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arXiv2014

Theory of extreme correlations using canonical Fermions and path integrals

The t-J model is studied using a novel and rigorous mapping of the Gutzwiller projected electrons, in terms of canonical electrons. The mapping has considerable similarity to the Dyson-Maleev transformation relating spin operators to canonical Bosons. This representation gives rise to a non Hermitean quantum theory, characterized by minimal redundancies. A path integral representation of the canonical theory is given. Using it, the salient results of the extremely correlated Fermi liquid (ECFL) theory, including the previously found Schwinger equations of motion, are easily rederived. Further a transparent physical interpretation of the previously introduced auxiliary Greens functions and the caparison factor is obtained. The low energy electron spectral function in this theory with a strong intrinsic asymmetry, is summarized in terms of a few expansion coefficients. These include an important emergent energy scale $Δ_0$ that shrinks to zero on approaching the insulating state, thereby making it difficult to access the underlying low energy Fermi liquid behavior. The scaled low frequency ECFL spectral function is related simply to the Fano line shape. The resulting energy dispersion (EDC or MDC) is a hybrid of a massive and a massless Dirac spectrum $ E^*_Q\sim γ\, Q- \sqrt{Γ_0^2 + Q^2} $, where the vanishing of $Q$, a momentum like variable, locates the kink. Therefore the quasiparticle velocity interpolates between $(γ\mp 1)$ over a width $Γ_0$ on the two sides of $Q=0$. The resulting kink strongly resembles a prominent low energy feature seen in angle resolved photoemission spectra (ARPES) of cuprate materials. We also propose novel ways of analyzing the ARPES data to isolate the predicted asymmetry between particle and hole excitations.

cond-mat.str-el↗

arXiv2010

Characterizing Complex Particle Morphologies Through Shape Matching: Descriptors, Applications, and Algorithms

Many standard structural quantities, such as order parameters and correlation functions, exist for common condensed matter systems, such as spherical and rod-like particles. However, these structural quantities are often insufficient for characterizing the unique and highly complex structures often encountered in the emerging field of nano and microscale self-assembly, or other disciplines involving complex structures such as computational biology. Computer science algorithms known as "shape matching" methods pose a unique solution to this problem by providing robust metrics for quantifying the similarity between pairs of arbitrarily complex structures. This pairwise matching operation, either implicitly or explicitly, lies at the heart of most standard structural characterization schemes for particle systems. By substituting more robust "shape descriptors" into these schemes we extend their applicability to structures formed from more complex building blocks. Here, we describe several structural characterization schemes and shape descriptors that can be used to obtain various types of structural information about particle systems. We demonstrate the application of shape matching algorithms to a variety of example problems, for topics including local and global structure identification and classification, automated phase diagram mapping, and the construction of spatial and temporal correlation functions. The methods are applicable to a wide range of systems, both simulated and experimental, provided particle positions are known or can be accurately imaged.

cond-mat.soft↗

arXiv2018

Taming the magnetoresistance anomaly in graphite

At low temperatures, graphite presents a magnetoresistance anomaly which manifests as a transition to a high-resistance state (HRS) above a certain critical magnetic field $\text{B}_\text{c}$. Such HRS is currently attributed to a c-axis charge-density-wave taking place only when the lowest Landau level is populated. By controlling the charge carrier concentration of a gated sample through its charge neutrality level (CNL), we were able to experimentally modulate the HRS in graphite for the first time. We demonstrate that the HRS is triggered both when electrons and holes are the majority carriers but is attenuated near the CNL. Taking screening into account, our results indicate that the HRS possess a strong in-plane component and can occur below the quantum limit, being at odds with the current understanding of the phenomenon. We also report the effect of sample thickness on the HRS.

cond-mat.str-el↗

arXiv2014

Hole sp3-character and delocalization in (Ga,Mn)As revised with pSIC and MLWF approaches - newly found spin-unpolarized gap states of s-type below 1% of Mn

The dilute magnetic semiconductor (Ga,Mn)As is ferromagnetic in accordance with the p-d Zener model. Hole density function (HDF) localization has been previously studied by means of the density functional theory (DFT) and non-standard DFT methods; however not for dopings near 1%. We have revised (Ga,Mn)As using the DFT with the pseudopotential self-interaction correction (pSIC) and maximally-localized Wannier functions (MLWFs), which show the sp3 character of a HDF. Nature of HDF is extended - for low dopings and the pSIC, 70% of the HDF is located within the inter-impurities region, and contribution of the 3d-Mn states is 3-5% for 1-3% of Mn with the pSIC, and 11% with the DFT. We found that for dopings below 1%, the spin-unpolarized s-type impurity states segregate from the conduction band to the energy gap - in contrast to earlier publications. This implies that donor co-doped dilute samples would be both insulating and nonmagnetic.

