Overview of the berry module

The berry module of postw90 is called by setting berry = true and choosing one or more of the available options for berry_task. The routines in the berry module which compute the \(k\)-space Berry curvature, orbital magnetization and spin Hall conductivity are also called when kpath = true and kpath_task = {curv,morb,shc}, or when kslice = true and kslice_task = {curv,morb,shc}.

Background: Berry connection and curvature

The Berry connection is defined in terms of the cell-periodic Bloch states \(\vert u_{n{\bf k}}\rangle=e^{-i{\bf k}\cdot{\bf r}}\vert \psi_{n{\bf k}}\rangle\) as \(\({\bf A}_n({\bf k})=\langle u_{n{\bf k}}\vert i\bm{\nabla}_{\bf k}\vert u_{n{\bf k}}\rangle,\)\) and the Berry curvature is the curl of the connection, \(\(\bm{\Omega}_n({\bf k})=\bm{\nabla}_{\bf k}\times {\bf A}_n({\bf k})= -{\rm Im} \langle \bm{\nabla}_{\bf k} u_{n{\bf k}}\vert \times \vert\bm{\nabla}_{\bf k} u_{n{\bf k}}\rangle.\)\) These two quantities play a central role in the description of several electronic properties of crystals ¹. In the following we will work with a matrix generalization of the Berry connection, \(\({\bf A}_{nm}({\bf k})=\langle u_{n{\bf k}}\vert i\bm{\nabla}_{\bf k}\vert u_{m{\bf k}}\rangle={\bf A}_{mn}^*({\bf k}), \label{eq:berry-connection-matrix}\)\) and write the curvature as an antisymmetric tensor, \(\(\label{eq:curv} \Omega_{n,\alpha\beta}({\bf k}) =\epsilon_{\alpha\beta\gamma} \Omega_{n,\gamma}({\bf k})=-2{\rm Im}\langle \nabla_{k_\alpha} u_{n\bf k}\vert \nabla_{k_\beta} u_{n\bf k}\rangle.\)\)

berry_task=kubo: optical conductivity and joint density of states

The Kubo-Greenwood formula for the optical conductivity of a crystal in the independent-particle approximation reads \(\(\sigma_{\alpha\beta}(\hbar\omega)=\frac{ie^2\hbar}{N_k\Omega_c} \sum_{\bf k}\sum_{n,m} \frac{f_{m{\bf k}}-f_{n{\bf k}}} {\varepsilon_{m{\bf k}}-\varepsilon_{n{\bf k}}} \frac{\langle\psi_{n{\bf k}}\vert v_\alpha\vert\psi_{m{\bf k}}\rangle \langle\psi_{m{\bf k}}\vert v_\beta\vert\psi_{n{\bf k}}\rangle} {\varepsilon_{m{\bf k}}-\varepsilon_{n{\bf k}}-(\hbar\omega+i\eta)}.\)\) Indices \(\alpha,\beta\) denote Cartesian directions, \(\Omega_c\) is the cell volume, \(N_k\) is the number of \(k\)-points used for sampling the Brillouin zone, and \(f_{n{\bf k}}=f(\varepsilon_{n{\bf k}})\) is the Fermi-Dirac distribution function. \(\hbar\omega\) is the optical frequency, and \(\eta>0\) is an adjustable smearing parameter with units of energy.

