how i can use wordpress function and plugin shortcut in ajax.php

function scroll_load(){
var num_next=current_company()
//alert(num_next)
        $('#loading').css('display', 'block')
        $.ajax({
            url: '<?php echo get_stylesheet_directory_uri().'/ajax.php?id=';?>'+num_next,
            data:{year:current_year},
            dataType: 'html',
            success: function(html) {

                $('#posts').append(html); 

                $('#loading').css('display', 'none')
            },
            error: function( jqXhr, textStatus, errorThrown ){
                console.log( errorThrown );
            }
        });

        }

this is my function i get data from ajax.php but when i use wordpress functions like get_header of short cut of plugins it say fatal error any one help me thanks

via Chebli Mohamed