cond-mat.str-el↗

arXiv2013

Pairing insights in iron-based superconductors from scanning tunneling microscopy

Scanning tunneling microscopy (STM) has made tremendous progress in the study and understanding of both classical and unconventional superconductors. This has motivated a rapidly growing effort to apply the same techniques to the iron-based high-Tc superconductors since their discovery in 2008. Five years have brought exciting advances in imaging and spectroscopic investigation of this new class of materials. In this review, we focus on several recent STM contributions to the identification of the gap symmetry and pairing glue. We highlight the unique capabilities and challenges still ahead for STM studies of iron-based superconductors.

cond-mat.supr-con↗

arXiv2013

Orbital-dependent effects of electron correlations in microscopic models for iron-based superconductors

The bad metal behavior in the normal state of the iron-based superconductors suggests an intimate connection between the superconductivity and a proximity to a Mott transition. At the same time, there is strong evidence for the multi-orbital nature of the electronic excitations. It is then important to understand the orbital-dependent effects of electron correlations. In this paper we review the recent theoretical progresses on the metal-to-insulator transition in multiorbital models for the iron-based superconductors. These include studies of models that contain at least the 3d xy and 3d xz/yz models, using a slave-spin technique. For commensurate filling corresponding to that of the parent iron pnictides and chalcogenideds, a Mott transition generally exists in all these models. Near the Mott transition, a strongly correlated metal exhibiting bad metal features and strong orbital selectivity is stabilized due to the interplay of Hund's coupling and orbital-degeneracy breaking. Particularly for the alkaline iron selenides, the ordered vacancies effectively reduce the kinetic energy, thereby pushing the system further into the Mott-insulating regime; in the metallic state, there exists an orbital-selective Mott phase in which the iron 3d xy orbital is Mott localized while the other 3d orbitals are still itinerant. An overall phase diagram for the alkaline iron selenides has been proposed, in which the orbital-selective Mott phase connects between the superconducting phase and the Mott-insulating parent state.

cond-mat.supr-con↗

arXiv2013

Improved Green's Function Measurement for Hybridization Expansion Quantum Monte Carlo

We present an algorithm for measurement of the Green's function in the hybridization expansion continuous-time quantum Monte-Carlo based on continuous estimators. Compared to the standard method, the present algorithm has similar or better accuracy with improvement notable especially at low perturbation orders and high frequencies. The resulting statistical noise is weakly correlated so high-accuracy data for the numerical analytical continuation can be produced.

cond-mat.str-el↗

arXiv2014

CheMPS2: a free open-source spin-adapted implementation of the density matrix renormalization group for ab initio quantum chemistry

The density matrix renormalization group (DMRG) has become an indispensable numerical tool to find exact eigenstates of finite-size quantum systems with strong correlation. In the fields of condensed matter, nuclear structure and molecular electronic structure, it has significantly extended the system sizes that can be handled compared to full configuration interaction, without losing numerical accuracy. For quantum chemistry (QC), the most efficient implementations of DMRG require the incorporation of particle number, spin and point group symmetries in the underlying matrix product state (MPS) ansatz, as well as the use of so-called complementary operators. The symmetries introduce a sparse block structure in the MPS ansatz and in the intermediary contracted tensors. If a symmetry is non-abelian, the Wigner-Eckart theorem allows to factorize a tensor into a Clebsch-Gordan coefficient and a reduced tensor. In addition, the fermion signs have to be carefully tracked. Because of these challenges, implementing DMRG efficiently for QC is not straightforward. Efficient and freely available implementations are therefore highly desired. In this work we present CheMPS2, our free open-source spin-adapted implementation of DMRG for ab initio QC. Around CheMPS2, we have implemented the augmented Hessian Newton-Raphson complete active space self-consistent field method, with exact Hessian. The bond dissociation curves of the 12 lowest states of the carbon dimer were obtained at the DMRG(28 orbitals, 12 electrons, D$_{\mathsf{SU(2)}}$=2500)/cc-pVDZ level of theory. The contribution of $1s$ core correlation to the $X^1Σ_g^+$ bond dissociation curve of the carbon dimer was estimated by comparing energies at the DMRG(36o, 12e, D$_{\mathsf{SU(2)}}$=2500)/cc-pCVDZ and DMRG-SCF(34o, 8e, D$_{\mathsf{SU(2)}}$=2500)/cc-pCVDZ levels of theory.