The off-diagonal velocity matrix elements can be expressed in terms of the connection matrix ², \(\(\label{eq:velocity_mat} \langle\psi_{n{\bf k}}\vert {\bf v} \vert\psi_{m{\bf k}}\rangle= -\frac{i}{\hbar}(\varepsilon_{m{\bf k}}-\varepsilon_{n{\bf k}}) {\bf A}_{nm}({\bf k})\,\,\,\,\,\,\,\,(m\not= n).\)\) The conductivity becomes \(\(\begin{aligned} \label{eq:sig-bz} \sigma_{\alpha\beta}(\hbar\omega)&= \frac{1}{N_k}\sum_{\bf k}\sigma_{{\bf k},\alpha\beta}(\hbar\omega)\\ \label{eq:sig-k} \sigma_{{\bf k},\alpha\beta}(\hbar\omega)&=\frac{ie^2}{\hbar\Omega_c}\sum_{n,m} (f_{m{\bf k}}-f_{n{\bf k}}) \frac{\varepsilon_{m{\bf k}}-\varepsilon_{n{\bf k}}} {\varepsilon_{m{\bf k}}-\varepsilon_{n{\bf k}}-(\hbar\omega+i\eta)} A_{nm,\alpha}({\bf k})A_{mn,\beta}({\bf k}). \end{aligned}\)\)

Let us decompose it into Hermitian (dissipative) and anti-Hermitean (reactive) parts. Note that \(\(\label{eq:lorentzian} \overline{\delta}(\varepsilon)=\frac{1}{\pi}{\rm Im} \left[\frac{1}{\varepsilon-i\eta}\right],\)\) where \(\overline{\delta}\) denotes a "broadended" delta-function. Using this identity we find for the Hermitean part \(\(\label{eq:sig-H} \sigma_{{\bf k},\alpha\beta}^{\rm H}(\hbar\omega)=-\frac{\pi e^2}{\hbar\Omega_c} \sum_{n,m}(f_{m{\bf k}}-f_{n{\bf k}}) (\varepsilon_{m{\bf k}}-\varepsilon_{n{\bf k}}) A_{nm,\alpha}({\bf k})A_{mn,\beta}({\bf k}) \overline{\delta}(\varepsilon_{m{\bf k}}-\varepsilon_{n{\bf k}}-\hbar\omega).\)\) Improved numerical accuracy can be achieved by replacing the Lorentzian ([eq:lorentzian]{reference-type="ref" reference="eq:lorentzian"}) with a Gaussian, or other shapes. The analytical form of \(\overline{\delta}(\varepsilon)\) is controlled by the keyword [kubo_]smr_type.

The anti-Hermitean part of Eq. ([eq:sig-k]{reference-type="ref" reference="eq:sig-k"}) is given by \(\(\label{eq:sig-AH} \sigma_{{\bf k},\alpha\beta}^{\rm AH}(\hbar\omega)=\frac{ie^2}{\hbar\Omega_c} \sum_{n,m}(f_{m{\bf k}}-f_{n{\bf k}}) {\rm Re}\left[ \frac{\varepsilon_{m{\bf k}}-\varepsilon_{n{\bf k}}} {\varepsilon_{m{\bf k}}-\varepsilon_{n{\bf k}} -(\hbar\omega+i\eta)} \right] A_{nm,\alpha}({\bf k})A_{mn,\beta}({\bf k}).\)\) Finally the joint density of states is \(\(\label{eq:jdos} \rho_{cv}(\hbar\omega)=\frac{1}{N_k}\sum_{\bf k}\sum_{n,m} f_{n{\bf k}}(1-f_{m{\bf k}}) \overline{\delta}(\varepsilon_{m{\bf k}}-\varepsilon_{n{\bf k}}-\hbar\omega).\)\)

Equations ([eq:lorentzian]{reference-type="ref" reference="eq:lorentzian"}--[eq:jdos]{reference-type="ref" reference="eq:jdos"}) contain the parameter \(\eta\), whose value can be chosen using the keyword\ [kubo_]smr_fixed_en_width. Better results can often be achieved by adjusting the value of \(\eta\) for each pair of states, i.e., \(\eta\rightarrow \eta_{nm\bf k}\). This is done as follows (see description of the keyword adpt_smr_fac) \(\(\eta_{nm{\bf k}}=\alpha\vert \bm{\nabla}_{\bf k} (\varepsilon_{m{\bf k}}-\varepsilon_{n{\bf k}})\vert \Delta k.\)\)

