insulator. Focussing on intermetallics such as silicides, antimonides, skutterudites, and Heusler compounds we showcase successes and challenges for the realistic simulation of transport properties in the presence of electronic correlations. Further, we advert to new avenues in which electronic correlations may contribute to the improvement of thermoelectric performance.

cond-mat.str-el↗

arXiv2017

Strongly correlated double Dirac fermions

Double Dirac fermions have recently been identified as possible quasiparticles hosted by three-dimensional crystals with particular non-symmorphic point group symmetries. Applying a combined approach of ab-initio methods and dynamical mean field theory, we investigate how interactions and double Dirac band topology conspire to form the electronic quantum state of Bi$_2$CuO$_4$. We derive a downfolded eight-band model of the pristine material at low energies around the Fermi level. By tuning the model parameters from the free band structure to the realistic strongly correlated regime, we find a persistence of the double Dirac dispersion until its constituting time reveral symmetry is broken due to the onset of magnetic ordering at the Mott transition. We analyze pressure as a promising route to realize a double-Dirac metal in Bi$_2$CuO$_4$.

cond-mat.str-el↗

arXiv2017

Ab initio dynamical vertex approximation

Diagrammatic extensions of dynamical mean field theory (DMFT) such as the dynamical vertex approximation (D$Γ$A) allow us to include non-local correlations beyond DMFT on all length scales and proved their worth for model calculations. Here, we develop and implement an AbinitioD$Γ$A approach for electronic structure calculations of materials. Starting point is the two-particle irreducible vertex in the two particle-hole channels which is approximated by the bare non-local Coulomb interaction and all local vertex corrections. From this we calculate the full non-local vertex and the non-local self-energy through the Bethe-Salpeter equation. The AbinitioD$Γ$A approach naturally generates all local DMFT correlations and all non-local $GW$ contributions, but also further non-local correlations beyond: mixed terms of the former two and non-local spin fluctuations. We apply this new methodology to the prototypical correlated metal SrVO$_3$.

cond-mat.str-el↗

arXiv2015

Large Seebeck Effect by Charge-Mobility Engineering

The Seebeck effect describes the generation of an electric potential in a conducting solid exposed to a temperature gradient. Besides fundamental relevance in solid state physics, it serves as a key quantity to determine the performance of functional thermoelectric materials. In most cases, it is dominated by an energy-dependent electronic density of states at the Fermi level, in line with the prevalent efforts toward superior thermoelectrics through the engineering of electronic structure. Here, we demonstrate an alternative source for the Seebeck effect based on charge-carrier relaxation: A charge mobility that changes rapidly with temperature can result in a sizeable addition to the Seebeck coefficient. This new Seebeck source is demonstrated explicitly for Ni-doped CoSb3, where a dramatic mobility change occurs due to the crossover between two different charge-relaxation regimes. Our findings unveil the origin of pronounced features in the Seebeck coefficient of many other elusive materials characterized by a significant mobility mismatch. As the physical origin for the latter can vary greatly, our proposal provides a unifying framework for the understanding of a large panoply of thermoelectric phenomena. When utilized appropriately, this effect can also provide a novel route to the design of improved thermoelectric materials for applications in solid-state cooling or power generation.

cond-mat.mtrl-sci↗

arXiv2015

A route to room temperature ferromagnetic ultrathin SrRuO$_3$ films

Experimental efforts to stabilize ferromagnetism in ultrathin films of transition metal oxides have so far failed, despite expectations based on density functional theory (DFT) and DFT+U. Here, we investigate one of the most promising materials, SrRuO$_3$, and include correlation effects beyond DFT by means of dynamical mean field theory. In agreement with experiment we find an intrinsic thickness limitation for metallic ferromagnetism in SrRuO$_3$ thin films. Indeed, we demonstrate that the realization of ultrathin ferromagnetic films is out of reach of standard thin-film techniques. Proposing charge carrier doping as a new route to manipulate thin films, we predict room temperature ferromagnetism in electron-doped SrRuO$_3$ ultra thin films.

cond-mat.str-el↗

arXiv2014

Dynamical correlations and screened exchange on the experimental bench: spectral properties of the cobalt pnictide BaCo2As2

Understanding the Fermi surface and low-energy excitations of iron or cobalt pnictides is crucial for assessing electronic instabilities such as magnetic or superconducting states. Here, we propose and implement a new approach to compute the low-energy properties of correlated electron materials, taking into account both screened exchange beyond the local density approximation and local dynamical correlations. The scheme allows us to resolve the puzzle of BaCo2As2, for which standard electronic structure techniques predict a ferromagnetic instability not observed in nature.

cond-mat.str-el↗