cond-mat.str-el↗

arXiv2018

An efficient GPU algorithm for tetrahedron-based Brillouin-zone integration

We report an efficient algorithm for calculating momentum-space integrals in solid state systems on modern graphics processing units (GPUs). Our algorithm is based on the tetrahedron method, which we demonstrate to be ideally suited for execution in a GPU framework. In order to achieve maximum performance, all floating point operations are executed in single precision. For benchmarking our implementation within the CUDA programming framework we calculate the orbital-resolved density of states in an iron-based superconductor. However, our algorithm is general enough for the achieved improvements to carry over to the calculation of other momentum integrals such as, e.g. susceptibilities. If our program code is integrated into an existing program for the central processing unit (CPU), i.e. when data transfer overheads exist, speedups of up to a factor $\sim130$ compared to a pure CPU implementation can be achieved, largely depending on the problem size. In case our program code is integrated into an existing GPU program, speedups over a CPU implementation of up to a factor $\sim165$ are possible, even for moderately sized workloads.

physics.comp-ph↗

arXiv2013

Recent developments in transport phenomena in Weyl semimetals

The last decade has witnessed great advancements in the science and engineering of systems with unconventional band structures, seeded by studies of graphene and topological insulators. While the band structure of graphene simulates massless relativistic electrons in two dimensions, topological insulators have bands that wind non-trivially over momentum space in a certain abstract sense. Over the last couple of years, enthusiasm has been burgeoning in another unconventional and topological (although, not quite in the same sense as topological insulators) phase -- the Weyl Semimetal. In this phase, electrons mimic Weyl fermions that are well-known in high-energy physics, and inherit many of their properties, including an apparent violation of charge conservation known as the Chiral Anomaly. In this review, we recap some of the unusual transport properties of Weyl semimetals discussed in the literature so far, focusing on signatures whose roots lie in the anomaly. We also mention several proposed realizations of this phase in condensed matter systems, since they were what arguably precipitated activity on Weyl semimetals in the first place.

cond-mat.str-el↗

arXiv2014

What is the simplest model which captures the basic experimental facts of the physics of underdoped cuprates?

I discuss here some well established experimental facts which have been shown to be generic of the pseudogap in underdoped cuprates. Some recent developments on Cellular Dynamic Mean Field Theories of the doped Hubbard model on a square lattice are nowadays allowing one to compute physical properties and k resolved single particle spectra. At high temperatures, recent results present qualitative features which perfectly fit the experimental observations done for more than two decades on the pseudogap in the cuprates. Most various broken symmetry states detected recently at lower temperatures than the pseudogap T*, certainly require more complicated modelling, and are not necessarily generic. I suggest that various perturbations due to the order (or disorder) of chemical dopants, which can hardly be avoided in actual materials, might drive the pseudogap electronic matter into specific ground states which exhibit interesting correlated electronic orders.

cond-mat.str-el↗

arXiv2013

Valence Fluctuation in CeMo2Si2C

We report on the valence fluctuation of Ce in CeMo$_{2}$Si$_{2}$C as studied by means of magnetic susceptibility $χ(T)$, specific heat $C(T)$, electrical resistivity $ρ(T)$ and x-ray absorption spectroscopy. Powder x-ray diffraction revealed that CeMo$_{2}$Si$_{2}$C crystallizes in CeCr$_{2}$Si$_{2}$C-type layered tetragonal crystal structure (space group \textit{P4/mmm}). The unit cell volume of CeMo$_{2}$Si$_{2}$C deviates from the expected lanthanide contraction, indicating non-trivalent state of Ce ions in this compound. The observed weak temperature dependence of the magnetic susceptibility and its low value indicate that Ce ions are in valence fluctuating state. The formal $L_{III}$ Ce valence in CeMo$_{2}$Si$_{2}$C $<$$\widetildeν$$>$ = 3.11 as determined from x-ray absorption spectroscopy measurement is well bellow the value $<$$\widetildeν$$> \simeq$ 3.4 in tetravalent Ce compound CeO$_{2}$. The temperature dependence of specific heat does not show any anomaly down to 1.8 K which rules out any magnetic ordering in the system. The Sommerfeld coefficient obtained from the specific heat data is $γ$ = 23.4 mJ/mol\,K$^{2}$. The electrical resistivity follows the $T{^2}$ behavior in the low temperature range below 35 K confirming a Fermi liquid behavior. Accordingly both the Kadowaki Wood ratio $A/γ^{2}$ and the Sommerfeld Wilson ratio $χ(0)/γ$ are in the range expected for Fermi-liquid systems. In order to get some information on the electronic states, we calculated the band structure within the density functional theory, eventhough this approach is not able to treat 4f electrons accurately. The non-$f$ electron states crossing the Fermi level have mostly Mo 4d character. They provide the states with which the 4f sates are strongly hybridized, leading to the intermediate valent state.