The energy eigenvalues \(\varepsilon_{n\bf k}\), band velocities \(\bm{\nabla}_{\bf k}\varepsilon_{n{\bf k}}\), and off-diagonal Berry connection \({\bf A}_{nm}({\bf k})\) entering the previous four equations are evaluated over a \(k\)-point grid by Wannier interpolation, as described in Refs. ³⁴. After averaging over the Brillouin zone, the Hermitean and anti-Hermitean parts of the conductivity are assembled into the symmetric and antisymmetric tensors \(\(\begin{aligned} \sigma^{\rm S}_{\alpha\beta}&= {\rm Re}\sigma^{\rm H}_{\alpha\beta}+i{\rm Im}\sigma^{\rm AH}_{\alpha\beta}\\ \sigma^{\rm A}_{\alpha\beta}&= {\rm Re}\sigma^{\rm AH}_{\alpha\beta}+i{\rm Im}\sigma^{\rm H}_{\alpha\beta}, \end{aligned}\)\) whose independent components are written as a function of frequency onto nine separate files.

berry_task=ahc: anomalous Hall conductivity

The antisymmetric tensor \(\sigma^{\rm A}_{\alpha\beta}\) is odd under time reversal, and therefore vanishes in non-magnetic systems, while in ferromagnets with spin-orbit coupling it is generally nonzero. The imaginary part \({\rm Im}\sigma^{\rm H}_{\alpha\beta}\) describes magnetic circular dichroism, and vanishes as \(\omega\rightarrow 0\). The real part \({\rm Re}\sigma^{\rm AH}_{\alpha\beta}\) describes the anomalous Hall conductivity (AHC), and remains finite in the static limit.

The intrinsic dc AHC is obtained by setting \(\eta=0\) and \(\omega=0\) in Eq. ([eq:sig-AH]{reference-type="ref" reference="eq:sig-AH"}). The contribution from point \({\bf k}\) in the Brillouin zone is \(\(\sigma^{\rm AH}_{{\bf k},\alpha\beta}(0)=\frac{2e^2}{\hbar\Omega_c} \sum_{n,m}f_{n\bf k}(1-f_{m\bf k}) {\rm Im}\langle \nabla_{k_\alpha} u_{n\bf k}\vert u_{m\bf k}\rangle \langle u_{m\bf k}\vert\nabla_{k_\beta} u_{n\bf k}\rangle,\)\) where we replaced \(f_{n\bf k}-f_{m\bf k}\) with \(f_{n\bf k}(1-f_{m\bf k})-f_{m\bf k}(1-f_{n\bf k})\).

This expression is not the most convenient for ab initio calculations, as the sums run over the complete set of occupied and empty states. In practice the sum over empty states can be truncated, but a relatively large number should be retained to obtain accurate results. Using the resolution of the identity \(1=\sum_m \vert u_{m\bf k}\rangle \langle u_{m\bf k}\vert\) and noting that the term \(\sum_{n,m}f_{n\bf k}f_{m\bf k}(\ldots)\) vanishes identically, we arrive at the celebrated formula for the intrinsic AHC in terms of the Berry curvature, \(\(\begin{aligned} \label{eq:ahc} \sigma^{\rm AH}_{\alpha\beta}(0)&=\frac{e^2}{\hbar} \frac{1}{N_k\Omega_c}\sum_{\bf k}(-1)\Omega_{\alpha\beta}({\bf k}),\\ %\sum_n (-1)f_{n\bf k}\Omega_{n,\alpha\beta}({\bf k}). \label{eq:curv-occ} \Omega_{\alpha\beta}({\bf k})&=\sum_n f_{n\bf k}\Omega_{n,\alpha\beta}({\bf k}). \end{aligned}\)\) Note that only occupied states enter this expression. Once we have a set of Wannier functions spanning the valence bands (together with a few low-lying conduction bands, typically) Eq. ([eq:ahc]) can be evaluated by Wannier interpolation as described in Refs. ³⁵, with no truncation involved.

berry_task=morb: orbital magnetization

The ground-state orbital magnetization of a crystal is given by ¹⁶ \(\(\begin{aligned} \label{eq:morb} {\bf M}^{\rm orb}&=\frac{e}{\hbar} %\int_{\rm BZ}\frac{d{\bf k}}{(2\pi)^3} \frac{1}{N_k\Omega_c}\sum_{\bf k}{\bf M}^{\rm orb}({\bf k}),\\ \label{eq:morb-k} {\bf M}^{\rm orb}({\bf k})&= \sum_n\,\frac{1}{2}f_{n{\bf k}}\, {\rm Im}\,\langle \bm{\nabla}_{\bf k}u_{n{\bf k}}\vert \times \left(H_{\bf k}+\varepsilon_{n{\bf k}}-2\varepsilon_F\right) \vert \bm{\nabla}_{\bf k}u_{n{\bf k}}\rangle, \end{aligned}\)\) where \(\varepsilon_F\) is the Fermi energy. The Wannier-interpolation calculation is described in Ref. ⁵. Note that the definition of \({\bf M}^{\rm orb}({\bf k})\) used here differs by a factor of \(-1/2\) from the one in Eq. (97) and Fig. 2 of that work.

berry_task=shc: spin Hall conductivity

The Kubo-Greenwood formula for the intrinsic spin Hall conductivity (SHC) of a crystal in the independent-particle approximation reads ⁷⁸⁹ \(\(\label{eq:kubo_shc} \sigma_{\alpha\beta}^{\text{spin}\gamma}(\omega) = \frac{\hbar}{\Omega_c N_k} \sum_{\bm{k}}\sum_{n} f_{n\bm{k}} \\ \sum_{m \neq n} \frac{2\operatorname{Im}[\langle n\bm{k}| \hat{j}_{\alpha}^{\gamma}|m\bm{k}\rangle \langle m\bm{k}| -e\hat{v}_{\beta}|n\bm{k}\rangle]} {(\epsilon_{n\bm{k}}-\epsilon_{m\bm{k}})^2-(\hbar\omega +i\eta)^2}.\)\) The spin current operator \(\hat{j}_{\alpha}^{\gamma}= \frac{1}{2}\{\hat{s}_{\gamma},\hat{v}_{\alpha}\}\) where the spin operator \(\hat{s}_{\gamma}=\frac{\hbar}{2}\hat{\sigma}_{\gamma}\). Indices \(\alpha,\beta\) denote Cartesian directions, \(\gamma\) denotes the direction of spin, commonly \(\alpha = x, \beta = y, \gamma = z\). \(\Omega_c\) is the cell volume, \(N_k\) is the number of \(k\)-points used for sampling the Brillouin zone, and \(f_{n{\bf k}}=f(\varepsilon_{n{\bf k}})\) is the Fermi-Dirac distribution function. \(\hbar\omega\) is the optical frequency, and \(\eta>0\) is an adjustable smearing parameter with unit of energy.

The velocity matrix element in the numerator is the same as Eq. ([eq:velocity_mat]{reference-type="ref" reference="eq:velocity_mat"}), so the only unknown quantity is the spin current matrix \(\langle n\bm{k}| \hat{j}_{\alpha}^{\gamma}|m\bm{k}\rangle\). We can use Wannier interpolation technique to efficiently calculate this matrix, and there are two derivation according to the degree of approximation. A noteworthy difference is the way in which two ab-initio matrix elements are evaluated, \(\(\langle u_{n{\bf k}}\vert\sigma_\gamma H_{\bf k}\vert u_{m{\bf k}+{\bf b}}\rangle, \langle u_{n{\bf k}}\vert\sigma_\gamma \vert u_{m{\bf k}+{\bf b}}\rangle, \gamma = x, y, z\)\) These are evaluated by pw2wannier90 using Ryoo's method. In contrast, Qiao's method does not require pw2wannier90, but it assumes an approximation \(1\approx\sum_{ l\in ab-initio{\rm \,bands}}|u_{l\bm{k}}\rangle \langle u_{l\bm{k}}|\). You can choose which method to evaluate this value with shc_method in the input file. For a full derivation please refer to Ref. ⁷ or Ref. ⁸.