cond-mat.mtrl-sci↗

arXiv2013

Magnetic anisotropy and lattice dynamics in FeAs studied by Mössbauer spectroscopy

Iron mono-arsenide in the powder form has been investigated by transmission 57Fe Moessbauer spectroscopy in the temperature range 4.2 - 1000 K. Additional spectra have been obtained at 20 K and 100 K applying external magnetic field of 7 T. It was found that the spin spiral propagating along the c-axis leads to the complex variation of the hyperfine magnetic field amplitude with the spin orientation varying in the a-b plane. The magnitude of the hyperfine field pointing in the direction of the local magnetic moment depends on the orientation of this moment in the a-b plane. Patterns are vastly different for iron located in the [0 k 0] positions and for iron in the [0 k+1/2 0] positions within the orthorhombic cell set to the Pnma symmetry. Lattice softens upon transition to the paramagnetic state at 69.2 K primarily in the a-c plane as seen by iron atoms. This effect is quite large considering lack of the structural transition. Two previously mentioned iron sites are discernible in the paramagnetic region till 300 K by different electron densities on the iron nuclei. The anisotropy of the iron vibrations developed at the transition to the paramagnetic state increases with the temperature in accordance with the harmonic approximation, albeit tends to saturation at high temperatures indicating gradual onset of the quasi-harmonic conditions. It seems that neither hyperfine fields nor magnetic moments are correct order parameters in light of the determined static critical exponents. Sample starts to loose arsenic at about 1000 K and under vacuum.

cond-mat.mtrl-sci↗

arXiv2014

Electric field gradient wave (EFGW) in iron-based superconductor Ba(0.6)K(0.4)Fe2As2 studied by Mössbauer spectroscopy

The optimally doped 122 iron-based superconductor Ba(0.6)K(0.4)Fe2As2 has been studied by 57Fe Moessbauer spectroscopy versus temperature ranging from 4.2 K till 300 K with particular attention paid to the superconducting transition around 38 K. The spectra do not contain magnetic components and they exhibit quasi-continuous distribution of quadrupole split doublets. A distribution follows the electric field gradient (EFG) spatial modulation (wave) - EFGW. The EFGW is accompanied by some charge density wave (CDW) having about an order of magnitude lesser influence on the spectrum. The EFGW could be modeled as widely separated narrow sheets with the EFG increasing from small till maximum value almost linearly and subsequently dropping back to the original value in a similar fashion - across the sheet. One encounters very small and almost constant EFG between sheets. The EFGW shape and amplitude as well as the amplitude of CDW are strongly affected by a superconducting transition. All modulations are damped significantly at transition (38 K) and recover at a temperature being about 14 K lower. The maximum quadrupole splitting at 4.2 K amounts to about 2.1 mm/s, while the dispersion of CDW seen on the iron nuclei could be estimated far away from the superconducting gap opening and at low temperature as 0.5 el./a.u.^3. It drops to about 0.3 el./a.u.^3 just below transition to the superconducting state.

cond-mat.supr-con↗

arXiv2014

Single crystal study of the layered heavy fermion compounds Ce$_2$PdIn$_8$, Ce$_3$PdIn$_{11}$, Ce$_2$PtIn$_8$ and Ce$_3$PtIn$_{11}$