The Eq. ([eq:kubo_shc]{reference-type="ref" reference="eq:kubo_shc"}) can be further separated into band-projected Berry curvature-like term \(\(\label{eq:kubo_shc_berry} \Omega_{n,\alpha\beta}^{\text{spin}\gamma}(\bm{k}) = {\hbar}^2 \sum_{ m\ne n}\frac{-2\operatorname{Im}[\langle n\bm{k}| \frac{1}{2}\{\hat{\sigma}_{\gamma},\hat{v}_{\alpha}\}|m\bm{k}\rangle \langle m\bm{k}| \hat{v}_{\beta}|n\bm{k}\rangle]} {(\epsilon_{n\bm{k}}-\epsilon_{m\bm{k}})^2-(\hbar\omega+i\eta)^2},\)\) \(k\)-resolved term which sums over occupied bands \(\(\label{eq:kubo_shc_berry_sum} \Omega_{\alpha\beta}^{\text{spin}\gamma}(\bm{k}) = \sum_{n} f_{n\bm{k}} \Omega_{n,\alpha\beta}^{\text{spin}\gamma}(\bm{k}),\)\) and the SHC is \(\(\sigma_{\alpha\beta}^{\text{spin}\gamma}(\omega) = -\frac{e^2}{\hbar}\frac{1}{\Omega_c N_k}\sum_{\bm{k}} \Omega_{\alpha\beta}^{\text{spin}\gamma}(\bm{k}).\)\) The unit of the \(\Omega_{n,\alpha\beta}^{\text{spin}\gamma}(\bm{k})\) is \(\text{length}^{2}\) (Angstrom\(^2\) or Bohr\(^2\), depending on your choice of berry_curv_unit in the input file), and the unit of \(\sigma_{\alpha\beta}^{\text{spin}\gamma}\) is \((\hbar/e)\)S/cm (the unit is written in the header of the output file). The case of \(\omega=0\) corresponds to direct current (dc) SHC while that of \(\omega\ne0\) corresponds to alternating current (ac) SHC or frequency-dependent SHC. Note in some papers Eq. ([eq:kubo_shc_berry]{reference-type="ref" reference="eq:kubo_shc_berry"}) is called as spin Berry curvature. However, it was pointed out by Ref. ¹⁰ that this name is misleading, so we use a somewhat awkward name "Berry curvature-like term" to refer to Eq. ([eq:kubo_shc_berry]{reference-type="ref" reference="eq:kubo_shc_berry"}). The \(k\)-resolved term Eq. ([eq:kubo_shc_berry_sum]{reference-type="ref" reference="eq:kubo_shc_berry_sum"}) can be used to draw kslice plot, and the band-projected Berry curvature-like term Eq. ([eq:kubo_shc_berry]{reference-type="ref" reference="eq:kubo_shc_berry"}) can be used to color the kpath plot.

Same as the case of optical conductivity, the parameter \(\eta\) contained in the Eq. ([eq:kubo_shc_berry]{reference-type="ref" reference="eq:kubo_shc_berry"}) can be chosen using the keyword [kubo_]smr_fixed_en_width. Also, adaptive smearing can be employed by the keyword [kubo_]adpt_smr (see Examples 29 and 30 in the Tutorial).