We report on single crystal growth and crystallographic parameters results of Ce$_2$PdIn$_8$, Ce$_3$PdIn$_{11}$, Ce$_2$PtIn$_8$ and Ce$_3$PtIn$_{11}$. The Pt-systems Ce$_2$PtIn$_8$ and Ce$_3$PtIn$_{11}$ are synthesized for the first time. All these compounds are member of the Ce$_n$T$_m$In$_{3n+2m}$ (n = 1, 2,..; m = 1, 2,.. and T = transition metal) to which the extensively studied heavy fermion superconductor CeCoIn$_5$ belongs. Single crystals have been grown by In self-flux method. Differential scanning calorimetry studies were used to derive optimal growth conditions. Evidently, the maximum growth conditions for these materials should not exceed 750 $^{\circ}$C. Single crystal x-ray data show that Ce$_2$TIn$_8$ compounds crystallize in the tetragonal Ho$_2$CoGa$_8$ phase (space group P4/mmm) with lattice parameters a =4.6898(3) $Å$ and c =12.1490(8) $Å$ for the Pt-based one (Pd: a = 4.6881(4) $Å$ and c = 12.2031(8) Å). The Ce$_3$TIn$_{11}$ compounds adopt the Ce$_3$PdIn$_{11}$ structure with a = 4.6874(4) $Å$ and c = 16.8422(12) $Å$ for the Pt-based one (Pd: a = 4.6896 $Å$ and c = 16.891 Å). Specific heat experiments on Ce$_3$PtIn$_{11}$ and Ce$_3$PdIn$_{11}$ have revealed that both compounds undergo two successive magnetic transitions at T$_1$ ~ 2.2 K followed by T$_N$ ~ 2.0 K and T$_1$ ~ 1.7 K and T$_N$ ~ 1.5 K, respectively. Additionally, both compounds exhibit enhanced Sommerfeld coefficients yielding γ$_{Pt}$ = 0.300 J/mol K$^2$ Ce (γ$_{Pd}$ = 0.290 J/mol K$^2$ Ce), hence qualifying them as heavy fermion materials.

cond-mat.str-el↗

arXiv2007

Spin-transfer physics and the model of ferromagnetism in (Ga,Mn)As

We describe recent progress and open questions in the physics of current-induced domain-wall displacement and creep in (Ga,Mn)As. Furthermore, the reasons are recalled why, despite strong disorder and localization, the p-d Zener model is suitable for the description of this system.

cond-mat.mtrl-sci↗

arXiv2013

Cluster glass magnetism in the phase-separated Nd2/3Ca1/3MnO3 perovskite

A detailed study of the low-temperature magnetic state and the relaxation in the phase-separated colossal magnetoresistance Nd2/3Ca1/3MnO3 perovskite has been carried out. Clear experimental evidence of the cluster-glass magnetic behavior of this compound has been revealed. Well defined maxima in the in-phase linear ac susceptibility $χ^{/}(T)$ were observed, indicative of the magnetic glass transition at $T_{g}$ = 60 K. Strongly divergent zero-field-cooled and field-cooled static magnetizations and frequency dependent ac susceptibility are evident of the glassy-like magnetic state of the compound at low temperatures. The frequency dependence of the cusp temperature Tmax of the $χ^{/}(T)$ susceptibility was found to follow the critical slowing down mechanism. The Cole-Cole analysis of the dynamic susceptibility at low temperature has shown extremely broad distribution of relaxation times, indicating that spins are frozen at "macroscopic" time scale. Slow relaxation in the zero-field-cooled magnetization has been experimentally revealed. The obtained results do not agree with a canonical spin-glass state and indicate a cluster glass magnetic state of the compound below $T_{g}$, associated with its antiferromagnetic-ferromagnetic nano-phase segregated state. It was found that the relaxation mechanisms below the cluster glass freezing temperature $T_{g}$ and above it are strongly different. Magnetic field up to about $μ_{0}$H = 0.4 T suppresses the glassy magnetic state of the compound.

cond-mat.mtrl-sci↗

arXiv2013

Magnetocaloric effect in {[Fe(pyrazole)$_4$]$_2$[Nb(CN)$_8$]$\cdot$4H$_2$O}$_n$ molecular magnet

Magnetocaloric effect in {[Fe(pyrazole)$_4$]$_2$[Nb(CN)$_8$]$\cdot$4H$_2$O}$_n$ molecular magnet is reported. It crystallizes in tetragonal I4$_1$/a space group. The compound exhibits a phase transition to a long range magnetically ordered state at $T_\mathrm{c}\approx$8.3 K. The magnetic entropy change $ΔS_\mathrm{M}$ as well as the adiabatic temperature change $ΔT_\mathrm{ad}$ due to applied field change $μ_0ΔH$=0.1, 0.2, 0.5, 1, 2, 5, 9 T as a function of temperature have been determined by the relaxation calorimetry measurements. The maximum value of $ΔS_\mathrm{M}$ for $μ_0ΔH=5$ T is 4.9 J mol$^{-1}$ K$^{-1}$ (4.8 J kg$^{-1}$ K$^{-1}$) at 10.3 K. The corresponding maximum value of $ΔT_\mathrm{ad}$ is 2.0 K at 8.9 K. The temperature dependence of the exponent $n$ characterizing the field dependence of $ΔS_\mathrm{M}$ has been estimated. It attains the value of 0.64 at the transition temperature, which is consistent with the 3D Heisenberg universality class.

cond-mat.mtrl-sci↗