Please cite the following paper ⁷ or  ⁸ when publishing SHC results obtained using this method:

Junfeng Qiao, Jiaqi Zhou, Zhe Yuan, and Weisheng Zhao,\ Calculation of intrinsic spin Hall conductivity by Wannier interpolation,\ Phys. Rev. B. 98, 214402 (2018), DOI:10.1103/PhysRevB.98.214402.

or

Ji Hoon Ryoo, Cheol-hwan Park, and Ivo Souza,\ Computation of intrinsic spin Hall conductivities from first principles using maximally localized Wannier functions,\ Phys. Rev. B. 99, 235113 (2019), DOI:10.1103/PhysRevB.99.235113.

berry_task=sc: shift current

The shift-current contribution to the second-order response is characterized by a frequency-dependent third-rank tensor ¹¹ \(\(\label{eq:shiftcurrent} \begin{split} \sigma^{abc}(0;\omega,-\omega)=&-\frac{i\pi e^3}{4\hbar^2 \Omega_c N_k} \sum_{\bm{k}} \sum_{n,m}(f_{n\bm{k}}-f_{m\bm{k}}) \times \left(r^b_{ mn}(\bm{k})r^{c;a}_{nm}(\bm{k}) + r^c_{mn}(\bm{k})r^{b;a}_{ nm}(\bm{k})\right)\\ &\times \left[\delta(\omega_{mn\bm{k}}-\omega)+\delta(\omega_{nm\bm{k}}-\omega)\right], \end{split}\)\) where \(a,b,c\) are spatial indexes and \(\omega_{mn\bm{k}}=(\epsilon_{n\bm{k}}-\epsilon_{m\bm{k}})/\hbar\). The expression in Eq. [eq:shiftcurrent]{reference-type="ref" reference="eq:shiftcurrent"} involves the dipole matrix element \(\(\label{eq:r} r^a_{ nm}(\bm{k})=(1-\delta_{nm})A^a_{ nm}(\bm{k}),\)\) and its generalized derivative \(\(\label{eq:gen-der} r^{a;b}_{nm}(\bm{k})=\partial_{k_{b}} r^a_{nm}(\bm{k}) -i\left(A^b_{nn}(\bm{k})-A^b_{ mm}(\bm{k})\right)r^a_{ nm}(\bm{k}).\)\) The first-principles evaluation of the above expression is technically challenging due to the presence of an extra \(k\)-space derivative. The implementation in wannier90 follows the scheme proposed in Ref. ¹², following the spirit of the Wannier-interpolation method for calculating the AHC ³ by reformulating \(k\cdot p\) perturbation theory within the subspace of wannierized bands. This strategy inherits the practical advantages of the sum-over-states approach, but without introducing the truncation errors usually associated with this procedure ¹¹.

As in the case of the optical conductivity, a broadened delta function can be applied in Eq. [eq:shiftcurrent]{reference-type="ref" reference="eq:shiftcurrent"} by means of the parameter \(\eta\) (see Eq. [eq:lorentzian]{reference-type="ref" reference="eq:lorentzian"}) using the keyword [kubo_]smr_fixed_en_width, and adaptive smearing can be employed using the keyword [kubo_]adpt_smr.

Please cite Ref. ¹² when publishing shift-current results using this method.

berry_task=kdotp: \(k\cdot p\) coefficients

Consider a Hamiltonian \(\(\label{eq:H} H=H^{0}+H^{\prime}\)\) where the eigenvalues \(E_{n}\) and eigenfunctions \(\vert n\rangle\) of \(H^{0}\) are known, and \(H^{\prime}\) is a perturbation. In a nutshell, quasi-degenerate perturbation theory assumes that the set of eigenfunctions of \(H^0\) can be divided into subsets \(A\) and \(B\) that are weakly coupled by \(H^{\prime}\), and that we are only interested in subset \(A\). This theory asserts that a transformed Hamiltonian \(\tilde{H}\) exists within subspace \(A\) such that \(\(\label{eq:pert-exp} \tilde{H}=\tilde{H}^{0}+\tilde{H}^{1}+\tilde{H}^{2} + \cdots\)\) where \(\tilde{H}^{j}\) contain matrix elements of \(H^{\prime}\) to the \(j\)th power. According to Appendix B of Ref ¹³, the first three terms are \(\(\begin{aligned} \label{eq:pert-matelem0} & \tilde{H}^{0}_{mm'} = H^{0}_{mm'},\\ \label{eq:pert-matelem1} & \tilde{H}^{1}_{mm'} = H^{'}_{mm'},\\ \label{eq:pert-matelem2} & \tilde{H}^{2}_{mm'} = \dfrac{1}{2}\sum_{l}H^{'}_{ml}H^{'}_{lm'} \left( \dfrac{1}{E_{m}-E_{l}}+\dfrac{1}{E_{m'}-E_{l}} \right), \end{aligned}\)\) where \(m,m'\in A\) and \(l\in B\). The approximation \(\tilde{H}\sim \tilde{H}^{0}+\tilde{H}^{1}\) amounts to truncating \(H\) to the \(A\) subspace. By further including \(\tilde{H}^{2}\), the coupling to the \(B\) subspace is incorporated approximately, "renormalizing" the elements of the truncated matrix.

We adopt the notation described in Sec. III.B of Ref. ³. We shift the origin of \(k\) space to the point where the band edge (or some other band extremum of interest) is located, and Taylor expand around that point the Wannier-gauge Hamiltonian, \(\(\label{eq:HW-exp} H^{(W)}(\bm{k})=H^{(W)}(0) +\sum_{a}H_{a}^{(W)}(0)k_{a} +\dfrac{1}{2}\sum_{ab}H_{ab}^{(W)}(0)k_{a}k_{b} + \mathcal{O}(k^{3})\)\) where \(a,b=x,y,z\), and \(\(\begin{aligned} &H_{a}^{(W)}(0)=\left. \dfrac{\partial H^{(W)}(\bm{k})}{\partial k_{a}}\right\rvert_{\bm{k}=0}\\ &H_{ab}^{(W)}(0)=\left. \dfrac{\partial^{2} H^{(W)}(\bm{k})}{\partial k_{a}\partial k_{b}}\right\rvert_{\bm{k}=0} \end{aligned}\)\)

We now apply to \(H^{(W)}(\bm{k})\) a similarity transformation \(U(0)\) that diagonalizes \(H^{(W)}(0)\), and call the transformed Hamiltonian \(H(\bm{k})\), \(\(\label{eq:Hbar} H(\bm{k})=\overbrace{\overline{H}}^{H^{0}} + \overbrace{\sum_{a}\overline{H}_{a}k_{a} +\dfrac{1}{2}\sum_{ab}\overline{H}_{ab}k_{a}k_{b}}^{H^{\prime}} + \mathcal{O}(k^{3}),\)\) where we introduced the notation \(\(\overline{\mathcal{O}}=U^{\dagger}(0)\mathcal{O}^{(W)}(0)U(0),\)\) and applied it to \(\mathcal{O}=H,{H}_{a},{H}_{ab}\). We can now apply quasi-degenerate perturbation theory by choosing the diagonal matrix \(\overline{H}\) as our \(H^{0}\), and the remaining (nondiagonal) terms in Eq. [eq:Hbar] as \(H^{\prime}\). Collecting terms in Eq. ([eq:pert-exp]{reference-type="ref" reference="eq:pert-exp"}) up to second order in \(k\) we get \(\(\label{eq:Htilde} \tilde{H}_{mm'}(\bm{k}) = \overline{H}_{mm'} + \sum_{a} \left(\overline{H}_{a}\right)_{mm'}k_{a} + \dfrac{1}{2}\sum_{a,b}\left[ \left(\overline{H}_{ab}\right)_{mm'} + \left({T}_{ab}\right)_{mm'} \right]k_{a}k_{b}+ \mathcal{O}(k^{3}),\)\) where \(m,m'\in A\) and we have defined the virtual-transition matrix \(\(\label{eq:Tab} \left({T}_{ab}\right)_{mm'}=\sum_{l\in B} \left(\overline{H}_{a}\right)_{ml}\left(\overline{H}_{b}\right)_{lm'} \times \left( \dfrac{1}{E_{m}-E_{l}}+\dfrac{1}{E_{m'}-E_{l}} \right) = \left({T}_{ab}\right)_{m'm}^{*}.\)\) (The \(T_{ab}\) term in Eq. [eq:Htilde] gives an Hermitean contribution to \(\tilde{H}(\bm{k})\) only after summing over \(a\) and \(b\), whereas the other terms are Hermitean already before summing.)

The implementation in wannier90 follows the scheme proposed in Ref. ¹⁴, and outputs \(\overline{H}_{mm'}\) in seedname-kdotp_0.dat, \(\left(\overline{H}_{a}\right)_{mm'}\) in seedname-kdotp_1.dat, and \(\left[\left(\overline{H}_{ab}\right)_{mm'} + \left({T}_{ab}\right)_{mm'}\right]/2\) in seedname-kdotp_2.dat.

Please cite Ref. ¹⁴ when publishing \(k\cdot p\) results using this method.

Needed matrix elements

All the quantities entering the formulas for the optical conductivity and AHC can be calculated by Wannier interpolation once the Hamiltonian and position matrix elements \(\langle {\bf 0}n\vert H\vert {\bf R}m\rangle\) and \(\langle {\bf 0}n\vert {\bf r}\vert {\bf R}m\rangle\) are known ³⁴. Those matrix elements are readily available at the end of a standard MLWF calculation with wannier90. In particular, \(\langle {\bf 0}n\vert {\bf r}\vert {\bf R}m\rangle\) can be calculated by Fourier transforming the overlap matrices in Eq. (1.7), \(\(\langle u_{n{\bf k}}\vert u_{m{\bf k}+{\bf b}}\rangle.\)\) Further Wannier matrix elements are needed for the orbital magnetization ⁵. In order to calculate them using Fourier transforms, one more piece of information must be taken from the \(k\)-space ab-initio calculation, namely, the matrices \(\(\langle u_{n{\bf k}+{\bf b}_1}\vert H_{\bf k}\vert u_{m{\bf k}+{\bf b}_2}\rangle\)\) over the ab-initio \(k\)-point mesh ⁵. These are evaluated by pw2wannier90, the interface routine between pwscf and wannier90, by adding to the input file seedname.pw2wan the line \(\({\tt %\begin{quote} write\_uHu = .true. %\end{quote} }\)\) The calculation of spin Hall conductivity needs the spin matrix elements \(\(\langle u_{n{\bf k}}\vert \sigma_\gamma \vert u_{m{\bf k}}\rangle, \gamma = x, y, z\)\) from the ab-initio \(k\)-point mesh. These are also evaluated by pw2wannier90 by adding to the input file seedname.pw2wan the line \(\({\tt %\begin{quote} write\_spn = .true. %\end{quote} }\)\) If one uses Ryoo's method to calculate spin Hall conductivity, the further matrix elements are needed: \(\(\langle u_{n{\bf k}}\vert \sigma_\gamma H_{\bf k}\vert u_{m{\bf k}+{\bf b}}\rangle, \langle u_{n{\bf k}}\vert \sigma_\gamma \vert u_{m{\bf k}+{\bf b}}\rangle, \gamma = x, y, z\)\) and these are evaluated by adding to the input file seedname.pw2wan the lines \(\({\tt write\_sHu = .true. }\)\) \(\({\tt write\_sIu = .true. }\)\)

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4. J. R. Yates, X. Wang, D. Vanderbilt, and I. Souza. Spectral and fermi surface properties from wannier interpolation. Phys. Rev. B, 75:195121, 2007. ↩↩

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7. Junfeng Qiao, Jiaqi Zhou, Zhe Yuan, and Weisheng Zhao. Calculation of intrinsic spin hall conductivity by wannier interpolation. Phys. Rev. B, 98:214402, Dec 2018. URL: https://link.aps.org/doi/10.1103/PhysRevB.98.214402, doi:10.1103/PhysRevB.98.214402. ↩↩↩

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